Subcellular Localization and Characterization of the ParAB System from Corynebacterium glutamicum

Faithful segregation of chromosomes and plasmids is a vital prerequisite to produce viable and genetically identical progeny. Bacteria use a specialized segregation system composed of the partitioning proteins ParA and ParB to segregate certain plasmids. Strikingly, homologues of ParA and ParB are found to be encoded in many chromosomes. Although mutations in the chromosomal Par system have effects on segregation efficiency, the exact mechanism by which the chromosomes are segregated into the daughter cells is not fully understood. We describe the polar localization of the ParB origin nucleoprotein complex in the actinomycete Corynebacterium glutamicum. ParB and the origin of replication were found to be stably localized to the cell poles. After replication, the origins move toward the opposite pole. Purified ParB was able to bind to the parS consensus sequence in vitro. C. glutamicum possesses two ParA-like partitioning ATPase proteins. Both proteins interact with ParB but show a slightly different subcellular localization and phenotype. While ParA might be part of a conventional partitioning system, PldP seems to play a role in division site selection.Bacterial cell division is a temporally and spatially tightly regulated process (1, 13, 16, 36, 37). Spatial regulation is achieved by division site selection and prevents fatal division across the nucleoids and aberrant division close to the cell poles (3, 40). Temporal control ensures that division does not precede chromosome replication and segregation. Replicated chromosomes are rapidly segregated into the daughter cells. However, the machinery that performs this active segregation is not fully elucidated. In contrast, plasmid segregation is somewhat better understood. Plasmids such as pB171 (8) encode a machinery composed of a tripartite system. Centromere-like DNA sequences, named parS sites, are composed of short inverted repeats. Centromere-binding proteins (ParB) are recruited to the parS sites, forming nucleoprotein complexes. Finally, a partitioning ATPase is recruited to the ParB-parS complex. The hydrolytic activity of ParA oligomers is believed to drive the active segregation process. Strikingly, many bacterial chromosomes encode orthologs of the plasmid partitioning genes parA and parB. A comparatively well-examined chromosomal partitioning system is that of Bacillus subtilis. B. subtilis encodes a ParA ATPase (called Soj) and a ParB protein (called Spo0J). B. subtilis contains eight parS sites that cluster around the oriC region and bind Spo0J. Subsequently Spo0J spreads across the DNA, thereby forming a huge nucleoprotein complex that could serve as a platform for anchoring the segregation machinery. The ParA protein Soj is a DNA-binding protein that dissociates from DNA upon ATP hydrolysis. A direct interaction of Soj and Spo0J has been described (35). Interestingly, analysis of knockout mutations revealed that only the loss of the ParB protein Spo0J increases the amount of anucleate cells slightly, while the loss of Soj has no significant effect on chromosome segregation (17, 18). However, knockout mutations in either parA or parB result only in subtle effects on chromosome segregation. Thus, although the two proteins might act together they have certainly multiple roles during chromosome segregation and cell division. Recently, it was shown that Spo0J (ParB) helps to recruit SMC proteins (for structural maintenance of chromosomes) to the oriC region, thereby ensuring correct chromosome organization, which seems essential for proper segregation (15, 39). The B. subtilis ParA homologue Soj was shown to play an role in the initiation of DNA replication by interacting with DnaA (32). Hence, the ParAB system is a central component connecting replication and segregation. Interestingly, Par proteins have been implicated with different developmental processes in other bacteria. In Caulobacter crescentus ParAB are involved in cell cycle progression and cell division. A ParA-like protein, MipZ, was shown to interact with ParB and directly inhibit FtsZ polymerization (42). Thus, chromosome segregation and cell division are directly coupled. Consequently, null mutations in ParA and ParB are lethal in C. crescentus. In Vibrio cholerae it was shown that ParA and ParB encoded on the large chromosome contribute to active chromosome segregation and anchor the oriC region of the chromosomes to the cell poles (10).Although these diverse properties of the Par system have been studied in some detail in the classical model organisms, the situation in other bacteria remains unknown. Corynebacteria are high GC Gram-positive bacteria and, depending on the growth medium, rod-shaped or club-shaped. A remarkable feature of corynebacteria and their close relatives is a special cell wall that has, in addition to the common peptidoglycan, an arabino-galactan and a mycolic acid layer. Notorious pathogens such as Mycobacterium tuberculosis, Mycobacterium leprae, and Corynebacterium diphtheriae are members of this family, and hence an understanding of fundamental cell biological mechanisms might reveal insights how to combat these organisms. We now report the subcellular localization of the chromosome partitioning system and the oriC in the actinomycete Corynebacterium glutamicum. We show localization and phenotypic consequences of the canonical ParAB proteins. Furthermore, we identified a ParA-like division protein (PldP) that plays a role in division site selection.     

Conjugative Plasmid Transfer and Adhesion Dynamics in an Escherichia coli Biofilm

A conjugative plasmid from the catheter-associated urinary tract infection strain Escherichia coli MS2027 was sequenced and annotated. This 42,644-bp plasmid, designated pMAS2027, contains 58 putative genes and is most closely related to plasmids belonging to incompatibility group X (IncX1). Plasmid pMAS2027 encodes two important virulence factors: type 3 fimbriae and a type IV secretion (T4S) system. Type 3 fimbriae, recently found to be functionally expressed in E. coli, played an important role in biofilm formation. Biofilm formation by E. coli MS2027 was specifically due to expression of type 3 fimbriae and not the T4S system. The T4S system, however, accounted for the conjugative ability of pMAS2027 and enabled a non-biofilm-forming strain to grow as part of a mixed biofilm following acquisition of this plasmid. Thus, the importance of conjugation as a mechanism to spread biofilm determinants was demonstrated. Conjugation may represent an important mechanism by which type 3 fimbria genes are transferred among the Enterobacteriaceae that cause device-related infections in nosocomial settings.Bacterial biofilms are complex communities of bacterial cells living in close association with a surface (17). Bacterial cells in these protected environments are often resistant to multiple factors, including antimicrobials, changes in the pH, oxygen radicals, and host immune defenses (19, 38). Biofilm formation is a property of many bacterial species, and a range of molecular mechanisms that facilitate this process have been described (2, 3, 11, 14, 16, 29, 33, 34). Often, the ability to form a biofilm is dependent on the production of adhesins on the bacterial cell surface. In Escherichia coli, biofilm formation is enhanced by the production of certain types of fimbriae (e.g., type 1 fimbriae, type 3 fimbriae, F1C, F9, curli, and conjugative pili) (14, 23, 25, 29, 33, 39, 46), cell surface adhesins (e.g., autotransporter proteins such as antigen 43, AidA, TibA, EhaA, and UpaG) (21, 34, 35, 40, 43), and flagella (22, 45).The close proximity of bacterial cells in biofilms creates an environment conducive for the exchange of genetic material. Indeed, plasmid-mediated conjugation in monospecific and mixed E. coli biofilms has been demonstrated (6, 18, 24, 31). The F plasmid represents the best-characterized conjugative system for biofilm formation by E. coli. The F pilus mediates adhesion to abiotic surfaces and stabilizes the biofilm structure through cell-cell interactions (16, 30). Many other conjugative plasmids also contribute directly to biofilm formation upon derepression of the conjugative function (16).One example of a conjugative system employed by gram-negative Enterobacteriaceae is the type 4 secretion (T4S) system. The T4S system is a multisubunit structure that spans the cell envelope and contains a secretion channel often linked to a pilus or other surface filament or protein (8). The Agrobacterium tumefaciens VirB-VirD4 system is the archetypical T4S system and is encoded by 11 genes in the virB operon and one gene (virD4) in the virD operon (7, 8). Genes with strong homology to genes in the virB operon have also been identified on other conjugative plasmids. For example, the pilX1 to pilX11 genes on the E. coli R6K IncX plasmid and the virB1 to virB11 genes are highly conserved at the nucleotide level (28).We recently described identification and characterization of the mrk genes encoding type 3 fimbriae in a uropathogenic strain of E. coli isolated from a patient with a nosocomial catheter-associated urinary tract infection (CAUTI) (29). The mrk genes were located on a conjugative plasmid (pMAS2027) and were strongly associated with biofilm formation. In this study we determined the entire sequence of plasmid pMAS2027 and revealed the presence of conjugative transfer genes homologous to the pilX1 to pilX11 genes of E. coli R6K (in addition to the mrk genes). We show here that biofilm formation is driven primarily by type 3 fimbriae and that the T4S apparatus is unable to mediate biofilm growth in the absence of the mrk genes. Finally, we demonstrate that conjugative transfer of pMAS2027 within a mixed biofilm confers biofilm formation properties on recipient cells due to acquisition of the type 3 fimbria-encoding mrk genes.     

