Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

We investigated the involvement of Tol proteins in the surface expression of lipopolysaccharide (LPS). tolQ, -R, -A and -B mutants of Escherichia coli K-12, which do not form a complete LPS-containing O antigen, were transformed with the O7+ cosmid pJHCV32. The tolA and tolQ mutants showed reduced O7 LPS expression compared with the respective isogenic parent strains. No changes in O7 LPS expression were found in the other tol mutants. The O7-deficient phenotype in the tolQ and tolA mutants was complemented with a plasmid encoding the tolQRA operon, but not with a similar plasmid containing a frameshift mutation inactivating tolA. Therefore, the reduction in O7 LPS was attributed to the lack of a functional tolA gene, caused either by a direct mutation of this gene or by a polar effect on tolA gene expression exerted by the tolQ mutation. Reduced surface expression of O7 LPS was not caused by changes in lipid A-core structure or downregulation of the O7 LPS promoter. However, an abnormal accumulation of radiolabelled mannose was detected in the plasma membrane. As mannose is a sugar unique to the O7 subunit, this result suggested the presence of accumulated O7 LPS biosynthesis intermediates. Attempts to construct a tolA mutant in the E. coli O7 wild-type strain VW187 were unsuccessful, suggesting that this mutation is lethal. In contrast, a polar tolQ mutation affecting tolA expression in VW187 caused slow growth rate and serum sensitivity in addition to reduced O7 LPS production. VW187 tolQ cells showed an elongated morphology and became permeable to the membrane-impermeable dye propidium iodide. All these phenotypes were corrected upon complementation with cloned tol genes but were not restored by complementation with the tolQRA operon containing the frameshift mutation in tolA. Our results demonstrate that the TolA protein plays a critical role in the surface expression of O antigen subunits by an as yet uncharacterized involvement in the processing of O antigen.     

Identification by genetic suppression of Escherichia coli TolB residues important for TolB-Pal interaction

The Tol-Pal system of Escherichia coli is involved in maintaining outer membrane stability. Mutations in tolQ, tolR, tolA, tolB, or pal genes result in sensitivity to bile salts and the leakage of periplasmic proteins. Moreover, some of the tol genes are necessary for the entry of group A colicins and the DNA of filamentous bacteriophages. TolQ, TolR, and TolA are located in the cytoplasmic membrane where they interact with each other via their transmembrane domains. TolB and Pal form a periplasmic complex near the outer membrane. We used suppressor genetics to identify the regions important for the interaction between TolB and Pal. Intragenic suppressor mutations were characterized in a domain of Pal that was shown to be involved in interactions with TolB and peptidoglycan. Extragenic suppressor mutations were located in tolB gene. The C-terminal region of TolB predicted to adopt a beta-propeller structure was shown to be responsible for the interaction of the protein with Pal. Unexpectedly, none of the suppressor mutations was able to restore a correct association between Pal and peptidoglycan, suggesting that interactions between Pal and other components such as TolB may also be important for outer membrane stability.     

Interplay of the Wzx translocase and the corresponding polymerase and chain length regulator proteins in the translocation and periplasmic assembly of lipopolysaccharide o antigen

Genetic evidence suggests that a family of bacterial and eukaryotic integral membrane proteins (referred to as Wzx and Rft1, respectively) mediates the transbilayer movement of isoprenoid lipid-linked glycans. Recent work in our laboratory has shown that Wzx proteins involved in O-antigen lipopolysaccharide (LPS) assembly have relaxed specificity for the carbohydrate structure of the O-antigen subunit. Furthermore, the proximal sugar bound to the isoprenoid lipid carrier, undecaprenyl-phosphate (Und-P), is the minimal structure required for translocation. In Escherichia coli K-12, N-acetylglucosamine (GlcNAc) is the proximal sugar of the O16 and enterobacterial common antigen (ECA) subunits. Both O16 and ECA systems have their respective translocases, WzxO16 and WzxE, and also corresponding polymerases (WzyO16 and WzyE) and O-antigen chain-length regulators (WzzO16 and WzzE), respectively. In this study, we show that the E. coli wzxE gene can fully complement a wzxO16 translocase deletion mutant only if the majority of the ECA gene cluster is deleted. In addition, we demonstrate that introduction of plasmids expressing either the WzyE polymerase or the WzzE chain-length regulator proteins drastically reduces the O16 LPS-complementing activity of WzxE. We also show that this property is not unique to WzxE, since WzxO16 and WzxO7 can cross-complement translocase defects in the O16 and O7 antigen clusters only in the absence of their corresponding Wzz and Wzy proteins. These genetic data are consistent with the notion that the translocation of O-antigen and ECA subunits across the plasma membrane and the subsequent assembly of periplasmic O-antigen and ECA Und-PP-linked polymers depend on interactions among Wzx, Wzz, and Wzy, which presumably form a multiprotein complex.     

