Enterococcus faecalis aggregation substance promotes opsonin-independent binding to human neutrophils via a complement receptor type 3-mediated mechanism.

Enterococcus faecalis aggregation substance (AS) mediates efficient adhesion between bacteria, thereby facilitating plasmid exchange as an integral part of a bacterial sex pheromone system. We examined the interaction of AS-bearing E. faecalis with human neutrophils (PMNs), an important component of the host defense system. AS promoted a markedly increased opsonin-independent bacterial binding to PMNs. Adhesion was dependent on the expression of the enterococcal Asc10 protein, which contains two Arg-Gly-Asp (RGD) sequences, and addition of exogenous RGD-containing peptides inhibited AS-mediated binding by 66%. AS-mediated adhesion was inhibited by 85% by anti-human complement receptor type 3 (CR3) monoclonal antibodies or by use of PMNs from a patient with leukocyte adhesion deficiency. However, AS-bearing E. faecalis cells were unable to bind to CHO-Mac-1 cells, expressing functionally active CR3, suggesting the potential need for additional PMN surface receptors for bacterial adhesion. Monoclonal antibodies against integrin-associated protein (CD47) and L-selectin, both of which may interact with CR3 and bind to ligands on E. faecalis, also inhibited AS-dependent binding. The non-opsonic binding of E. faecalis to PMNs may play an important role in this organism's pathogenesis.     

Heterologous inducible expression of Enterococcus faecalis pCF10 aggregation substance asc10 in Lactococcus lactis and Streptococcus gordonii contributes to cell hydrophobicity and adhesion to fibrin

Aggregation substance proteins encoded by the sex pheromone plasmid family of Enterococcus faecalis have been shown previously to contribute to the formation of a stable mating complex between donor and recipient cells and have been implicated in the virulence of this increasingly important nosocomial pathogen. In an effort to characterize the protein further, prgB, the gene encoding the aggregation substance Asc10 on pCF10, was cloned in a vector containing the nisin-inducible nisA promoter and its two-component regulatory system. Expression of aggregation substance after nisin addition to cultures of E. faecalis and the heterologous bacteria Lactococcus lactis and Streptococcus gordonii was demonstrated. Electron microscopy revealed that Asc10 was presented on the cell surfaces of E. faecalis and L. lactis but not on that of S. gordonii. The protein was also found in the cell culture supernatants of all three species. Characterization of Asc10 on the cell surfaces of E. faecalis and L. lactis revealed a significant increase in cell surface hydrophobicity upon expression of the protein. Heterologous expression of Asc10 on L. lactis also allowed the recognition of its binding ligand (EBS) on the enterococcal cell surface, as indicated by increased transfer of a conjugative transposon. We also found that adhesion of Asc10-expressing bacterial cells to fibrin was elevated, consistent with a role for the protein in the pathogenesis of enterococcal endocarditis. The data demonstrate that Asc10 expressed under the control of the nisA promoter in heterologous species will be an useful tool in the detailed characterization of this important enterococcal conjugation protein and virulence factor.     

Acceleration of Enterococcus faecalis biofilm formation by aggregation substance expression in an ex vivo model of cardiac valve colonization

Infectious endocarditis involves formation of a microbial biofilm in vivo. Enterococcus faecalis Aggregation Substance (Asc10) protein enhances the severity of experimental endocarditis, where it has been implicated in formation of large vegetations and in microbial persistence during infection. In the current study, we developed an ex vivo porcine heart valve adherence model to study the initial interactions between Asc10(+) and Asc10(-)E. faecalis and valve tissue, and to examine formation of E. faecalis biofilms on a relevant tissue surface. Scanning electron microscopy of the infected valve tissue provided evidence for biofilm formation, including growing masses of bacterial cells and the increasing presence of exopolymeric matrix over time; accumulation of adherent biofilm populations on the cardiac valve surfaces during the first 2-4 h of incubation was over 10-fold higher than was observed on abiotic membranes incubated in the same culture medium. Asc10 expression accelerated biofilm formation via aggregation between E. faecalis cells; the results also suggested that in vivo adherence to host tissue and biofilm development by E. faecalis can proceed by Asc10-dependent or Asc10-independent pathways. Mutations in either of two Asc10 subdomains previously implicated in endocarditis virulence reduced levels of adherent bacterial populations in the ex vivo system. Interference with the molecular interactions involved in adherence and initiation of biofilm development in vivo with specific inhibitory compounds could lead to more effective treatment of infectious endocarditis.     