Plasmid R1 Conjugative DNA Processing Is Regulated at the Coupling Protein Interface

Selective substrate uptake controls initiation of macromolecular secretion by type IV secretion systems in gram-negative bacteria. Type IV coupling proteins (T4CPs) are essential, but the molecular mechanisms governing substrate entry to the translocation pathway remain obscure. We report a biochemical approach to reconstitute a regulatory interface between the plasmid R1 T4CP and the nucleoprotein relaxosome dedicated to the initiation stage of plasmid DNA processing and substrate presentation. The predicted cytosolic domain of T4CP TraD was purified in a predominantly monomeric form, and potential regulatory effects of this protein on catalytic activities exhibited by the relaxosome during transfer initiation were analyzed in vitro. TraDΔN130 stimulated the TraI DNA transesterase activity apparently via interactions on both the protein and the DNA levels. TraM, a protein interaction partner of TraD, also increased DNA transesterase activity in vitro. The mechanism may involve altered DNA conformation as TraM induced underwinding of oriT plasmid DNA in vivo (ΔL_k = −4). Permanganate mapping of the positions of duplex melting due to relaxosome assembly with TraDΔN130 on supercoiled DNA in vitro confirmed localized unwinding at nic but ruled out formation of an open complex compatible with initiation of the TraI helicase activity. These data link relaxosome regulation to the T4CP and support the model that a committed step in the initiation of DNA export requires activation of TraI helicase loading or catalysis.Type IV secretion systems (T4SS) in gram-negative bacteria mediate translocation of macromolecules out of the bacterial cell (14). The transmission of effector proteins and DNA into plant cells or other bacteria via cell-cell contact is one example of their function, and conjugation systems as well as the transferred DNA (T-DNA) delivery system of the phytopathogen Agrobacterium tumefaciens are prototypical of the T4SS family. Macromolecular translocation is achieved by a membrane-spanning protein machinery comprised of 12 gene products, VirB1 to VirB11 and an associated factor known as the coupling protein (VirD4) (66). The T4SS-associated coupling protein (T4CP) performs a crucial function in recognition of appropriate secretion substrates and governing entry of those molecules to the translocation pathway (7, 8, 10, 30, 41). In conjugation systems substrate recognition is applied to the relaxosome, a nucleoprotein complex of DNA transfer initiator proteins assembled specifically at the plasmid origin of transfer (oriT). In current models, initiation of the reactions that provide the single strand of plasmid (T-strand) DNA for secretion to recipient bacteria is expected to resemble the initiation of chromosomal replication (for reviews, see references 18, 54, and 81). Controlled opening of the DNA duplex is required to permit entry of the DNA processing machinery. The task of remodeling the conjugative oriT is generally ascribed to two or three relaxosome auxiliary factors, of host and plasmid origin, which occupy specific DNA binding sites at this locus. Intrinsic to the relaxosome is also a site- and strand-specific DNA transesterase activity that breaks the phosphodiester backbone at nic (5). Upon cleavage, the transesterase enzyme (also called relaxase) forms a reversible phosphotyrosyl linkage to the 5′ end of the DNA. Duplex unwinding initiating from this site produces the single-stranded T strand to be exported. A wealth of information is available supporting the importance of DNA sequence recognition and binding by relaxosome components at oriT to the transesterase reaction in vitro and for effective conjugative transfer (for reviews, see references 18, 54, and 81). On the other hand, the mechanisms controlling release of the 3′-OH generated at nic and the subsequent DNA unwinding stage remain obscure.Equally little is known about the process of nucleoprotein uptake by the transport channel. DNA-independent translocation of the relaxases TrwC (R388), MobA (RSF1010), and VirD2 (Ti plasmid) has been demonstrated; thus, current models propose that the relaxase component of the protein-DNA adduct is the substrate actively secreted by the transport system after interaction with the T4CP (42, 66). Cotransport of the covalently linked single-stranded T strand occurs concurrently (42). The mechanisms underlying relaxosome recognition by T4CPs are not understood. Direct interactions have been observed biochemically between the RP4 TraG protein and relaxase proteins of the cognate plasmid (65) and heterologous relaxosomes that it mobilizes (73, 76). TrwB of R388 interacts in vitro with relaxase TrwC and an auxiliary component, TrwA (44). TraD proteins of plasmid R1 and F are known to interact with the auxiliary relaxosome protein TraM (20) via a cluster of C-terminal amino acids (3, 62). Extensive mutagenic analyses (45) plus recent three-dimensional structural data for a complex of the TraM tetramerization domain and the C-terminal tail of TraD (46) have provided more detailed models for the intermolecular contacts involved in recognition.Application of the Cre recombinase assay for translocation of conjugative relaxases as well as effector proteins to eukaryotic cells is currently the most promising approach to elucidate protein motifs recognized by T4CPs (56, 68, 78, 79). Despite that progress, the nature of the interactions between a T4CP and its target protein that initiate secretion and the mechanisms controlling this step remain obscure. In contrast to systems dedicated specifically to effector protein translocation, conjugation systems mobilize nucleoprotein complexes that additionally exhibit catalytic activities, which can be readily monitored. These models are therefore particularly well suited to investigate aspects of regulation occurring at the physical interface of a T4CP and its secretion substrate. For this purpose the MOB_F family of DNA-mobilizing systems is additionally advantageous, since DNA processing within this family features the fusion of a dedicated conjugative helicase to the DNA transesterase enzyme within a single bifunctional protein. The TraI protein of F-like plasmids, originally described as Escherichia coli DNA helicase I (1, 2, 23), and the related TrwC protein of plasmid R388 (25) are well characterized (reviewed in reference 18). Early work by Llosa et al. revealed a complex domain arrangement for TrwC (43). Similar analyses with TraI identified nonoverlapping transesterase and helicase domains (6, 77), while the remaining intermediate and C-terminal regions of the protein additionally provide functions essential to effective conjugative transfer (49, 71). The ability to physically separate the catalytic domains of TraI and TrwC has facilitated a detailed biochemical characterization of their DNA transesterase, ATPase, and DNA-unwinding reactions. Nonetheless, failure of the physically disjointed polypeptides to complement efficient conjugative transfer when coexpressed indicates a role(s) for these proteins in the strand transfer process that goes beyond the need for their dual catalytic activities (43, 50). The assignment of additional functional properties to regions within TraI is a focus of current investigation (16, 29, 49).In all systems studied thus far, conditions used to reconstitute relaxosomes on a supercoiled oriT plasmid have not supported the initiation steps necessary to enable duplex unwinding by a conjugative helicase. The question remains open whether additional protein components are required and/or whether the pathway of initiation is subject to specific repression. In the present study, we applied the IncFII plasmid R1 paradigm to investigate the potential for interaction between purified components of the relaxosome and its cognate T4CP, TraD, to exert regulatory effects on relaxosome activities in vitro. In this and in the accompanying report (72), we present evidence for wide-ranging stimulatory effects of the cytoplasmic domain of TraD protein and its interaction partner TraM on multiple aspects of relaxosome function.     