Lateral transfer of rfb genes: a mobilizable ColE1-type plasmid carries the rfbO:54 (O:54 antigen biosynthesis) gene cluster from Salmonella enterica serovar Borreze.

Plasmid pWQ799 is a 6.9-kb plasmid isolated from Salmonella enterica serovar Borreze. Our previous studies have shown that the plasmid contains a functional biosynthetic gene cluster for the expression of the O:54 lipopolysaccharide O-antigen of this serovar. The minimal replicon functions of pWQ799 have been defined, and a comparison with nucleotide and protein databases revealed this replicon to be virtually identical to ColE1. This is the first report of involvement of ColE1-related plasmids in O-antigen expression. The replicon of pWQ799 is predicted to encode two RNA molecules, typical of other ColE1-type plasmids. RNAII, the putative replication primer from pWQ799, shares regions of homology with RNAII from ColE1. RNA1 is an antisense regulator of DNA replication in ColE1-related plasmids. The coding region for RNAI from pWQ799 shares no homology with any other known RNAI sequence but is predicted to adopt a secondary structure characteristic of RNAI molecules. pWQ799 may therefore represent a new incompatibility group within this family. pWQ799 also possesses cer, rom, and mob determinants, and these differ minimally from those of ColE1. The plasmid is mobilizable in the presence of either the broad-host-range helper plasmid pRK2013 or the IncI1 plasmid R64drd86. Mobilization and transfer of pWQ799 to other organisms provides the first defined mechanism for lateral transfer of O-antigen biosynthesis genes in S. enterica and explains both the distribution of related plasmids and coexpression of the O:54 factor with other O-factors in different Salmonella serovars. The base composition of the pWQ799 replicon sequences gives an average percent G+C value typical of Salmonella spp. In contrast, the percent G+C value is dramatically lower with rfb0:54, consistent with the possibility that the cluster was acquired from an organism with much lower G+C composition.     

TolB protein of Escherichia coli K-12 interacts with the outer membrane peptidoglycan-associated proteins Pal, Lpp and OmpA

The Tol–Pal proteins of Escherichia coli are involved in maintaining outer membrane integrity. Transmembrane domains of TolQ, TolR and TolA interact in the cytoplasmic membrane, while TolB and Pal form a complex near the outer membrane. TolB and the central domain of TolA interact in vitro with the outer membrane porins. In this study, both genetic and biochemical analyses were carried out to analyse the links between TolB, Pal and other components of the cell envelope. It was shown that TolB could be cross-linked in vivo with Pal, OmpA and Lpp, while Pal was associated with TolB and OmpA. The isolation of pal and tolB mutants disrupting some interactions between these proteins represents a first approach to characterizing the residues contributing to the interactions. We propose that TolB and Pal are part of a multiprotein complex that links the peptidoglycan to the outer membrane. The Tol–Pal proteins might form transenvelope complexes that bring the two membranes into close proximity and help some outer membrane components to reach their final destination.     