Analysis of Functional Domains of the Enterococcus faecalis Pheromone-Induced Surface Protein Aggregation Substance

Pheromone-inducible aggregation substance (AS) proteins of Enterococcus faecalis are essential for high-efficiency conjugation of the sex pheromone plasmids and also serve as virulence factors during host infection. A number of different functions have been attributed to AS in addition to bacterial cell aggregation, including adhesion to host cells, adhesion to fibrin, increased cell surface hydrophobicity, resistance to killing by polymorphonuclear leukocytes and macrophages, and increased vegetation size in an experimental endocarditis model. Relatively little information is available regarding the structure-activity relationship of AS. To identify functional domains, a library of 23 nonpolar 31-amino-acid insertions was constructed in Asc10, the AS encoded by the plasmid pCF10, using the transposons TnlacZ/in and TnphoA/in. Analysis of these insertions revealed a domain necessary for donor-recipient aggregation that extends further into the amino terminus of the protein than previously reported. In addition, insertions in the C terminus of the protein also reduced aggregation. As expected, the ability to aggregate correlates with efficient plasmid transfer. The results also indicated that an increase in cell surface hydrophobicity resulting from AS expression is not sufficient to mediate bacterial aggregation.     

Role of the pheromone-inducible surface protein Asc10 in mating aggregate formation and conjugal transfer of the Enterococcus faecalis plasmid pCF10.

The high transfer frequency of pheromone-inducible conjugative plasmids of Enterococcus faecalis in liquid culture is due in part to the formation of mating aggregates. These aggregates result from the interaction of two surface components, aggregation substance (AS), which is plasmid encoded, and the chromosomally encoded binding substance (BS). In the accompanying paper (S.-M. Kao, S. B. Olmsted, A. S. Viksnins, J.C. Gallo, G. M. Dunny, J. Bacteriol, 173:7650-7664, 1991), the sequence of the prgB gene encoding the AS molecule (Asc10) produced by pheromone-induced cells carrying plasmid pCF10 is presented. Here we report the results of genetic and immunological experiments which define the role of Asc10 in aggregation and plasmid transfer. These data indicate expression of AS on the surface of an E. faecalis cell and its binding to BS expressed on a second cell are required for the formation of a mating pair and the efficient transfer of pCF10 in liquid matings. However, the orientation of the receptors was not critical for transfer; ie., AS expressed on recipient cells could facilitate plasmid transfer via binding to BS on the donor. Our results suggest that additional (as yet unidentified) products are involved in forming the channel that ultimately serves to transfer the DNA, with AS-BS binding serving primarily to generate the initial attachment between cells. The putative prgC gene product, identified by DNA sequencing (data presented in the accompanying paper), could be involved in transfer events occurring subsequent to aggregation.     

A region of the Yersinia pseudotuberculosis invasin protein enhances integrin-mediated uptake into mammalian cells and promotes self-association

Invasin allows efficient entry into mammalian cells by Yersinia pseudotuberculosis. It has been shown that the C-terminal 192 amino acids of invasin are essential for binding of beta1 integrin receptors and subsequent uptake. By analyzing the internalization of latex beads coated with invasin derivatives, an additional domain of invasin was shown to be required for efficient bacterial internalization. A monomeric derivative encompassing the C-terminal 197 amino acids was inefficient at promoting entry of latex beads, whereas dimerization of this derivative by antibody significantly increased uptake. By using the DNA-binding domain of lambda repressor as a reporter for invasin self-interaction, we have demonstrated that a region of the invasin protein located N-terminal to the cell adhesion domain of invasin is able to self-associate. Chemical cross-linking studies of purified and surface-exposed invasin proteins, and the dominant-interfering effect of a non-functional invasin derivative are consistent with the presence of a self-association domain that is located within the region of invasin that enhances bacterial uptake. We conclude that interaction of homomultimeric invasin with multiple integrins establishes tight adherence and receptor clustering, thus providing a signal for internalization.     