Characterization of the Replication,Transfer, and Plasmid/Lytic Phage Cycle of the Streptomyces Plasmid-Phage pZL12

We report here the isolation and recombinational cloning of a large plasmid, pZL12, from endophytic Streptomyces sp. 9R-2. pZL12 comprises 90,435 bp, encoding 112 genes, 30 of which are organized in a large operon resembling bacteriophage genes. A replication locus (repA) and a conjugal transfer locus (traA-traC) were identified in pZL12. Surprisingly, the supernatant of a 9R-2 liquid culture containing partially purified phage particles infected 9R-2 cured of pZL12 (9R-2X) to form plaques, and a phage particle (φZL12) was observed by transmission electron microscopy. Major structural proteins (capsid, portal, and tail) of φZL12 virions were encoded by pZL12 genes. Like bacteriophage P1, linear φZL12 DNA contained ends from a largely random pZL12 sequence. There was also a hot end sequence in linear φZL12. φZL12 virions efficiently infected only one host, 9R-2X, but failed to infect and form plaques in 18 other Streptomyces strains. Some 9R-2X spores rescued from lysis by infection of φZL12 virions contained a circular pZL12 plasmid, completing a cycle comprising autonomous plasmid pZL12 and lytic phage φZL12. These results confirm pZL12 as the first example of a plasmid-phage in Streptomyces.Streptomyces species, a major source of antibiotics and pharmacologically active metabolites, are Gram-positive, mycelial bacteria with high G+C content in their DNA (15). They usually harbor conjugative circular and/or linear plasmids, propagating in autonomous and/or chromosomally integrated forms (14). Most Streptomyces circular plasmids reported are small (8 to 14 kb), including rolling-circle-replication (RCR) plasmids (pIJ101, pJV1, pSG5, pSN22, pSVH1, pSB24.2, pSY10, pSNA1, pSLG33, pEN2701, etc.) (12, 14) and chromosomally integrating/autonomous plasmids (SLP1 and pSAM2) (4, 27, 28). Some theta replication plasmids are of intermediate size (31 to 39 kb), such as SCP2, pFP1, and pFP11 (13, 40). These theta replication loci comprise a rep gene and an adjacent noncoding or iteron sequence, to which Rep protein binds specifically in vitro (10, 40). The occurrence of an ∼163-kb large plasmid, pSV1, in Streptomyces violaceoruber SANK95570 was confirmed (1, 37), but this plasmid could not be physically isolated by standard procedures for plasmid preparation (17). In contrast to more than 30 genes for conjugal transfer on the Escherichia coli F plasmid (20), Streptomyces plasmids usually need a single tra gene (encoding a DNA translocase containing a cell division FtsK/SpoIIIE domain) (15, 29). The transfer of Streptomyces circular plasmids involves binding of the nonnicked double-stranded DNA (dsDNA) by multimers of Tra proteins at a noncoding sequence and ATP hydrolysis-dependent translocation of this DNA through the hyphal tips of the Streptomyces mycelium (15, 32).Numerous Streptomyces phages have been described, including φC31 (22), SAt1 (26), TG1 (11), FP43 (24), φSPK1 (19), φSC623 (34), DAH2/DAH4/DAH5/DAH6 (6), and mu1/6 (9). They range in size from 36 kb (19) to 121 kb (6), with 50 to 71.2% GC content (9, 23, 35). Streptomyces phages often have a wide host range; for example, 16 of 27 Streptomyces strains are susceptible to infection by φSPK1 (19), and phage FP43 transduces species of Streptoverticillium, Chainia, and Sacchropolyspora (24). φC31 is the most-studied Streptomyces phage and cloning vector (8). The sequences of the φC31 head proteins (e.g., portal, capsid, and head protease) resemble those of other bacterial dsDNA phages, suggesting evolutionary relationships to other viruses (35).We report here the isolation and recombinational cloning of a 90,435-bp plasmid, pZL12, from endophytic Streptomyces sp. 9R-2 and the characterization of its replication and transfer. Surprisingly, the supernatant of 9R-2 liquid culture infected 9R-2 cured of pZL12 to form plaques. A cycle comprising autonomous plasmid pZL12 and lytic phage φZL12 is described.     