Energy-Dependent Conformational Change in the TolA Protein of Escherichia coli Involves Its N-Terminal Domain, TolQ, and TolR

TolQ, TolR, and TolA inner membrane proteins of Escherichia coli are involved in maintaining the stability of the outer membrane. They share homology with the ExbB, ExbD, and TonB proteins, respectively. The last is involved in energy transduction between the inner and the outer membrane, and its conformation has been shown to depend on the presence of the proton motive force (PMF), ExbB, and ExbD. Using limited proteolysis experiments, we investigated whether the conformation of TolA was also affected by the PMF. We found that dissipation of the PMF by uncouplers led to the formation of a proteinase K digestion fragment of TolA not seen when uncouplers are omitted. This fragment was also detected in Delta tolQ, Delta tolR, and tolA(H22P) mutants but, in contrast to the parental strain, was also seen in the absence of uncouplers. We repeated those experiments in outer membrane mutants such as lpp, pal, and Delta rfa mutants: the behavior of TolA in lpp mutants was similar to that observed with the parental strain. However, the proteinase K-resistant fragment was never detected in the Delta rfa mutant. Altogether, these results suggest that TolA is able to undergo a PMF-dependent change of conformation. This change requires TolQ, TolR, and a functional TolA N-terminal domain. The potential role of this energy-dependent process in the stability of the outer membrane is discussed.     

Pal Lipoprotein of Escherichia coli Plays a Major Role in Outer Membrane Integrity

The Tol-Pal system of gram-negative bacteria is composed of five proteins. TolA, TolQ, and TolR are inner membrane proteins, TolB is a periplasmic protein, and Pal, the peptidoglycan-associated lipoprotein, is anchored to the outer membrane. In this study, the roles of Pal and major lipoprotein Lpp were compared in Escherichia coli. lpp and tol-pal mutations have previously been found to perturb the outer membrane permeability barrier and to cause the release of periplasmic proteins and the formation of outer membrane vesicles. In this study, we showed that the overproduction of Pal is able to restore the outer membrane integrity of an lpp strain but that overproduced Lpp has no effect in a pal strain. Together with the previously reported observation that overproduced TolA complements an lpp but not a pal strain, these results indicate that the cell envelope integrity is efficiently stabilized by an epistatic Tol-Pal system linking inner and outer membranes. The density of Pal was measured and found to be lower than that of Lpp. However, Pal was present in larger amounts compared to TolA and TolR proteins. The oligomeric state of Pal was determined and a new interaction between Pal and Lpp was demonstrated.     

O-antigen polymerase adopts a distributive mechanism for lipopolysaccharide biosynthesis

Bacterial lipopolysaccharide (LPS) is an essential cell envelope component for gram-negative bacteria. As the most variable region of LPS, O antigens serve as important virulence determinants for many bacteria and represent a promising carbohydrate source for glycoconjugate vaccines. In the Wzy-dependent O-antigen biosynthetic pathway, the integral membrane protein Wzy was shown to be the sole enzyme responsible for polymerization of O-repeat unit. Its catalytic mechanism, however, remains elusive. Herein, Wzy was successfully overexpressed in Escherichia coli with an N-terminal His₁₀-tag. Blue native polyacrylamide gel electrophoresis (BN-PAGE) revealed that the Wzy protein exists in its native confirmation as a dimer. Subsequently, we chemo-enzymatically synthesized the substrates of Wzy, the lipid-PP-linked repeat units. Together with an optimized O-antigen visualization method, we monitored the production of reaction intermediates at varying times. We present here our result as the first biochemical evidence that Wzy functions in a distributive manner.     

Proton motive force drives the interaction of the inner membrane TolA and outer membrane pal proteins in Escherichia coli

The Tol-Pal system of the Escherichia coli envelope is formed from the inner membrane TolQ, TolR and TolA proteins, the periplasmic TolB protein and the outer membrane Pal lipoprotein. Any defect in the Tol-Pal proteins or in the major lipoprotein (Lpp) results in the loss of outer membrane integrity giving hypersensitivity to drugs and detergents, periplasmic leakage and outer membrane vesicle formation. We found that multicopy plasmid overproduction of TolA was able to complement the membrane defects of an lpp strain but not those of a pal strain. This result indicated that overproduced TolA has an envelope-stabilizing effect when Pal is present. We demonstrate that Pal and TolA formed a complex using in vivo cross-linking and immunoprecipitation experiments. These results, together with in vitro experiments with purified Pal and TolA derivatives, allowed us to show that Pal interacts with the TolA C-terminal domain. We also demonstrate using protonophore, K+ carrier valinomycin, nigericin, arsenate and fermentative conditions that the proton motive force was coupled to this interaction.     