Sequence analysis of Enterococcus faecalis aggregation substance encoded by the sex pheromone plasmid pAD1

The location of the structural gene for aggregation substance on the sex pheromone plasmid pAD1 of Enterococcus faecalis was determined using an oligonucleotide deduced from the N-terminal amino acid sequence of the purified protein. The nucleotide sequence was determined for the corresponding region and two open reading frames (ORFs) could be identified. ORF1 codes for a small (Mr 13,160) acidic protein of unknown function. The gene for aggregation substance (named asa1) was found to code for a protein of 1296 amino acids (Mr 142,248). The protein has a signal peptide of 43 amino acids (the resulting Mr for mature aggregation substance is 137,429) and contains in its C-terminal region a proline-rich sequence, previously characterized as being involved in cell wall association, which is followed by a membrane anchor. The membrane anchor showed significant similarity to that of other Gram-positive organisms, but no other similarities to surface proteins from Gram-positive bacteria were found. In particular, no repeats on the DNA or protein level could be detected for pAD1-specific aggregation substance. The protein contains the amino acid motifs Arg-Gly-Asp-Ser and Arg-Gly-Asp-Val (once each), which, it is proposed, play a crucial role in adherence to eukaryotic cells.     

Two functional domains of the epsilon subunit of DNA polymerase III

The epsilon subunit is the 3'-->5' proofreading exonuclease that associates with the alpha and theta subunits in the E. coli DNA polymerase III. Two fragments of the epsilon protein were prepared, and binding of these epsilon fragments with alpha and theta was investigated using gel filtration chromatography and exonuclease stimulation assays. The N-terminal fragment of epsilon, containing amino acids 2-186 (epsilon186), is a relatively protease-resistant core domain of the exonuclease. The purified recombinant epsilon186 protein catalyzes the cleavage of 3' terminal nucleotides, demonstrating that the exonuclease domain of epsilon is present in the N-terminal region of the protein. The absence of the C-terminal 57 amino acids of epsilon in the epsilon186 protein reduces the binding affinity of epsilon186 for alpha by at least 400-fold relative to the binding affinity of epsilon for alpha. In addition, stimulation of the epsilon186 exonuclease by alpha using a partial duplex DNA is about 50-fold lower than stimulation of the epsilon exonuclease by alpha. These results indicate that the C-terminal region of epsilon is required in the epsilonalpha association. To directly demonstrate that the C-terminal region of epsilon contains the alpha-association domain fusion protein, constructs containing the maltose-binding protein (MBP) and fragments of the C-terminal region of epsilon were prepared. Gel filtration analysis demonstrates that the alpha-association domain of epsilon is contained within the C-terminal 40 amino acids of epsilon. Also, the epsilon186 protein forms a tight complex with theta, demonstrating that the association of theta with epsilon is localized to the N-terminal region of epsilon. Association of epsilon186 and theta is further supported by the stimulation of the epsilon186 exonuclease in the presence of theta. These data support the concept that epsilon contains a catalytic domain located within the N-terminal region and an alpha-association domain located within the C-terminal region of the protein.     

Cooperative binding of the E2 protein of bovine papillomavirus to adjacent E2-responsive sequences.

The DNA-binding properties of purified full-length E2 protein from bovine papillomavirus type 1 have been investigated by utilizing a quantitative gel shift analysis. By using a recombinant baculovirus which express the E2 open reading frame from the polyhedrin promoter, the full-length E2 protein was synthesized in insect cells and purified to homogeneity by using an E2 binding site (ACCGN4CGGT)-specific oligonucleotide column. The Kd of E2 binding to a 41-bp oligonucleotide containing a single binding site was found to be 2 x 10(-11) M. When two binding sites were included on an oligonucleotide, cooperative binding to these sites by the E2 protein was observed. A cooperativity parameter of 8.5 was determined for E2 binding to two sites. An 86-amino-acid peptide encompassing the C terminus of the protein retains the ability to bind E2 binding sites with a Kd of 4 x 10(-10) M but exhibits slight cooperativity of binding to two adjacent sites. A major determinant for cooperative binding of the full-length E2 protein is thus encoded by the N-terminal amino acids outside the minimal DNA binding domain.     

pAM401-based shuttle vectors that enable overexpression of promoterless genes and one-step purification of tag fusion proteins directly from Enterococcus faecalis

