Localization of the terminal steps of O-antigen synthesis in Salmonella typhimurium

Repeat-Unit Elongations To Produce Bacterial Complex Long Polysaccharide Chains, an O-Antigen Perspective

The O-antigen, a long polysaccharide that constitutes the distal part of the outer membrane-anchored lipopolysaccharide, is one of the critical components in the protective outer membrane of Gram-negative bacteria. Most species produce one of the structurally diverse O-antigens, with nearly all the polysaccharide components having complex structures made by the Wzx/Wzy pathway. This pathway produces repeat-units of mostly 3-8 sugars on the cytosolic face of the cytoplasmic membrane that is translocated by Wzx flippase to the periplasmic face and polymerized by Wzy polymerase to give long-chain polysaccharides. The Wzy polymerase is a highly diverse integral membrane protein typically containing 10-14 transmembrane segments. Biochemical evidence confirmed that Wzy polymerase is the sole driver of polymerization, and recent progress also began to demystify its interacting partner, Wzz, shedding some light to speculate how the proteins may operate together during polysaccharide biogenesis. However, our knowledge of how the highly variable Wzy proteins work as part of the O-antigen processing machinery remains poor. Here, we discuss the progress to the current understanding of repeat-unit polymerization and propose an updated model to explain the formation of additional short chain O-antigen polymers found in the lipopolysaccharide of diverse Gram-negative species and their importance in the biosynthetic process.

Lipopolysaccharide as an antibiotic target

Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii are amongst the highest priority drug-resistant pathogens, for which new antibiotics are urgently needed. Whilst antibiotic drug development is inherently challenging, this is particularly true for Gram-negative bacteria due to the presence of the outer membrane, a highly selective permeability barrier that prevents the ingress of several classes of antibiotic. This selectivity is largely due to an outer leaflet composed of the glycolipid lipopolysaccharide (LPS), which is essential for the viability of almost all Gram-negative bacteria. This essentiality, coupled with the conservation of the synthetic pathway across species and recent breakthroughs in our understanding of transport and membrane homeostasis has made LPS an attractive target for novel antibiotic drug development. Several different targets have been explored and small molecules developed that show promising activity in vitro. However, these endeavours have met limited success in clinical testing and the polymyxins, discovered more than 70 years ago, remain the only LPS-targeting drugs to enter the clinic thus far. In this review, we will discuss efforts to develop therapeutic inhibitors of LPS synthesis and transport and the reasons for limited success, and explore new developments in understanding polymyxin mode of action and the identification of new analogues with reduced toxicity and enhanced activity.

O-Polysaccharides of LPS Modulate E. coli Uptake by Acanthamoeba castellanii

Protozoan grazing is a major cause of bacterial mortality and controls bacterial population size and composition in the natural environment. To enhance their survival, bacteria evolved many defense strategies to avoid grazing by protists. Cell wall modification is one of the defense strategies that helps bacteria escape from recognition and/or internalization by its predators. Lipopolysaccharide (LPS) is the major component of Gram-negative bacterial cell wall. LPS is divided into three regions: lipid A, oligosaccharide core and O-specific polysaccharide. O-polysaccharide as the outermost region of E. coli LPS provides protection against predation by Acanthamoeba castellanii; however, the characteristics of O-polysaccharide contribute to this protection remain unknown. Here, we investigate how length, structure and composition of LPS affect E. coli recognition and internalization by A. castellanii. We found that length of O-antigen does not play a significant role in regulating bacterial recognition by A. castellanii. However, the composition and structure of O-polysaccharide play important roles in providing resistance to A. castellanii predation.

Cracking outer membrane biogenesis

The outer membrane is a distinguishing feature of the Gram-negative envelope. It lies on the external face of the peptidoglycan sacculus and forms a robust permeability barrier that protects extracytoplasmic structures from environmental insults. Overcoming the barrier imposed by the outer membrane presents a significant hurdle towards developing novel antibiotics that are effective against Gram-negative bacteria. As the outer membrane is an essential component of the cell, proteins involved in its biogenesis are themselves promising antibiotic targets. Here, we summarize key findings that have built our understanding of the outer membrane. Foundational studies describing the discovery and composition of the outer membrane as well as the pathways involved in its construction are discussed.