Protein and DNA Effectors Control the TraI Conjugative Helicase of Plasmid R1

The mechanisms controlling progression of conjugative DNA processing from a preinitiation stage of specific plasmid strand cleavage at the transfer origin to a stage competent for unwinding the DNA strand destined for transfer remain obscure. Linear heteroduplex substrates containing double-stranded DNA binding sites for plasmid R1 relaxosome proteins and various regions of open duplex for TraI helicase loading were constructed to model putative intermediate structures in the initiation pathway. The activity of TraI was compared in steady-state multiple turnover experiments that measured the net production of unwound DNA as well as transesterase-catalyzed cleavage at nic. Helicase efficiency was enhanced by the relaxosome components TraM and integration host factor. The magnitude of stimulation depended on the proximity of the specific protein binding sites to the position of open DNA. The cytoplasmic domain of the R1 coupling protein, TraDΔN130, stimulated helicase efficiency on all substrates in a manner consistent with cooperative interaction and sequence-independent DNA binding. Variation in the position of duplex opening also revealed an unsuspected autoinhibition of the unwinding reaction catalyzed by full-length TraI. The activity reduction was sequence dependent and was not observed with a truncated helicase, TraIΔN308, lacking the site-specific DNA binding transesterase domain. Given that transesterase and helicase domains are physically tethered in the wild-type protein, this observation suggests that an intramolecular switch controls helicase activation. The data support a model where protein-protein and DNA ligand interactions at the coupling protein interface coordinate the transition initiating production and uptake of the nucleoprotein secretion substrate.Controlled duplex DNA unwinding is a crucial prerequisite for the expression and maintenance of genomes. Genome-manipulating and -regulating proteins are central to that biological function in recognizing appropriate DNA targets at initiation sequences and unwinding the complementary strands to provide single-stranded DNA (ssDNA) templates for nucleic acid synthesis and other processing reactions. The protein machineries involved include nucleic acid helicases. DNA helicases are powerful enzymes that convert the energy of nucleoside triphosphate hydrolysis to directional DNA strand translocation and separation of the double helix into its constituent single strands (for reviews, see references 13, 14, 16, 38, 55, and 64). By necessity, these enzymes interact with DNA strands via mechanisms independent of sequence recognition. At replication initiation helicases gain controlled access to the double-stranded genome at positions determined by the DNA binding properties of initiator proteins that comprise an origin recognition complex (1, 9, 17, 31, 45, 66). The mechanisms supporting localized unwinding within the complex include initiator-induced DNA looping, wrapping, and bending and feature regions of low thermodynamic stability. The exposed ssDNA mediates helicase binding followed by directional translocation along that strand until the enzyme engages the duplex for unwinding.In the MOB_F family of conjugation systems, the plasmid DNA strand destined for transfer (T strand) is unwound from its complement by a dedicated conjugative helicase, TraI of F-like plasmids or TrwC of the IncW paradigm. These enzymes are remarkable in that the same polypeptides additionally harbor in a distinct domain a DNA transesterase activity. That function is required to recognize and cleave the precise phosphodiester bond, nic, in the T strand where unwinding of the secretion substrate begins. In current models the conjugative helicases are thus targeted to the transfer origin (oriT) of their cognate plasmid by the high-affinity DNA sequence interactions of their N-terminal DNA transesterase domains. In the bacterial cell, recruitment and activation of the conjugative helicase occur not on naked DNA but within an initiator complex called the relaxosome (67). For the F-like plasmid R1, sequence-specific DNA binding properties of the plasmid proteins TraI, TraY, TraM, and the host integration factor (IHF) direct assembly of the relaxosome at oriT (10, 12, 29, 33, 51, 52). Integration of protein TraM confers recognition features to the relaxosome, which permit its selective docking to TraD, the coupling protein associated with the conjugative type IV secretion system (T4CP) (2, 15, 49). In current models, the T4CP forms a hexameric translocation pore at the cytoplasmic membrane that not only governs substrate entry to the envelope spanning type IV secretion machinery but also provides energy for macromolecular transport via ATP hydrolysis (36, 50). These models propose that T4CPs provide not only a physical bridge between the plasmid and the type IV transporter but also a unique control function in distinguishing one plasmid (relaxosome) from another (7, 8). Before the current study (see accompanying report [41]), evidence indicating that regulation of the initiation of conjugative DNA processing also takes place at this interface had not been reported.F plasmid TraI protein, originally named Escherichia coli DNA helicase I, was initially characterized in the Hoffman-Berling laboratory (19). The purified enzyme exhibits properties in vitro consistent with its function in conjugative DNA strand transfer including a very high 1,100-bp/s rate of duplex unwinding, high processivity, and a 5′-to-3′ directional bias (relative to the strand to which it is bound) (34, 54). Together these features should readily support the observed rate of conjugative DNA translocation as well as concomitant replacement synthesis of the mobilized T strand from the 3′ OH product of nic cleavage.Comparatively little is known about the mechanisms of initiating TraI helicase activity. The enzyme requires ssDNA 5′ to the duplex junction (32), and a minimum length of 30 nucleotides (nt) is necessary to promote efficient duplex unwinding on substrates lacking oriT (11, 54). To our knowledge, oriT is the only sequence where the helicase activity is naturally initiated, however. Moreover, the unique fusion of a helicase to the site- and strand-specific DNA transesterase domains within MOB_F enzymes is expected to pose intriguing regulatory challenges during initiation. The combination within a single polypeptide of a site-specific DNA binding capacity with a helical motor activity would seem counterproductive. The extraordinary efficiency of these proteins in intercellular DNA strand transfer belies this prediction and instead hints strongly at a coordinated progression of the initiation pathway. Since relaxosome assembly is thus far insufficient to initiate helicase activity on supercoiled oriT substrates in vitro, we have developed a series of heteroduplex DNA substrates which support the unwinding reaction and model possible intermediate structures of R1 plasmid strand transfer initiation (10). In this system linear double-stranded DNA (dsDNA) substrates with a central region of sequence heterogeneity trap defined lengths of R1 oriT sequence in unwound conformation. Unexpectedly, efficient helicase activity initiated from a melted oriT duplex required ssDNA twice as long (60 nt) as that previously observed on substrates lacking this sequence (11).In the current report, we describe an application of these models where variation in the position of duplex opening in the vicinity of nic, as well as the additional presence of auxiliary relaxosome proteins, has revealed novel insights into control of a conjugative helicase involving both DNA and protein interactions. Moreover, we observe a sequence-independent stimulation of the unwinding reaction in the presence of T4CP TraD. These results support a model where docking of the preinitiation relaxosome assembly to the T4CP alters the composition and architecture of the complex in a manner essential to the subsequent initiation of T-strand unwinding.     

Novel Antibiotic-Free Plasmid Selection System Based on Complementation of Host Auxotrophy in the NAD De Novo Synthesis Pathway