Structural and genetic relationships of two pairs of closely related O-antigens of Escherichia coli and Salmonella enterica: E. coli O11/S. enterica O16 and E. coli O21/S. enterica O38

The O-antigen is a part of the lipopolysaccharide molecule present in the outer membrane of Gram-negative bacteria, and is essential for the full function of the microorganisms. Salmonella enterica and Escherichia coli are taxonomically closely related species. In this study, the O-antigen structures of S. enterica O16 and O38 and E. coli O11 were determined. Salmonella enterica O38 and E. coli O21 were found to have identical O-antigen structures, whereas S. enterica O16 and E. coli O11 had closely related structures, differing only in the presence of a lateral glucose residue and O-acetylation of a mannose residue in the former. The O-antigen gene clusters of S. enterica O16 and O38 and E. coli O11 were sequenced and analyzed together with that of E. coli O21 retrieved from the GenBank. Each S. enterica/E. coli pair was found to contain the same set of genes organized in the same manner and to share 56-78% overall DNA identity. These data suggest that the O-antigen gene clusters of each pair studied originated from a common ancestor. Thus, it has become evident that in the past, the degree of relatedness between the O-antigens of S. enterica and E. coli was underestimated.     

Impairment of cell division in tolA mutants of Escherichia coli at low and high medium osmolarities.

Mutations in the tolA gene of Escherichia coli cause the cell to become sensitive to detergents and to some antibiotics, to release periplasmic enzymes and to be resistant to group A colicins; tolA mutations also lead to mucoid phenotype. TolA is a three-domain protein anchored in the inner membrane by its N-terminal domain. The second domain is proposed to span the periplasmic space and to interact with trimeric porins of the outer membrane. TolA proteins are considered to be located in the adhesion zones between inner and outer membranes. Our observations by confocal and electron microscopy have revealed that tolA mutants show modified morphology and produce DNA-free cells. Increasing or decreasing medium osmolarity amplifies these defects; mutants become essentially unable to locate the division site properly so that cells of highly unequal lengths are produced. Moreover, septation is impaired with asymmetric constrictions and oblique septa. These results suggest that TolA could play a role in positioning the division sites via the organisation of either the outer membrane or the possible adhesion zones.     

The activity of a putative polyisoprenol-linked sugar translocase (Wzx) involved in Escherichia coli O antigen assembly is independent of the chemical structure of the O repeat

During O antigen lipopolysaccharide (LPS) synthesis in bacteria, transmembrane migration of undecaprenylpyrophosphate (Und-P-P)-bound O antigen subunits occurs before their polymerization and ligation to the rest of the LPS molecule. Despite the general nature of the translocation process, putative O-antigen translocases display a low level of amino acid sequence similarity. In this work, we investigated whether complete O antigen subunits are required for translocation. We demonstrate that a single sugar, GlcNAc, can be incorporated to LPS of Escherichia coli K-12. This incorporation required the functions of two O antigen synthesis genes, wecA (UDP-GlcNAc:Und-P GlcNAc-1-P transferase) and wzx (O-antigen translocase). Complementation experiments with putative O-antigen translocases from E. coli O7 and Salmonella enterica indicated that translocation of O antigen subunits is independent of the chemical structure of the saccharide moiety. Furthermore, complementation with putative translocases involved in synthesis of exopolysaccharides demonstrated that these proteins could not participate in O antigen assembly. Our data indicate that recognition of a complete Und-P-P-bound O antigen subunit is not required for translocation and suggest a model for O antigen synthesis involving recognition of Und-P-P-linked sugars by a putative complex made of Wzx translocase and other proteins involved in the processing of O antigen.     