The Mutation in wbaP cps Gene Cluster Selected by Phage-Borne Depolymerase Abolishes Capsule Production and Diminishes the Virulence of Klebsiella pneumoniae

Klebsiella pneumoniae is considered one of the most critical multidrug-resistant pathogens and urgently requires new therapeutic strategies. Capsular polysaccharides (CPS), lipopolysaccharides (LPS), and exopolysaccharides (EPS) are the major virulence factors protecting K. pneumoniae against the immune response and thus may be targeted by phage-based therapeutics such as polysaccharides-degrading enzymes. Since the emergence of resistance to antibacterials is generally considered undesirable, in this study, the genetic and phenotypic characteristics of resistance to the phage-borne CPS-degrading depolymerase and its effect on K. pneumoniae virulence were investigated. The K63 serotype targeting depolymerase (KP36gp50) derived from Klebsiella siphovirus KP36 was used as the selective agent during the treatment of K. pneumoniae 486 biofilm. Genome-driven examination combined with the surface polysaccharide structural analysis of resistant mutant showed the point mutation and frameshift in the wbaP gene located within the cps gene cluster, resulting in the loss of the capsule. The sharp decline in the yield of CPS was accompanied by the production of a larger amount of smooth LPS. The modification of the surface polysaccharide layers did not affect bacterial fitness nor the insensitivity to serum complement; however, it made bacteria more prone to phagocytosis combined with the higher adherence and internalization to human lung epithelial cells. In that context, it was showed that the emerging resistance to the antivirulence agent (phage-borne capsule depolymerase) results in beneficial consequences, i.e., the sensitization to the innate immune response.

The low level of O antigen in Salmonella enterica Paratyphi A is due to inefficiency of the glycosyltransferase WbaV

The group A O antigen is the major surface polysaccharide of Salmonella enterica serovar Paratyphi A (SPA), and the focal point for most current vaccine development efforts. The SPA O-antigen repeat (O unit) is structurally similar to the group D1 O unit of S. enterica serovar Typhi, differing only in the presence of a terminal side-branch paratose (Par) in place of tyvelose (Tyv), both of which are attached by the glycosyltransferase WbaV. The two O-antigen gene clusters are also highly similar, but with a loss-of-function mutation in the group A tyv gene and the tandem amplification of wbaV in most SPA strains. In this study, we show that SPA strains consistently produce less O antigen than their group D1 counterparts and use an artificial group A strain (D1 Δtyv) to show this is due to inefficient Par attachment by WbaV. We also demonstrate that group A O-antigen production can be increased by overexpression of the wbaV gene in both the D1 Δtyv strain and two multi-wbaV SPA strains. These findings should be broadly applicable in ongoing vaccine development pipelines, where efficient isolation and purification of large quantities of O antigen is of critical importance.

Undecaprenol kinase: Function, mechanism and substrate specificity of a potential antibiotic target

The bifunctional undecaprenol kinase/phosphatase (UdpK) is a small, prokaryotic, integral membrane kinase, homologous with Escherichia coli diacylglycerol kinase and expressed by the dgkA gene. In Gram-positive bacteria, UdpK is involved in the homeostasis of the bacterial undecaprenoid pool, where it converts undecaprenol to undecaprenyl phosphate (C₅₅P) and also catalyses the reverse process. C₅₅P is the universal lipid carrier and critical to numerous glycopolymer and glycoprotein biosynthetic pathways in bacteria. DgkA gene expression has been linked to facilitating bacterial growth and survival in response to environmental stressors, as well being implicated as a resistance mechanism to the topical antibiotic bacitracin, by providing an additional route to C₅₅P. Therefore, identification of UdpK inhibitors could lead to novel antibiotic treatments. A combination of homology modelling and mutagenesis experiments on UdpK have been used to identify residues that may be involved in kinase/phosphatase activity. In this review, we will summarise recent work on the mechanism and substrate specificity of UdpK.