The use of antibiotic resistance genes in plasmids causes potential biosafety and clinical hazards, such as the possibility of horizontal spread of resistance genes or the rapid emergence of multidrug-resistant pathogens. This paper introduces a novel auxotrophy complementation system that allowed plasmids and host cells to be effectively selected and maintained without the use of antibiotics. An Escherichia coli strain carrying a defect in NAD de novo biosynthesis was constructed by knocking out the chromosomal quinolinic acid phosphoribosyltransferase (QAPRTase) gene. The resistance gene in the plasmids was replaced by the QAPRTase gene of E. coli or the mouse. As a result, only expression of the QAPRTase gene from plasmids can complement and rescue E. coli host cells in minimal medium. This is the first time that a vertebrate gene has been used to construct a nonantibiotic selection system, and it can be widely applied in DNA vaccine and gene therapy. As the QAPRTase gene is ubiquitous in species ranging from bacteria to mammals, the potential environmental biosafety problems caused by horizontal gene transfer can be eliminated.Antibiotic resistance genes are the most commonly used markers for selecting and maintaining recombinant plasmids in hosts, such as Escherichia coli. However, the use of these genes has several drawbacks. For example, horizontal transfer of the antibiotic resistance gene can potentially contribute to the rapid emergence of multidrug-resistant organisms (e.g., superbacteria) (11, 29). Another significant concern is that the antibiotic resistance genes in DNA vaccines may become integrated into human chromosomes (23). The possibility arises, although the probability is low, that once the antibiotic resistance gene is translated into a functional protein, the vaccinee might be resistant to the corresponding antibiotic. This would add to the difficulty of curing diseases caused by infectious pathogens. Accordingly, the use of antibiotic resistance genes is undesirable in many areas of biotechnology, especially in gene therapy products and genetically engineered microorganisms (17, 23, 28). Furthermore, the addition of antibiotics is costly in large-scale cultivation, and there are risks of contamination of the final product with antibiotics (2, 3). Finally, the constitutively expressed antibiotic resistance genes impose a metabolic burden on the host cells, resulting in reduced growth rate and cell density (4, 27). An alternative strategy is to utilize antibiotic-free host-plasmid balanced lethal systems to select and maintain the recombinant plasmids.To date, several such systems have been developed to replace traditional antibiotic selection systems. They include auxotrophy complementation (AC), postsegregational killing (PSK), and operator-repressor titration (ORT) (8). The AC system is based on a strain auxotrophic for an essential metabolite, obtained by mutating or knocking out the corresponding chromosomal gene, which can be complemented with the plasmid-borne selection gene. The choice of the essential gene used for complementation of host auxotrophy is critical, and it is mainly involved in DNA precursor, amino acid, or cell wall biosynthetic pathways. Various essential genes, such as asd, thyA, and glnA, have been utilized to construct AC systems (5, 9, 21, 22, 24, 26, 28). However, all of these systems require extra nutrients or expensive reagents. The PSK system relies on the balance between toxin and antitoxin, expressed from genome and plasmid, respectively. If a cell loses the plasmid, the corresponding antitoxin is degraded and the toxin then kills the cell. Unfortunately, this system has proven ineffective for plasmid maintenance during prolonged culture (6, 14). The ORT system utilizes plasmids with the lac operator to derepress a modified essential chromosomal gene. Loss of these types of plasmids no longer titrates the repressor and leads to the death of the bacterium. This system requires short, nonexpressed lac operator functions as the vector-borne selection marker and enables the selection and maintenance of plasmids free from expressed selectable marker genes (7, 8, 15, 30). Additionally, several other nonantibiotic selection systems (e.g., the fabI-triclosan system) have recently been developed (12, 17, 18).Among the antibiotic-free selection systems that have been developed, the AC system has drawn much attention and has now been applied in numerous bacterial species, such as Lactococcus lactis, Salmonella spp., Vibrio cholerae, Mycobacterium bovis, and E. coli (5, 16, 21, 22, 24). However, all of the AC systems utilize plasmid-borne bacterial-origin genes to complement the auxotrophy. These systems may suffer from a potential risk that the bacterial-origin genes may be integrated into human chromosome when they are used in transgenic products, such as DNA vaccines. Therefore, a better strategy would be to use the genes of the vaccinees themselves to construct an AC system. Not only would this type of approach select and maintain plasmids in bacteria, but it could also be widely applied in the production of safer DNA vaccines.In the present study, we successfully developed a novel antibiotic-free plasmid selection system based on complementation of host auxotrophy in the NAD synthesis pathway. The NAD synthesis pathway, including de novo and salvage pathways, differs among species. However, by comparison of NAD metabolism in different species, quinolinic acid phosphoribosyltransferase (QAPRTase) appears to be a common enzyme for de novo NAD biosynthesis in both prokaryotes and eukaryotes (13). Therefore, the QAPRTase gene was viewed as a favorable candidate that could potentially be utilized to construct a new AC system.     

Conserved Symbiotic Plasmid DNA Sequences in the Multireplicon Pangenomic Structure of Rhizobium etli

Strains of the same bacterial species often show considerable genomic variation. To examine the extent of such variation in Rhizobium etli, the complete genome sequence of R. etli CIAT652 and the partial genomic sequences of six additional R. etli strains having different geographical origins were determined. The sequences were compared with each other and with the previously reported genome sequence of R. etli CFN42. DNA sequences common to all strains constituted the greater part of these genomes and were localized in both the chromosome and large plasmids. About 700 to 1,000 kb of DNA that did not match sequences of the complete genomes of strains CIAT652 and CFN42 was unique to each R. etli strain. These sequences were distributed throughout the chromosome as individual genes or chromosomal islands and in plasmids, and they encoded accessory functions, such as transport of sugars and amino acids, or secondary metabolism; they also included mobile elements and hypothetical genes. Sequences corresponding to symbiotic plasmids showed high levels of nucleotide identity (about 98 to 99%), whereas chromosomal sequences and the sequences with matches to other plasmids showed lower levels of identity (on average, about 90 to 95%). We concluded that R. etli has a pangenomic structure with a core genome composed of both chromosomal and plasmid sequences, including a highly conserved symbiotic plasmid, despite the overall genomic divergence.It is becoming clear that bacterial genomes of strains of the same species vary widely both in size and in gene composition (39). An unexpected degree of genomic diversity has been found by comparing whole genomes (39). For instance, in Escherichia coli strains, differences of up to 1,400 kb account for some strain-specific pathogenic traits (5, 56). The extent of intraspecies genome diversity varies in different bacterial lineages. Some species have a wide range of variation; these species include E. coli (42), Streptococcus agalactiae (53), and Haloquadratum walsbyi (34). Other bacteria display only limited gene content diversity; an example is Ureaplasma urealyticum (1, 54). Tettelin and colleagues have suggested that bacterial species can be characterized by the presence of a pangenome consisting of a core genome containing genes present in all strains and a dispensable genome consisting of partially shared and strain-specific genes (53, 54). This concept is rooted in the earlier ideas of Reanney (43) and Campbell (7) concerning the structure of bacterial populations, and it indicates both that there is a pool of accessory genetic information in bacterial species and that strains of the same or even different species can obtain this information by horizontal transfer mechanisms (7, 43).Genome size and diversity are related to bacterial lifestyle. Small genomes are typical of strict pathogens such as Rickettsia prowazekii (2) and endosymbionts such as Buchnera aphidicola (44a). In contrast, free-living bacteria, such as Pseudomonas syringae and Streptomyces coelicolor, have large genomes (4, 6). The bacteria with the largest genomes are common inhabitants of heterogeneous environments, such as soil, where energy sources are limited but diverse (32). An increase in genome size is attributable mainly to expansion of functions such as secondary metabolism, transport of metabolites, and gene regulation. All these features are common to the nitrogen-fixing symbiotic bacteria of legumes, which are collectively known as rhizobia, and their close relative the plant pathogen Agrobacterium. The genomes of such bacterial species have diverse architectures with circular chromosomes that are different sizes or linear chromosomes, like that in Agrobacterium species, and the organisms contain variable numbers of large plasmids (31, 49). Comparative genomic studies have highlighted the conservation of gene content and order among the chromosomes of some species of rhizobia (22, 23, 25, 40). Furthermore, Guerrero and colleagues (25) observed that most essential genes occur in syntenic arrangements and display a higher level of sequence identity than nonsyntenic genes. In contrast, plasmids, including symbiotic plasmids and symbiotic chromosomal islands (like those in Mesorhizobium loti and Bradyrhizobium japonicum) are poorly conserved in terms of both gene content and gene order (21). It is not clear what evolutionary advantage, if any, is provided by multipartite genomes, but some authors have speculated that such genomes may allow further accumulation of genes independent of the chromosome. Recently, Slater and coworkers (46) proposed a model for the origin of secondary chromosomes. Their idea is based on the notion of intragenomic gene transfers that might occur from primary chromosomes to ancestral plasmids of the repABC type. Observations of conservation of clusters of genes in secondary chromosomes or in large plasmids that retain synteny with respect to the main chromosome support this hypothesis (46).We have been studying Rhizobium etli as a multipartite genome model species (23). This organism is a free-living soil bacterium that is able to form nodules and fix nitrogen in the roots of bean plants. The genome of R. etli is partitioned into several replicons, a circular chromosome, and several large plasmids. In the reference strain R. etli CFN42, the genome is composed of a circular chromosome consisting of about 4,381 kb and 6 large plasmids whose total size is 2,148 kb (23). A 371-kb plasmid, termed pSym or the symbiotic plasmid, contains most of the genes required for symbiosis (21). Previous studies have described the high level of genetic diversity among geographically different R. etli isolates (41). The strains are also variable with respect to the number and size of plasmids. Nevertheless, there has been no direct measurement of diversity at the genomic level, nor have comparative studies of shared and particular genomic features of R. etli strains been reported. Therefore, to assess the degrees of genomic difference and genomic similarity in R. etli, we obtained the complete genomic sequence of an additional R. etli strain and partial genomic sequences of six other R. etli strains isolated worldwide. Our results support the concept of a pangenomic structure at the multireplicon level and show that a highly conserved symbiotic plasmid is present in divergent R. etli isolates.     