Two novel Enterococcus faecalis-Escherichia coli shuttle vectors that utilize the promoter and ribosome binding site of bacA on the E. faecalis plasmid pPD1 were constructed. The vectors were named pMGS100 and pMGS101. pMGS100 was designed to overexpress cloned genes in E. coli and E. faecalis and encodes the bacA promoter followed by a cloning site and stop codon. pMGS101 was designed for the overexpression and purification of a cloned protein fused to a Strep-tag consisting of 9 amino acids at the carboxyl terminus. The Strep-tag provides the cloned protein with an affinity to immobilized streptavidin that facilitates protein purification. We cloned a promoterless beta-galactosidase gene from E. coli and cloned the traA gene of the E. faecalis plasmid pAD1 into the vectors to test gene expression and protein purification, respectively. beta-Galactosidase was expressed in E. coli and E. faecalis at levels of 10(3) and 10 Miller units, respectively. By cloning the pAD1 traA into pMGS101, the protein could be purified directly from a crude lysate of E. faecalis or E. coli with an immobilized streptavidin matrix by one-step affinity chromatography. The ability of TraA to bind DNA was demonstrated by the DNA-associated protein tag affinity chromatography method using lysates prepared from both E. coli and E. faecalis that overexpress TraA. The results demonstrated the usefulness of the vectors for the overexpression and cis/trans analysis of regulatory genes, purification and copurification of proteins from E. faecalis, DNA binding analysis, determination of translation initiation site, and other applications that require proteins purified from E. faecalis.     

The Tra domain of the lactococcal CluA surface protein is a unique domain that contributes to sex factor DNA transfer

CluA is a cell surface-presented protein that causes cell aggregation and is essential for a high-efficiency conjugation process in Lactococcus lactis. We know from previous work that in addition to promoting cell-to-cell contact, CluA is involved in sex factor DNA transfer. To define the CluA domains involved in aggregation and in transfer, we first performed random mutagenesis of the cluA gene using a modified mini-Tn7 element which generated five amino acid insertions located throughout the encoded protein. Thirty independent cluA insertion mutants expressing modified CluA proteins at the cell surface were isolated and characterized further. The level of aggregation of each mutant was determined. The cell binding capacity of CluA was affected strongly when the protein had a mutation in its N-terminal region, which defined an aggregation domain extending from amino acid 153 to amino acid 483. Of the cluA mutants that still exhibited aggregation, eight showed an attenuated ability to conjugate, and six mutations were located in a 300-amino-acid C-terminal region of the protein defining a transfer domain (Tra). This result was confirmed by a phenotypic analysis of an additional five mutants obtained using site-directed mutagenesis in which charged amino acids of the Tra domain were replaced by alanine residues. Two distinct functional domains of the CluA protein were defined in this work; the first domain is involved in cell binding specificity, and the Tra domain is probably involved in the formation of the DNA transport machinery. This is the first report of a protein involved in conjugation that actively contributes to DNA transfer and mediates contact between donor and recipient strains.     

N-terminus of the photosystem II manganese stabilizing protein: effects of sequence elongation and truncation

The importance of the N-terminal domain of manganese stabilizing protein in binding to photosystem II has been previously demonstrated [Eaton-Rye and Murata (1989) Biochim. Biophys. Acta 977, 219-226; Odom and Bricker (1992) Biochemistry 31, 5616-5620]. In this paper, we report results from a systematic study of functional and structural consequences of N-terminal elongation and truncation of manganese stabilizing protein. Precursor manganese stabilizing protein is the unprocessed wild-type protein, which carries an N-terminal extension of 84 amino acids in the form of its chloroplastic signal peptide. Despite its increased size, this protein is able to reconstitute O(2) evolution activity to levels observed with the mature, processed protein, but it also binds nonspecifically to PSII. Truncation of wild-type manganese stabilizing protein by site-directed mutagenesis to remove three N-terminal amino acids, resulting in a mutant called DeltaG3M, causes no loss of activity reconstitution, but this protein also exhibits nonspecific binding. Further truncation of the wild-type protein by ten N-terminal amino acids, producing DeltaE10M, limits binding of manganese stabilizing protein to 1 mol/mol of photosystem II and decreases activity reconstitution to about 65% of that obtained with the wild-type protein. Because two copies of wild type normally bind to photosystem II, amino acids in the domain (4)K-(10)E must be involved in the binding of one copy of manganese stabilizing protein to photosystem II. Spectroscopic analysis (CD and UV spectra) reveals that N-terminal elongation and deletion of manganese stabilizing protein influence its overall conformation, even though secondary structure content is not perturbed. Our data suggest that the solution structure of manganese stabilizing protein attains a more compact solution structure upon removal of N-terminal amino acids.     