Structure and genetics of Escherichia coli O antigens

Escherichia coli includes clonal groups of both commensal and pathogenic strains, with some of the latter causing serious infectious diseases. O antigen variation is current standard in defining strains for taxonomy and epidemiology, providing the basis for many serotyping schemes for Gram-negative bacteria. This review covers the diversity in E. coli O antigen structures and gene clusters, and the genetic basis for the structural diversity. Of the 187 formally defined O antigens, six (O31, O47, O67, O72, O94 and O122) have since been removed and three (O34, O89 and O144) strains do not produce any O antigen. Therefore, structures are presented for 176 of the 181 E. coli O antigens, some of which include subgroups. Most (93%) of these O antigens are synthesized via the Wzx/Wzy pathway, 11 via the ABC transporter pathway, with O20, O57 and O60 still uncharacterized due to failure to find their O antigen gene clusters. Biosynthetic pathways are given for 38 of the 49 sugars found in E. coli O antigens, and several pairs or groups of the E. coli antigens that have related structures show close relationships of the O antigen gene clusters within clades, thereby highlighting the genetic basis of the evolution of diversity.

Function and Biogenesis of Lipopolysaccharides

The cell envelope is the first line of defense between a bacterium and the world-at-large. Often, the initial steps that determine the outcome of chemical warfare, bacteriophage infections, and battles with other bacteria or the immune system greatly depend on the structure and composition of the bacterial cell surface. One of the most studied bacterial surface molecules is the glycolipid known as lipopolysaccharide (LPS), which is produced by most Gram-negative bacteria. Much of the initial attention LPS received in the early 1900s was owed to its ability to stimulate the immune system, for which the glycolipid was commonly known as endotoxin. It was later discovered that LPS also creates a permeability barrier at the cell surface and is a main contributor to the innate resistance that Gram-negative bacteria display against many antimicrobials. Not surprisingly, these important properties of LPS have driven a vast and still prolific body of literature for more than a hundred years. LPS research has also led to pioneering studies in bacterial envelope biogenesis and physiology, mostly using Escherichia coli and Salmonella as model systems. In this review, we will focus on the fundamental knowledge we have gained from studies of the complex structure of the LPS molecule and the biochemical pathways for its synthesis, as well as the transport of LPS across the bacterial envelope and its assembly at the cell surface.

Wzx flippases exhibiting complex O-unit preferences require a new model for Wzx-substrate interactions

The Wzx flippase is a critical component of the O‐antigen biosynthesis pathway, being responsible for the translocation of oligosaccharide O units across the inner membrane in Gram‐negative bacteria. Recent studies have shown that Wzx has a strong preference for its cognate O unit, but the types of O‐unit structural variance that a given Wzx can accommodate are poorly understood. In this study, we identified two Yersinia pseudotuberculosis Wzx that can distinguish between different terminal dideoxyhexose sugars on a common O‐unit main‐chain, despite both being able to translocate several other structurally‐divergent O units. We also identified other Y. pseudotuberculosis Wzx that can translocate a structurally divergent foreign O unit with high efficiency, and thus exhibit an apparently relaxed substrate preference. It now appears that Wzx substrate preference is more complex than previously suggested, and that not all O‐unit residues are equally important determinants of translocation efficiency. We propose a new “Structure‐Specific Triggering” model in which Wzx translocation proceeds at a low level for a wide variety of substrates, with high‐frequency translocation only being triggered by Wzx interacting with one or more preferred O‐unit structural elements found on its cognate O unit(s).

Molecular serogrouping ofEscherichia coli

O-antigens present on the surface ofEscherichia coliprovide antigenic specificity for the strain and are the main components for O-serogroup designation. Serotyping using O-group-specific antisera for the identification ofE. coliO-serogroups has been traditionally the gold-standard for distinguishingE. colistrains. Knowledge of the O-group is important for determining pathogenic lineage, classifyingE. colifor epidemiological studies, for determining virulence, and for tracing outbreaks of diseases and sources of infection. However, serotyping has limitations, as the antisera generated against each specific O-group may cross-react, many strains are non-typeable, and others can autoagglutinate or be rough (lacking an O-antigen). Currently, the nucleotide sequences are available for most of the 187 designatedE. coliO-groups. Public health and other laboratories are considering whole genome sequencing to develop genotypic methods to determine O-groups. These procedures require instrumentation and analysis that may not be accessible and may be cost-prohibitive at this time. In this review, we have identified unique gene sequences within the O-antigen gene clusters and have targeted these genes for identification of O-groups using the polymerase chain reaction. This information can be used to distinguish O-groups by developing other platforms forE. colidiagnostics in the future.