Predicting Plasmid Promiscuity Based on Genomic Signature

Despite the important contribution of self-transmissible plasmids to bacterial evolution, little is understood about the range of hosts in which these plasmids have evolved. Our goal was to infer this so-called evolutionary host range. The nucleotide composition, or genomic signature, of plasmids is often similar to that of the chromosome of their current host, suggesting that plasmids acquire their hosts’ signature over time. Therefore, we examined whether the evolutionary host range of plasmids could be inferred by comparing their trinucleotide composition to that of all completely sequenced bacterial chromosomes. The diversity of candidate hosts was determined using taxonomic classification and genetic distance. The method was first tested using plasmids from six incompatibility (Inc) groups whose host ranges are generally thought to be narrow (IncF, IncH, and IncI) or broad (IncN, IncP, and IncW) and then applied to other plasmid groups. The evolutionary host range was found to be broad for IncP plasmids, narrow for IncF and IncI plasmids, and intermediate for IncH and IncN plasmids, which corresponds with their known host range. The IncW plasmids as well as several plasmids from the IncA/C, IncP, IncQ, IncU, and PromA groups have signatures that were not similar to any of the chromosomal signatures, raising the hypothesis that these plasmids have not been ameliorated in any host due to their promiscuous nature. The inferred evolutionary host range of IncA/C, IncP-9, and IncL/M plasmids requires further investigation. In this era of high-throughput sequencing, this genomic signature method is a useful tool for predicting the host range of novel mobile elements.Comparative genomics has clearly shown that bacterial evolution occurs not only through genetic changes that are vertically inherited but also by extensive horizontal gene transfer between closely and distantly related bacteria (9). Mobile genetic elements such as plasmids and phages serve as important agents of horizontal gene transfer that can exchange genetic material between chromosomes (26). Plasmids also play a critical role in rapid bacterial adaptation to local environmental changes, as best exemplified by the alarmingly rapid spread of plasmid-encoded multidrug resistance in human pathogens (44, 66). In spite of this, very little is understood about the range of bacterial hosts in which these plasmids may have resided and evolved in natural or clinical environments over time, i.e., their potential “evolutionary host range.” Understanding the evolutionary history of virulence, catabolic, and other plasmids may help us to reconstruct the plasmid transfer network among microorganisms and track the pathways of gene dissemination.A plasmid''s host range can be defined in different ways, but it is typically understood as the range of hosts in which a plasmid can replicate (replication host range, or from here on simply called “host range”). This host range is often narrower than the range of hosts to which the plasmid can transfer by conjugation (transfer host range) (32, 72) but wider than the range in which it can be stably maintained (long-term host range) (16). The host range of a plasmid is often determined by mating assays, wherein that plasmid is transferred into a set of recipient strains followed by selection for transconjugant clones that can express one of the traits encoded by the plasmid (40, 47). Ideally, the physical presence of the plasmids is then verified to confirm independent replication. Sometimes the host range is also inferred from the observed natural range of hosts in which a plasmid is found in various habitats (24, 72). The plasmid host range is known to be highly variable among plasmids, and the terms “narrow host range” and “broad host range” are used as qualitative indicators (18, 49, 62). For example, it has been generally considered that incompatibility (Inc) groups IncF, IncH, and IncI contain self-transmissible narrow-host-range plasmids, while IncN, IncP, and IncW plasmids transfer and replicate in a broad range of hosts (13, 49, 62). This oldest system of plasmid classification into Inc groups is based on the inability of plasmids from the same group to be maintained in the same host due to similarity in replication or partitioning systems (11, 53). We note that IncP plasmids are also called IncP-1 in the Pseudomonas classification system, but they are here referred to as IncP. The entire range of hosts, including ancestral forms and extant bacteria, in which a plasmid has replicated at some point during its evolutionary history is of course unknown but expected to be narrower than its replication range. Here, we designate this range the “evolutionary host range.”To understand the contributions of plasmids to horizontal gene transfer and bacterial evolution, it is not sufficient to know the hosts in which plasmids can potentially replicate and be maintained when tested in the laboratory or the field. While very valid, such experiments (13, 17, 40, 47, 56, 72) do not allow us to evaluate which plasmids have in fact spread among the widest range of hosts in the past and therefore contributed most so far to horizontal gene transfer across distantly related bacteria. We also need to gain insight into the range of hosts in which they have actually resided over evolutionary time—their evolutionary host range. This insight into the evolutionary history of plasmids will also shed light on the reservoirs of the many unwanted drug resistance and virulence plasmids (65). Previous studies have shown that the dinucleotide composition (2-mer genomic signatures) of plasmids tend to be similar to those of the chromosomes of their known host, suggesting that the plasmids acquire the host''s genomic signature (7, 67). It has previously been suggested that host-specific mutational biases homogenize the nucleotide compositions of genetic elements that are being replicated in the same host (plasmids, phages, and DNA fragments inserted in the chromosome); this phenomenon has been designated “genome amelioration” (7, 43). In addition, due to the potential DNA exchange between chromosomes and plasmids by recombination and transposition (8, 42), acquisition of large sections of chromosomal DNA by plasmids may also result in similar signatures between plasmids and their evolutionary hosts. It thus follows that a similar genomic signature between a plasmid and a host''s chromosome may indicate residence of the plasmid in that or a closely related host during its evolutionary history. Therefore, it should be possible to infer the evolutionary host range for plasmids whose genome sequences have been determined, based on the similarity in genomic signature with that of completely sequenced bacterial chromosomes.The goal of this study was to infer the evolutionary host range of various plasmids based on their genomic signatures. Specifically, we postulate (i) that known broad-host-range plasmids from Proteobacteria have evolved in a wider range of hosts than narrow-host-range plasmids and (ii) that our genomic signature approach can be used to assess the promiscuity of sequenced but uncharacterized plasmids and other mobile elements. To develop our approach, we chose self-transmissible plasmids belonging to six incompatibility groups, whose host ranges have been studied intensively and are thought to be narrow (IncF, IncH, and IncI) or broad (IncN, IncP, and IncW). To propose candidate evolutionary hosts of these plasmids, we compared the genomic signature of each plasmid with those of 817 chromosomes of prokaryotes for which complete sequences were available. Our results suggest that the evolutionary host range is broad for IncP plasmids, narrow for IncF and IncI plasmids, and intermediate for IncH and IncN plasmids. The lack of hosts with signatures similar to the IncW plasmids raises the hypothesis that they have not been ameliorated for any host due to their promiscuity. We then used the same method to infer the evolutionary host range of additional plasmid groups, such as IncA/C (also called IncP-3), IncL/M, IncP-9, IncQ (IncP-4), IncU, and PromA and plasmids Ri and Ti from Agrobacterium sp. (designated Ri/Ti). The similarities and discrepancies between our findings and previous knowledge on plasmid host range are discussed.     