Characterization of adhesive epitopes with the OmpS display system.

OmpS is an outer membrane protein of Vibrio cholerae where it forms trimeric pores that function in the uptake of maltose and maltodextrins. Based on sequence similarity to LamB proteins, a model of OmpS folding in the outer membrane has been constructed. According to this model, OmpS contains 18 transmembrane beta-strands and nine surface-accessible loops. Adhesive epitopes can, when inserted into surface-accessible loop 4 (L4) and expressed in Escherichia coli, retain their functional characteristics. We inserted three D-repeats from the Staphylococcus aureus fibronectin-binding protein FnBPA into L4 of OmpS and showed that E. coli cells expressing these hybrids bind fibronectin. DNA fragments covering the N-terminal half of the globoside-binding P-fimbrial adhesin class II PapG of E. coli were cloned into the same surface accessible loop (L4) of OmpS. Fragments of papG encoding 53 or 186 amino acids from the N-terminal end of class II PapG adhesin were found to confer bacterial adhesiveness to globoside. Removal of 23 amino acids from the N-terminus of PapG did not affect receptor binding, but removal of 31 amino acids abolished it. The newly developed night sky image technique was also used to demonstrate the binding properties of membrane vesicles carrying the hybrid proteins. We raised antibodies against the purified hybrid protein containing 53 amino acids from PapG. This antiserum recognized the P-fimbriae on E. coli cells. These data provide evidence that the N-terminal first 53 amino acids of class II PapG contain the receptor-binding domain.     

Functional analysis of AtlA, the major N-acetylglucosaminidase of Enterococcus faecalis

The major peptidoglycan hydrolase of Enterococcus faecalis, AtlA, has been identified, but its enzyme activity remains unknown. We have used tandem mass spectrometry analysis of peptidoglycan hydrolysis products obtained using the purified protein to show that AtlA is an N-acetylglucosaminidase. To gain insight into the regulation of its enzyme activity, the three domains of AtlA were purified alone or in combination following expression of truncated forms of the atlA gene in Escherichia coli or partial digestion of AtlA by proteinase K. The central domain of AtlA was catalytically active, but its activity was more than two orders of magnitude lower than that of the complete protein. Partial proteolysis of AtlA was detected in vivo: zymograms of E. faecalis extracts revealed two catalytically active protein bands of 62 and 72 kDa that were both absent in extracts from an atlA null mutant. Limited digestion of AtlA by proteinase K in vitro suggested that the proteolytic cleavage of AtlA in E. faecalis extracts corresponds to the truncation of the N-terminal domain, which is rich in threonine and glutamic acid residues. We show that the truncation of the N-terminal domain from recombinant AtlA has no impact on enzyme activity. The C-terminal domain of the protein, which contains six LysM modules bound to highly purified peptidoglycan, was required for optimal enzyme activity. These data indicate that AtlA is not produced as a proenzyme and that control of the AtlA glucosaminidase activity is likely to occur at the level of LysM-mediated binding to peptidoglycan.     

Mannitol-specific phosphoenolpyruvate-dependent phosphotransferase system of Enterococcus faecalis: molecular cloning and nucleotide sequences of the enzyme IIIMtl gene and the mannitol-1-phosphate dehydrogenase gene, expression in Escherichia coli, and comparison of the gene products with similar enzymes.