Rapid customised operon assembly by yeast recombinational cloning

We have developed a system called the Operon Assembly Protocol (OAP), which takes advantage of the homologous recombination DNA repair pathway in Saccharomyces cerevisiae to assemble full-length operons from a series of overlapping PCR products into a specially engineered yeast-Escherichia coli shuttle vector. This flexible, streamlined system can be used to assemble several operon clones simultaneously, and each clone can be expressed in the same E. coli tester strain to facilitate direct functional comparisons. We demonstrated the utility of the OAP by assembling and expressing a series of E. coli O1A O-antigen gene cluster clones containing various gene deletions or replacements. We then used these constructs to assess the substrate preferences of several Wzx flippases, which are responsible for translocation of oligosaccharide repeat units (O units) across the inner membrane during O-antigen biosynthesis. We were able to identify several O unit structural features that appear to be important determinants of Wzx substrate preference. The OAP system should be broadly applicable for the genetic manipulation of any bacterial operon and can be modified for use in other host species. It could also have potential uses in fields such as glycoengineering.

Genome-Wide Survey of Genes Under Positive Selection in Avian Pathogenic Escherichia coli Strains

The ability to obtain bacterial genomes from the same host has allowed for comparative studies that help in the understanding of the molecular evolution of specific pathotypes. Avian pathogenic Escherichia coli (APEC) is a group of extraintestinal strains responsible for causing colibacillosis in birds. APEC is also suggested to possess a role as a zoonotic agent. Despite its importance, APEC pathogenesis still has several cryptic pathogenic processes that need to be better understood. In this work, a genome-wide survey of eight APEC strains for genes with evidence of recombination revealed that ∼14% of the homologous groups evaluated present signs of recombination. Enrichment analyses revealed that nine Gene Ontology (GO) terms were significantly more represented in recombinant genes. Among these GO terms, several were noted to be ATP-related categories. The search for positive selection in these APEC genomes revealed 32 groups of homologous genes with evidence of positive selection. Among these groups, we found several related to cell metabolism, as well as several uncharacterized genes, beyond the well-known virulence factors ompC, lamB, waaW, waaL, and fliC. A GO term enrichment test showed a prevalence of terms related to bacterial cell contact with the external environment (e.g., viral entry into host cell, detection of virus, pore complex, bacterial-type flagellum filament C, and porin activity). Finally, the genes with evidence of positive selection were retrieved from genomes of non-APEC strains and tested as were done for APEC strains. The result revealed that none of the groups of genes presented evidence of positive selection, confirming that the analysis was effective in inferring positive selection for APEC and not for E. coli in general, which means that the study of the genes with evidence of positive selection identified in this study can contribute for the better understanding of APEC pathogenesis processes.

Lipopolysaccharide transport and assembly at the outer membrane: the PEZ model

Gram-negative bacteria have a double-membrane cellular envelope that enables them to colonize harsh environments and prevents the entry of many clinically available antibiotics. A main component of most outer membranes is lipopolysaccharide (LPS), a glycolipid containing several fatty acyl chains and up to hundreds of sugars that is synthesized in the cytoplasm. In the past two decades, the proteins that are responsible for transporting LPS across the cellular envelope and assembling it at the cell surface in Escherichia coli have been identified, but it remains unclear how they function. In this Review, we discuss recent advances in this area and present a model that explains how energy from the cytoplasm is used to power LPS transport across the cellular envelope to the cell surface.