Identification of Diverse Antimicrobial Resistance Determinants Carried on Bacterial,Plasmid, or Viral Metagenomes from an Activated Sludge Microbial Assemblage

Using both sequence- and function-based metagenomic approaches, multiple antibiotic resistance determinants were identified within metagenomic libraries constructed from DNA extracted from bacterial chromosomes, plasmids, or viruses within an activated sludge microbial assemblage. Metagenomic clones and a plasmid that in Escherichia coli expressed resistance to chloramphenicol, ampicillin, or kanamycin were isolated, with many cloned DNA sequences lacking any significant homology to known antibiotic resistance determinants.Activated sludge in wastewater treatment plants is an open system with a dynamic and phylogenetically diverse microbial community (2, 3, 6, 7, 10, 11). Since the activated sludge process promotes cellular interactions among diverse microorganisms, there is great potential for the lateral transfer of antibiotic resistance genes between microbes in activated sludge and in downstream environments. Several studies have previously identified antibiotic resistance determinants from wastewater communities that are carried on bacterial chromosomes (1, 4, 14) and plasmids (9, 12, 13), but to our knowledge, a simultaneous metagenomic survey of antibiotic resistance determinants from all three genetic reservoirs (i.e., chromosomes, plasmids, and viruses) has never been performed within the same environment. To achieve a more comprehensive assessment of antibiotic resistance genes in the activated sludge microbial community, this study used both function- and sequence-based metagenomic approaches to identify antibiotic resistance determinants carried on bacterial chromosomes, plasmids, or viruses within an activated sludge microbial assemblage.     

A New Borrelia Species Defined by Multilocus Sequence Analysis of Housekeeping Genes

Analysis of Lyme borreliosis (LB) spirochetes, using a novel multilocus sequence analysis scheme, revealed that OspA serotype 4 strains (a rodent-associated ecotype) of Borrelia garinii were sufficiently genetically distinct from bird-associated B. garinii strains to deserve species status. We suggest that OspA serotype 4 strains be raised to species status and named Borrelia bavariensis sp. nov. The rooted phylogenetic trees provide novel insights into the evolutionary history of LB spirochetes.Multilocus sequence typing (MLST) and multilocus sequence analysis (MLSA) have been shown to be powerful and pragmatic molecular methods for typing large numbers of microbial strains for population genetics studies, delineation of species, and assignment of strains to defined bacterial species (4, 13, 27, 40, 44). To date, MLST/MLSA schemes have been applied only to a few vector-borne microbial populations (1, 6, 30, 37, 40, 41, 47).Lyme borreliosis (LB) spirochetes comprise a diverse group of zoonotic bacteria which are transmitted among vertebrate hosts by ixodid (hard) ticks. The most common agents of human LB are Borrelia burgdorferi (sensu stricto), Borrelia afzelii, Borrelia garinii, Borrelia lusitaniae, and Borrelia spielmanii (7, 8, 12, 35). To date, 15 species have been named within the group of LB spirochetes (6, 31, 32, 37, 38, 41). While several of these LB species have been delineated using whole DNA-DNA hybridization (3, 20, 33), most ecological or epidemiological studies have been using single loci (5, 9-11, 29, 34, 36, 38, 42, 51, 53). Although some of these loci have been convenient for species assignment of strains or to address particular epidemiological questions, they may be unsuitable to resolve evolutionary relationships among LB species, because it is not possible to define any outgroup. For example, both the 5S-23S intergenic spacer (5S-23S IGS) and the gene encoding the outer surface protein A (ospA) are present only in LB spirochete genomes (36, 43). The advantage of using appropriate housekeeping genes of LB group spirochetes is that phylogenetic trees can be rooted with sequences of relapsing fever spirochetes. This renders the data amenable to detailed evolutionary studies of LB spirochetes.LB group spirochetes differ remarkably in their patterns and levels of host association, which are likely to affect their population structures (22, 24, 46, 48). Of the three main Eurasian Borrelia species, B. afzelii is adapted to rodents, whereas B. valaisiana and most strains of B. garinii are maintained by birds (12, 15, 16, 23, 26, 45). However, B. garinii OspA serotype 4 strains in Europe have been shown to be transmitted by rodents (17, 18) and, therefore, constitute a distinct ecotype within B. garinii. These strains have also been associated with high pathogenicity in humans, and their finer-scale geographical distribution seems highly focal (10, 34, 52, 53).In this study, we analyzed the intra- and interspecific phylogenetic relationships of B. burgdorferi, B. afzelii, B. garinii, B. valaisiana, B. lusitaniae, B. bissettii, and B. spielmanii by means of a novel MLSA scheme based on chromosomal housekeeping genes (30, 48).     

Improved Strains and Plasmid Vectors for Conditional Overexpression of His-Tagged Proteins in Haloferax volcanii

Research into archaea will not achieve its full potential until systems are in place to carry out genetics and biochemistry in the same species. Haloferax volcanii is widely regarded as the best-equipped organism for archaeal genetics, but the development of tools for the expression and purification of H. volcanii proteins has been neglected. We have developed a series of plasmid vectors and host strains for conditional overexpression of halophilic proteins in H. volcanii. The plasmids feature the tryptophan-inducible p.tnaA promoter and a 6×His tag for protein purification by metal affinity chromatography. Purification is facilitated by host strains, where pitA is replaced by the ortholog from Natronomonas pharaonis. The latter lacks the histidine-rich linker region found in H. volcanii PitA and does not copurify with His-tagged recombinant proteins. We also deleted the mrr restriction endonuclease gene, thereby allowing direct transformation without the need to passage DNA through an Escherichia coli dam mutant.Over the past century, our understanding of fundamental biological processes has grown exponentially, and this would have been impossible without the use of organisms that are amenable to experimental manipulation. Model species, such as Escherichia coli, Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, Mus musculus, and Arabidopsis thaliana, have become a byword for scientific progress (15). The rational choice of a model organism is critically important, and certain features are taken for granted, such as ease of cultivation, a short generation time, and systems for genetic manipulation. This list has now grown to include a genome sequence and methods for biochemical analysis of purified proteins in vitro.Research into archaea has lagged behind work on bacteria and eukaryotes but has nonetheless yielded profound insights (2). One hurdle has been the paucity of archaeal organisms suitable for both biochemistry and genetics. For example, Methanothermobacter thermautotrophicus is a stalwart of archaeal biochemistry but has proved resistant to even the most rudimentary genetic manipulation (2). Progress has recently been made with another biochemical workhorse, Sulfolobus spp., and a few genetic tools are now available (6, 13, 37). Methanosarcina spp. and Thermococcus kodakaraensis offer alternative systems with an increasing array of techniques (16, 35, 36), but sophisticated genetics has traditionally been the preserve of haloarchaea, of which Haloferax volcanii is the organism of choice (39). It is easy to culture, the genome has been sequenced (19), and there are several selectable markers and plasmids for transformation and gene knockout (3, 7, 31), including a Gateway system (14), as well as reporter genes (20, 33) and a tightly controlled inducible promoter (26).The genetic prowess of H. volcanii is not yet fully matched by corresponding systems for protein overexpression and purification. Like other haloarchaea, H. volcanii grows in high salt concentrations (2 to 5 M NaCl), and to cope with the osmotic potential of such environments, it accumulates high intracellular concentrations of potassium ions (12). Consequently, halophilic proteins are adapted to function at high salt concentrations and commonly feature a large excess of acidic amino acids; the negative surface charge is thought to be critical to solubility (28). This can pose problems for expression in heterologous hosts, such as E. coli, since halophilic proteins can misfold and aggregate under conditions of low ionic strength. The purification of misfolded halophilic enzymes from E. coli has relied on the recovery of insoluble protein from inclusion bodies, followed by denaturation and refolding in hypersaline solutions (8, 11). This approach is feasible only where the protein is well characterized and reconstitution of the active form can be monitored (for example, by an enzymatic assay). Furthermore, archaeal proteins expressed in heterologous bacterial hosts lack posttranslational modifications, such as acetylation or ubiquitination (4, 22), which are critical to understanding their biological function.Systems for expression of halophilic proteins in a native haloarchaeal host are therefore required. A number of studies have successfully purified recombinant proteins with a variety of affinity tags after overexpression in H. volcanii. For example, Humbard et al. employed tandem affinity tagging to purify 20S proteasomal core particles from the native host (23). However, the protein expression constructs used in these studies were custom made and somewhat tailored to the application in question. We report here the development of “generic” plasmid vectors and host strains for conditional overexpression of halophilic proteins in H. volcanii. The plasmids feature a tryptophan-inducible promoter derived from the tnaA gene of H. volcanii (26). We demonstrate the utility of these vectors by overexpressing a hexahistidine-tagged recombinant version of the H. volcanii RadA protein. Purification was greatly facilitated by a host strain in which the endogenous pitA gene was replaced by an ortholog from Natronomonas pharaonis. The latter protein lacks the histidine-rich linker region found in H. volcanii PitA (5) and therefore does not copurify with His-tagged recombinant proteins. Finally, we deleted the mrr gene of H. volcanii, which encodes a restriction enzyme that cleaves foreign DNA methylated at GATC residues. The mrr deletion strain allows direct transformation of H. volcanii without the need to passage plasmid DNA through an E. coli dam mutant (21).     