Functional characterization of Gne (UDP-N-acetylglucosamine-4-epimerase), Wzz (chain length determinant), and Wzy (O-antigen polymerase) of Yersinia enterocolitica serotype O:8

The lipopolysaccharide (LPS) O-antigen of Yersinia enterocolitica serotype O:8 is formed by branched pentasaccharide repeat units that contain N-acetylgalactosamine (GalNAc), L-fucose (Fuc), D-galactose (Gal), D-mannose (Man), and 6-deoxy-D-gulose (6d-Gul). Its biosynthesis requires at least enzymes for the synthesis of each nucleoside diphosphate-activated sugar precursor; five glycosyltransferases, one for each sugar residue; a flippase (Wzx); and an O-antigen polymerase (Wzy). As this LPS shows a characteristic preferred O-antigen chain length, the presence of a chain length determinant protein (Wzz) is also expected. By targeted mutagenesis, we identify within the O-antigen gene cluster the genes encoding Wzy and Wzz. We also present genetic and biochemical evidence showing that the gene previously called galE encodes a UDP-N-acetylglucosamine-4-epimerase (EC 5.1.3.7) required for the biosynthesis of the first sugar of the O-unit. Accordingly, the gene was renamed gne. Gne also has some UDP-glucose-4-epimerase (EC 5.1.3.2) activity, as it restores the core production of an Escherichia coli K-12 galE mutant. The three-dimensional structure of Gne was modeled based on the crystal structure of E. coli GalE. Detailed structural comparison of the active sites of Gne and GalE revealed that additional space is required to accommodate the N-acetyl group in Gne and that this space is occupied by two Tyr residues in GalE whereas the corresponding residues present in Gne are Leu136 and Cys297. The Gne Leu136Tyr and Cys297Tyr variants completely lost the UDP-N-acetylglucosamine-4-epimerase activity while retaining the ability to complement the LPS phenotype of the E. coli galE mutant. Finally, we report that Yersinia Wzx has relaxed specificity for the translocated oligosaccharide, contrary to Wzy, which is strictly specific for the O-unit to be polymerized.     

Genetic analysis of the O-specific lipopolysaccharide biosynthesis region (rfb) of Escherichia coli K-12 W3110: identification of genes that confer group 6 specificity to Shigella flexneri serotypes Y and 4a.

We recently reported a novel genetic locus located in the sbcB-his region of the chromosomal map of Escherichia coli K-12 which directs the expression of group 6-positive phenotype in Shigella flexneri lipopolysaccharide, presumably due to the transfer of O-acetyl groups onto rhamnose residues of the S. flexneri O-specific polysaccharide (Z. Yao, H. Liu, and M. A. Valvano, J. Bacteriol. 174:7500-7508, 1992). In this study, we identified the genetic region encoding group 6 specificity as part of the rfb gene cluster of E. coli K-12 strain W3110 and established the DNA sequence of most of this cluster. The rfbBDACX block of genes, located in the upstream region of the rfb cluster, was found to be strongly conserved in comparison with the corresponding region in Shigella dysenteriae type 1 and Salmonella enterica. Six other genes, four of which were shown to be essential for the expression of group 6 reactivity in S. flexneri serotypes Y and 4a, were identified downstream of rfbX. One of the remaining two genes showed similarities with rfc (O-antigen polymerase) of S. enterica serovar typhimurium, whereas the other, located in the downstream end of the cluster next to gnd (gluconate-6-phosphate dehydrogenase), had an IS5 insertion. Recently, it has been reported that the IS5 insertion mutation (rfb-50) can be complemented, resulting in the formation of O16-specific polysaccharide by E. coli K-12 (D. Liu and P. R. Reeves, Microbiology 140:49-57, 1994). We present immunochemical evidence suggesting that S. flexneri rfb genes also complement the rfb-50 mutation; in the presence of rfb genes of E. coli K-12, S. flexneri isolates express O16-specific polysaccharide which is also acetylated in its rhamnose residues, thereby eliciting group 6 specificity.     