Model for the Controlled Synthesis of O-Antigen Repeat Units Involving the WaaL Ligase

The Wzx/Wzy O-antigen pathway involves synthesis of a repeat unit (O unit) consisting of 3 to 8 sugars on an inner-membrane-embedded lipid carrier. These O units are translocated across the membrane to its periplasmic face by Wzx, while retaining linkage to the carrier, and then polymerized by Wzy to O-antigen polymer, which WaaL ligase transfers to a lipopolysaccharide precursor to complete lipopolysaccharide synthesis, concomitantly releasing the lipid carrier. This lipid carrier is also used for peptidoglycan assembly, and sequestration is known to be toxic. Thus, O-unit synthesis must involve precise regulation to meet demand but avoid overproduction. Here we show that loss of WaaL reverses a known growth defect in a Salmonella mutant that otherwise accumulates O-unit intermediates and propose that WaaL is also involved in a novel feedback mechanism to regulate O-unit synthesis, based on the availability of O units on the periplasmic face of the membrane.

Inefficient translocation of a truncated O unit by a Salmonella Wzx affects both O-antigen production and cell growth

Bacterial Wzx flippases translocate (flip) short oligosaccharide repeat units (O units) across the inner membrane into the periplasm, which is a critical step in the assembly of many O antigens, capsules and other surface polysaccharides. There is enormous diversity in O antigens and capsules in particular, even within species. Wzx proteins are similarly diverse, but it has been widely accepted that they have significant specificity only for the first sugar of an O unit. In this study, we analysed the Wzx from the Salmonella enterica group C2 O antigen gene cluster, which is a unique and divergent member of a set of gene clusters that produce galactose-initiated O antigens. We demonstrate that this Wzx has a strong preference for the presence of an abequose side-branch, which manifests in a reduction of long-chain O antigen and a major growth defect. This contributes to a growing body of evidence that, contrary to earlier proposals, Wzx flippases commonly exhibit a strong preference for the structure of their native O unit.

Synthesis of bacterial polysaccharides via the Wzx/Wzy-dependent pathway

The surfaces of bacteria mediate a multitude of functions in the environment and in an infected host, including adhesion to both biotic and abiotic substrata, motility, immune system interaction and (or) activation, biofilm formation, and cell–cell communication, with many of these features directly influenced by cell-surface glycans. In both Gram-negative and Gram-positive bacteria, the majority of cell-surface polysaccharides are produced via the Wzx/Wzy-dependent assembly pathway; these glycans include heteropolymeric O-antigen, enterobacterial common antigen, exopolysaccharide, spore coat, and capsule in diverse bacteria. The key components of this assembly pathway are the integral inner membrane Wzx flippase, Wzy polymerase, and Wzz chain-length regulator proteins, which until recently have resisted detailed structural and functional characterization. In this review, we have provided a comprehensive synthesis of the latest structural and mechanistic data for each protein, as well as an examination of substrate specificity for each assembly step and complex formation between the constituent proteins. To complement the unprecedented explosion of genomic-sequencing data for bacteria, we have also highlighted both classical and state-of-the-art methods by which encoded Wzx, Wzy, and Wzz proteins can be reliably identified and annotated, using the model Gram-negative bacterium Pseudomonas aeruginosa as an example data set. Lastly, we outline future avenues of research, with the aim of stimulating researchers to take the next steps in investigating the function of, and interplay between, the constituents of this widespread assembly scheme.

The capsular polysaccharide of Staphylococcus aureus is attached to peptidoglycan by the LytR-CpsA-Psr (LCP) family of enzymes

Envelope biogenesis in bacteria involves synthesis of intermediates that are tethered to the lipid carrier undecaprenol-phosphate. LytR-CpsA-Psr (LCP) enzymes have been proposed to catalyze the transfer of undecaprenol-linked intermediates onto the C6-hydroxyl of MurNAc in peptidoglycan, thereby promoting attachment of wall teichoic acid (WTA) in bacilli and staphylococci and capsular polysaccharides (CPS) in streptococci. S. aureus encodes three lcp enzymes, and a variant lacking all three genes (Δlcp) releases WTA from the bacterial envelope and displays a growth defect. Here, we report that the type 5 capsular polysaccharide (CP5) of Staphylococcus aureus Newman is covalently attached to the glycan strands of peptidoglycan. Cell wall attachment of CP5 is abrogated in the Δlcp variant, a defect that is best complemented via expression of lcpC in trans. CP5 synthesis and peptidoglycan attachment are not impaired in the tagO mutant, suggesting that CP5 synthesis does not involve the GlcNAc-ManNAc linkage unit of WTA and may instead utilize another Wzy-type ligase to assemble undecaprenyl-phosphate intermediates. Thus, LCP enzymes of S. aureus are promiscuous enzymes that attach secondary cell wall polymers with discrete linkage units to peptidoglycan.