tISCpe8, an IS1595-Family Lincomycin Resistance Element Located on a Conjugative Plasmid in Clostridium perfringens

Clostridium perfringens is a normal gastrointestinal organism that is a reservoir for antibiotic resistance genes and can potentially act as a source from which mobile elements and their associated resistance determinants can be transferred to other bacterial pathogens. Lincomycin resistance in C. perfringens is common and is usually encoded by erm genes that confer macrolide-lincosamide-streptogramin B resistance. In this study we identified strains that are lincomycin resistant but erythromycin sensitive and showed that the lincomycin resistance determinant was plasmid borne and could be transferred to other C. perfringens isolates by conjugation. The plasmid, pJIR2774, is the first conjugative C. perfringens R-plasmid to be identified that does not confer tetracycline resistance. Further analysis showed that resistance was encoded by the lnuP gene, which encoded a putative lincosamide nucleotidyltransferase and was located on tISCpe8, a functional transposable genetic element that was a member of the IS1595 family of transposon-like insertion sequences. This element had significant similarity to the mobilizable lincomycin resistance element tISSag10 from Streptococcus agalactiae. Like tISSag10, tISCpe8 carries a functional origin of transfer within the resistance gene, allowing the element to be mobilized by the conjugative transposon Tn916. The similarity of these elements and the finding that they both contain an oriT-like region support the hypothesis that conjugation may result in the movement of DNA modules that are not obviously mobile since they are not linked to conjugation or mobilization functions. This process likely plays a significant role in bacterial adaptation and evolution.There has been increasing concern about the emergence of multiply antibiotic-resistant strains of many common bacterial pathogens. The development of multiple resistance phenotypes has already led to compromises in the ability to successfully treat infected patients and to increased treatment costs (15). The emergence of resistant bacteria is often the result of excessive or inappropriate use of antibiotics and the ability of antibiotic resistance genes to be transferred from resistant to susceptible bacteria, either within a bacterial species, between different species within the same genus, or between different genera (14). Different types of mobile genetic elements, including conjugative plasmids, conjugative transposons, mobilizable plasmids, mobilizable transposons, nonconjugative plasmids, and integrons, may contain the resistance genes (14). All of these elements have the ability to mediate the transfer of resistance genes within and between bacterial cells, either independently or cooperatively, which has significant implications for the transfer and evolution of antibiotic resistance, particularly in pathogenic bacterial species.Clostridium perfringens is a normal gastrointestinal organism that causes food poisoning, necrotic enteritis, and gas gangrene (29). It is a proven reservoir for antibiotic resistance determinants. For example, the catP chloramphenicol resistance determinant, which is located on the Tn4451/Tn4453 family of integrative mobilizable elements in C. perfringens and Clostridium difficile, has been detected in clinical isolates of Neisseria meningitidis (20, 23, 41). Similarly, genetically related variants of the macrolide-lincosamide-streptogramin B (MLS) resistance determinant Erm(B) from C. perfringens have been found in Enterococcus faecalis, Streptococcus agalactiae, and C. difficile (19). It is likely that the C. perfringens determinant is the progenitor of the C. difficile determinant (18, 19, 44). Significantly, both determinants can be transferred into recipient cells by conjugation, although the processes are different (12, 19, 43). The pathogenic clostridia also carry other uncharacterized MLS resistance determinants and can potentially act as a source from which these resistance determinants may be transferred to other bacterial pathogens (10, 18).Lincomycin belongs to the lincosamide group of antibiotics, which also includes clindamycin. The spectrum of activity of lincosamides predominantly encompasses gram-positive bacteria, and these antimicrobial agents are often used for treatment of infections caused by anaerobic bacteria (45). These antibiotics inhibit protein synthesis by blocking the peptidyltransferase site of the 23S rRNA component of the 50S subunit of the bacterial ribosome (17). Although cross-resistance to MLS antibiotics most commonly involves N6 dimethylation of the A2058 residue of 23S rRNA and is catalyzed by an erm-encoded rRNA methyltransferase (24, 34, 47), specific resistance to the lincosamides is the result of modification and inactivation by a lincosamide nucleotidyltransferase encoded by members of the lnu (previously lin) gene family (5, 34, 45). This type of resistance gene is found in staphylococci and streptococci, where it is often located on plasmids or transposons (5, 45).Lincomycin resistance in C. perfringens is relatively common, but it is usually conferred as MLS resistance by erm(B) or erm(Q) genes (10, 11). Recent studies have shown that there has been an increase in lincomycin resistance in C. perfringens strains isolated from chickens in Belgium (28). The researchers reported two strains that conferred resistance to lincomycin and carried the lnu(A) or lnu(B) gene, the first such strains reported for C. perfringens.In the current study we analyzed several multiply antibiotic-resistant isolates of C. perfringens and identified strains that were lincomycin resistant but were susceptible to erythromycin. We characterized these isolates and their lincomycin resistance determinant(s) and showed that resistance could be transferred to other C. perfringens isolates. Detailed analysis of the lincomycin-resistant strain 95-949 showed that resistance was encoded by the lnuP gene, which was located on a transposable genetic element, tISCpe8, that was located on a conjugative plasmid, pJIR2774. This plasmid is the first conjugative C. perfringens R-plasmid to be identified that does not confer tetracycline resistance.