A single nucleotide exchange in the wzy gene is responsible for the semirough O6 lipopolysaccharide phenotype and serum sensitivity of Escherichia coli strain Nissle 1917

Structural analysis of lipopolysaccharide (LPS) isolated from semirough, serum-sensitive Escherichia coli strain Nissle 1917 (DSM 6601, serotype O6:K5:H1) revealed that this strain's LPS contains a bisphosphorylated hexaacyl lipid A and a tetradecasaccharide consisting of one E. coli O6 antigen repeating unit attached to the R1-type core. Configuration of the GlcNAc glycosidic linkage between O-antigen oligosaccharide and core (beta) differs from that interlinking the repeating units in the E. coli O6 antigen polysaccharide (alpha). The wa(*) and wb(*) gene clusters of strain Nissle 1917, required for LPS core and O6 repeating unit biosyntheses, were subcloned and sequenced. The DNA sequence of the wa(*) determinant (11.8 kb) shows 97% identity to other R1 core type-specific wa(*) gene clusters. The DNA sequence of the wb(*) gene cluster (11 kb) exhibits no homology to known DNA sequences except manC and manB. Comparison of the genetic structures of the wb(*)(O6) (wb(*) from serotype O6) determinants of strain Nissle 1917 and of smooth and serum-resistant uropathogenic E. coli O6 strain 536 demonstrated that the putative open reading frame encoding the O-antigen polymerase Wzy of strain Nissle 1917 was truncated due to a point mutation. Complementation with a functional wzy copy of E. coli strain 536 confirmed that the semirough phenotype of strain Nissle 1917 is due to the nonfunctional wzy gene. Expression of a functional wzy gene in E. coli strain Nissle 1917 increased its ability to withstand antibacterial defense mechanisms of blood serum. These results underline the importance of LPS for serum resistance or sensitivity of E. coli.     

Pseudomonas aeruginosa B-band O-antigen chain length is modulated by Wzz (Ro1).

The wbp gene cluster, encoding the B-band lipopolysaccharide O antigen of Pseudomonas aeruginosa serotype O5 strain PAO1, was previously shown to contain a wzy (rfc) gene encoding the O-antigen polymerase. This study describes the molecular characterization of the corresponding wzz (rol) gene, responsible for modulating O-antigen chain length. P. aeruginosa O5 Wzz has 19 to 20% amino acid identity with Wzz of Escherichia coli, Salmonella enterica, and Shigella flexneri. Knockout mutations of the wzz gene in serotypes O5 and O16 (which has an O antigen structurally related to that of O5) yielded mutants expressing O antigens with a distribution of chain lengths differing markedly from that of the parent strains. Unlike enteric wzz mutants, the P. aeruginosa wzz mutants continued to display some chain length modulation. The P. aeruginosa O5 wzz gene complemented both O5 and O16 wzz mutants as well as an E. coli wzz mutant. Coexpression of E. coli and P. aeruginosa wzz genes in a rough strain of E. coli carrying the P. aeruginosa wbp cluster resulted in the expression of two populations of O-antigen chain lengths. Sequence analysis of the region upstream of wzz led to identification of the genes rpsA and himD, encoding 30S ribosomal subunit protein S1 and integration host factor, respectively. This finding places rpsA and himD adjacent to wzz and the wbp cluster at 37 min on the PAO1 chromosomal map and completes the delineation of the O5 serogroup-specific region of the wbp cluster.     

Gene products required for surface expression of the capsular form of the group 1 K antigen in Escherichia coli (O9a:K30)

The group 1 K30 antigen from Escherichia coli (O9a:K30) is present on the cell surface as both a capsular structure composed of high-molecular-weight K30 polysaccharide and as short K30 oligosaccharides linked to lipid A-core in a lipopolysaccharide molecule (K30LPS). To determine the molecular processes that are responsible for the two forms of K antigen, the 16 kb chromosomal cps region has been characterized. This region encodes 12 gene products required for the synthesis, polymerization and translocation of the K30 antigen. The gene products include four glycosyltransferases responsible for synthesis of the K30 repeat unit; a PST (1) exporter (Wzx), required to transfer lipid-linked K30 units across the plasma membrane to the periplasmic space; and a K30-antigen polymerase (Wzy). These gene products are typical of those seen in O-antigen biosynthesis gene clusters and they interact with the lipopolysaccharide translocation pathway to express K30LPS on the cell surface. The same gene products also provide the biosynthetic intermediates for the capsule assembly pathway, although they are not in themselves sufficient for synthesis of the K30 capsule. Three additional genes, wza, wzb and wzc, encode homologues to proteins that are encoded by gene clusters involved in expression of a variety of bacterial exopolysaccharides. Mutant analysis indicates that Wza and Wzc are required for wild-type surface expression of the capsular structure but are not essential for polymerization and play no role in the translocation of K30LPS. These surface expression components provide the key feature that distinguishes the assembly systems for O antigens and capsules.     