Enzyme IIIMtl is part of the mannitol phosphotransferase system of Enterococcus faecalis. It is phosphorylated in a reaction sequence requiring enzyme I and heat-stable phosphocarrier protein (HPr). The phospho group is transferred from enzyme IIIMtl to enzyme IIMtl, which then catalyzes the uptake and concomitant phosphorylation of mannitol. The internalized mannitol-1-phosphate is oxidized to fructose-6-phosphate by mannitol-1-phosphate dehydrogenase. In this report we describe the cloning of the mtlF and mtlD genes, encoding enzyme IIIMtl and mannitol-1-phosphate dehydrogenase of E. faecalis, by a complementation system designed for cloning of gram-positive phosphotransferase system genes. The complete nucleotide sequences of mtlF, mtlD, and flanking regions were determined. From the gene sequences, the primary translation products are deduced to consist of 145 amino acids (enzyme IIIMtl) and 374 amino acids (mannitol-1-phosphate dehydrogenase). Amino acid sequence comparison confirmed a 41% similarity of E. faecalis enzyme IIIMtl to the hydrophilic enzyme IIIMtl-like portion of enzyme IIMtl of Escherichia coli and 45% similarity to enzyme IIIMtl of Staphylococcus carnosus. The putative N-terminal NAD+ binding domain of mannitol-1-phosphate dehydrogenase of E. faecalis shows a high degree of similarity with the N terminus of E. coli mannitol-1-phosphate dehydrogenase (T. Davis, M. Yamada, M. Elgort, and M. H. Saier, Jr., Mol. Microbiol. 2:405-412, 1988) and the N-terminal part of the translation product of S. carnosus mtlD, which was also determined in this study. There is 40% similarity between the dehydrogenases of E. faecalis and E. coli over the whole length of the enzymes. The organization of mannitol-specific genes in E. faecalis seems to be similar to the organization in S. carnosus. The open reading frame for enzyme IIIMtl E. faecalis is followed by a stem-loop structure, analogous to a typical Rho-independent terminator. We conclude that the mannitol-specific genes are organized in an operon and that the gene order is mtlA orfX mtlF mtlD.     

Identification of a receptor-binding domain of the spike glycoprotein of human coronavirus HCoV-229E

Human coronavirus HCoV-229E uses human aminopeptidase N (hAPN) as its receptor (C. L. Yeager et al., Nature 357:420-422, 1992). To identify the receptor-binding domain of the viral spike glycoprotein (S), we expressed soluble truncated histidine-tagged S glycoproteins by using baculovirus expression vectors. Truncated S proteins purified by nickel affinity chromatography were shown to be glycosylated and to react with polyclonal anti-HCoV-229E antibodies and monoclonal antibodies to the viral S protein. A truncated protein (S(547)) that contains the N-terminal 547 amino acids bound to 3T3 mouse cells that express hAPN but not to mouse 3T3 cells transfected with empty vector. Binding of S(547) to hAPN was blocked by an anti-hAPN monoclonal antibody that inhibits binding of virus to hAPN and blocks virus infection of human cells and was also blocked by polyclonal anti-HCoV-229E antibody. S proteins that contain the N-terminal 268 or 417 amino acids did not bind to hAPN-3T3 cells. Antibody to the region from amino acid 417 to the C terminus of S blocked binding of S(547) to hAPN-3T3 cells. Thus, the data suggest that the domain of the spike protein between amino acids 417 and 547 is required for the binding of HCoV-229E to its hAPN receptor.     

Identification of Two Binding Domains, One for Peptidoglycan and Another for a Secondary Cell Wall Polymer, on the N-Terminal Part of the S-Layer Protein SbsB from Bacillus stearothermophilus PV72/p2

First studies on the structure-function relationship of the S-layer protein from B. stearothermophilus PV72/p2 revealed the coexistence of two binding domains on its N-terminal part, one for peptidoglycan and another for a secondary cell wall polymer (SCWP). The peptidoglycan binding domain is located between amino acids 1 to 138 of the mature S-layer protein comprising a typical S-layer homologous domain. The SCWP binding domain lies between amino acids 240 to 331 and possesses a high serine plus glycine content.     

Development of a method for markerless genetic exchange in Enterococcus faecalis and its use in construction of a srtA mutant