Characterization of the tol-pal region of Escherichia coli K-12: translational control of tolR expression by TolQ and identification of a new open reading frame downstream of pal encoding a periplasmic protein.

The TolQ, TolR, TolA, TolB, and Pal proteins appear to function in maintaining the integrity of the outer membrane, as well as facilitating the uptake of the group A colicins and the DNA of the infecting filamentous bacteriophages. Sequence data showed that these genes are clustered in a 6-kb segment of DNA with the gene order orf1 tolQ tolR tolA tolB pal orf2 (a newly identified open reading frame encoding a 29-kD9 protein). Like those containing orf1, bacteria containing an insertion mutation in this gene showed no obvious phenotype. Analysis of beta-galactosidase activity from fusion constructs in which the lac operon was fused to various genes in the cluster showed that the genes in this region constitute two separate operons: orf1 tolQRA and tolB pal orf2. In the orf1 tolQRA operon, translation of MR was dependent on translation of the upstream tolQ region. Consistent with this result, no functional ribosome-binding site for TolR synthesis was detected.     

The Escherichia coli O111 and Salmonella enterica O35 gene clusters: gene clusters encoding the same colitose-containing O antigen are highly conserved

O antigen is part of the lipopolysaccharide present in the outer membrane of gram-negative bacteria. Escherichia coli and Salmonella enterica each have many forms of O antigen, but only three are common to the two species. It has been found that, in general, O-antigen genes are of low GC content. This deviation in GC content from that of typical S. enterica or E. coli genes (51%) is thought to indicate that the O-antigen DNA originated in species other than S. enterica or E. coli and was captured by lateral transfer. The O-antigen structure of Salmonella enterica O35 is identical to that of E. coli O111, commonly found in enteropathogenic E. coli strains. This O antigen, which has been shown to be a virulence factor in E. coli, contains colitose, a 3,6-dideoxyhexose found only rarely in the Enterobacteriaceae. Sequencing of the O35-antigen gene cluster of S. enterica serovar Adelaide revealed the same gene order and flanking genes as in E. coli O111. The divergence between corresponding genes of these two gene clusters at the nucleotide level ranges from 21.8 to 11.7%, within the normal range of divergence between S. enterica and E. coli. We conclude that the ancestor of E. coli and S. enterica had an O antigen identical to the O111 and O35 antigens, respectively, of these species and that the gene cluster encoding it has survived in both species.     

Synthesis of a Klebsiella pneumoniae O-antigen heteropolysaccharide (O12) requires an ABC 2 transporter

A recombinant clone encoding enzymes for Klebsiella pneumoniae O12-antigen lipopolysaccharide (LPS) was found when we screened for serum resistance of a cosmid-based genomic library of K. pneumoniae KT776 (O12:K80) introduced into Escherichia coli DH5alpha. A total of eight open reading frames (ORFs) (wb(O12) gene cluster) were necessary to produce K. pneumoniae O12-antigen LPS in E. coli K-12. A complete analysis of the K. pneumoniae wb(O12) cluster revealed an interesting coincidence with the wb(O4) cluster of Serratia marcescens from ORF5 to ORF8 (or WbbL to WbbA). This prompted us to generate mutants of K. pneumoniae strain KT776 (O12) and to study complementation between the two enterobacterial wb clusters using mutants of S. marcescens N28b (O4) obtained previously. Both wb gene clusters are examples of ABC 2 transporter-dependent pathways for O-antigen heteropolysaccharides. The wzm-wzt genes and the wbbA or wbbB genes were not interchangeable between the two gene clusters despite their high level of similarity. However, introduction of three cognate genes (wzm-wzt-wbbA or wzm-wzt-wbbB) into mutants unable to produce O antigen allowed production of the specific O antigen. The K. pneumoniae O12 WbbL protein performs the same function as WbbL from S. marcescens O4 in either the S. marcescens O4 or E. coli K-12 genetic background.