Enterococcus faecalis is a gram-positive commensal bacterium of the gastrointestinal tract and an important opportunistic pathogen. Despite the increasing clinical significance of the enterococci, genetic analysis of these organisms has thus far been limited in scope due to the lack of advanced genetic tools. To broaden the repertoire of genetic tools available for manipulation of E.faecalis, we investigated the use of phosphoribosyl transferases as elements of a counterselection strategy. We report here the development of a counterselectable markerless genetic exchange system based on the upp-encoded uracil phosphoribosyl transferase of E. faecalis. Whereas wild-type E. faecalis is sensitive to growth inhibition by the toxic base analog 5-fluorouracil (5-FU), a mutant bearing an in-frame deletion of upp is resistant to 5-FU. When a cloned version of upp was ectopically introduced into the deletion mutant, sensitivity to 5-FU growth inhibition was restored, thereby providing the basis for a two-step integration and excision strategy for the transfer of mutant alleles to the enterococcal chromosome by recombination. This method was validated by the construction of a DeltasrtA mutant of E. faecalis and by the exchange of the surface protein Asc10, encoded on the pheromone-responsive conjugative plasmid pCF10, with a previously isolated mutant allele. Analysis of the DeltasrtA mutant indicated that SrtA anchors Asc10 to the enterococcal cell wall, facilitating the pheromone-induced aggregation of E. faecalis cells required for high-frequency conjugative plasmid transfer in liquid matings. The system of markerless exchange reported here will facilitate detailed genetic analysis of these important pathogens.     

Localization of functional domains in the Escherichia coli coprogen receptor FhuE and the Pseudomonas putida ferric-pseudobactin 358 receptor PupA

Transport of ferric-siderophores across the outer membrane of gram-negative bacteria is mediated by specific outer membrane receptors. To localize the substrate-binding domain of the ferric-pseudobactin 358 receptor, PupA, of Pseudomonas putida WCS358, we constructed chimeric receptors in which different domains of PupA were replaced by the corresponding domains of the related ferric-pseudobactin receptors PupB and PupX, or the coprogen receptor FhuE of Escherichia coli. None of the chimeric proteins composed of pseudobactin receptor domains facilitated growth on any of the original substrates, or they showed only an extremely low efficiency. However, these receptors enabled cells of Pseudomonas BN8 to grow on media supplemented with uncharacterized siderophore preparations. These siderophore preparations were isolated from the culture supernatant of WCS358 cells carrying plasmids that contain genes of Pseudomonas B10 required for the biosynthesis of pseudobactin B10. Hybrid proteins that contained at least the amino-terminal 516 amino acids of mature FhuE were active as a receptor for coprogen and interacted with the E. coli TonB protein. A chimeric PupA-FhuE protein, containing the amino-terminal 94 amino acids of mature PupA, was also active as a coprogen receptor, but only in the presence of Pseudomonas TonB. It is concluded that the carboxy-terminal domain of ferric-pseudobactin receptors is important, but not sufficient, for ligand interaction, whereas binding of coprogen by the FhuE receptor is not dependent on this domain. Apparently, the ligand-binding sites of different receptors are located in different regions of the proteins. Furthermore, species-specific TonB binding by the PupA receptor is dependent on the amino-terminal domain of the receptor.     

Localization of functional domains in the Escherichia coli coprogen receptor FhuE and the Pseudomonas putida ferric-pseudobactin 358 receptor PupA

Transport of ferric-siderophores across the outer membrane of gram-negative bacteria is mediated by specific outer membrane receptors. To localize the substrate-binding domain of the ferric-pseudobactin 358 receptor, PupA, of Pseudomonas putida WCS358, we constructed chimeric receptors in which different domains of PupA were replaced by the corresponding domains of the related ferric-pseudobactin receptors PupB and PupX, or the coprogen receptor FhuE of Escherichia coli. None of the chimeric proteins composed of pseudobactin receptor domains facilitated growth on any of the original substrates, or they showed only an extremely low efficiency. However, these receptors enabled cells of Pseudomonas BN8 to grow on media supplemented with uncharacterized siderophore preparations. These siderophore preparations were isolated from the culture supernatant of WCS358 cells carrying plasmids that contain genes of Pseudomonas B10 required for the biosynthesis of pseudobactin B10. Hybrid proteins that contained at least the amino-terminal 516 amino acids of mature FhuE were active as a receptor for coprogen and interacted with the E. coli TonB protein. A chimeric PupA-FhuE protein, containing the amino-terminal 94 amino acids of mature PupA, was also active as a coprogen receptor, but only in the presence of Pseudomonas TonB. It is concluded that the carboxy-terminal domain of ferric-pseudobactin receptors is important, but not sufficient, for ligand interaction, whereas binding of coprogen by the FhuE receptor is not dependent on this domain. Apparently, the ligand-binding sites of different receptors are located in different regions of the proteins. Furthermore, species-specific TonB binding by the PupA receptor is dependent on the amino-terminal domain of the receptor.