Biosynthesis and export of bacterial lipopolysaccharides

Lipopolysaccharide molecules represent a unique family of glycolipids based on a highly conserved lipid moiety known as lipid A. These molecules are produced by most gram-negative bacteria, in which they play important roles in the integrity of the outer-membrane permeability barrier and participate extensively in host-pathogen interplay. Few bacteria contain lipopolysaccharide molecules composed only of lipid A. In most forms, lipid A is glycosylated by addition of the core oligosaccharide that, in some bacteria, provides an attachment site for a long-chain O-antigenic polysaccharide. The complexity of lipopolysaccharide structures is reflected in the processes used for their biosynthesis and export. Rapid growth and cell division depend on the bacterial cell's capacity to synthesize and export lipopolysaccharide efficiently and in large amounts. We review recent advances in those processes, emphasizing the reactions that are essential for viability.

Diversity of o-antigen repeat unit structures can account for the substantial sequence variation of wzx translocases

The most common system for synthesis of cell surface polysaccharides is the Wzx/Wzy-dependent pathway, which involves synthesis, on the cytoplasmic face of the cell membrane, of repeat units, which are then translocated to the periplasmic face by a Wzx translocase and then polymerized by Wzy to generate the polysaccharide. One such polysaccharide is O antigen, which is incorporated into lipopolysaccharide (LPS). The O antigen is extremely variable, with over 186 forms in Escherichia coli. Wzx proteins are also very diverse, but they have been thought to be specific only for the first sugar of the repeat units. However, recent studies demonstrated examples in which Wzx translocases have considerable preference for their native repeat unit, showing that specificity can extend well beyond the first sugar. These results appear to be in conflict with the early conclusions, but they involved specificity for side branch residues and could be a special case. Here we take six Wzx translocases that were critical in the earlier studies on the importance of the first sugar and assess their ability to translocate the Escherichia coli O16 and O111 repeat units. We use gene replacements to optimize maintenance of expression level and show that under these conditions the native translocases are the most effective for their native repeat unit, being, respectively, 64-fold and 4-fold more effective than the next best. We conclude that Wzx translocases are commonly adapted to their native repeat unit, which provides an explanation for the great diversity of wzx genes.

The WbaK acetyltransferase of Salmonella enterica group E gives insights into O antigen evolution

O antigens are polysaccharides consisting of repeat units of three to eight sugars, generally assembled by genes in a discrete O antigen gene cluster. Salmonella enterica produces 46 forms of O antigen, and most of the variation is determined by genes in the gene cluster. However in some cases the structures are modified by enzymes encoded outside of the gene cluster, and several such modifications have been reported for Salmonella enterica group E, some with the genes on bacteriophages and one gene at a distant chromosomal site. We identified the enzyme, WbaK, that is responsible for O-acetylating the subgroup E1 O antigen, and found that the gene is located just downstream of the gene cluster as currently known. The wbaK gene appears to have been imported by a recombination event that also replaced the last 37 bp of the wbaP gene, indicating that homologous recombination was involved. Some of the group E strains we studied must have the original gene cluster, as they lack wbaK and the sequence downstream of wbaP is very similar to that in several other S. enterica O antigen gene clusters. In effect the gene cluster was extended by one gene in subgroup E1. It appears that a function that is usually encoded by a gene outside of the gene cluster has been added to the gene cluster, in this case giving an example of how such gene clusters can evolve.

Structural diversity in Salmonella O antigens and its genetic basis

This review covers the structures and genetics of the 46 O antigens of Salmonella, a major pathogen of humans and domestic animals. The variation in structures underpins the serological specificity of the 46 recognized serogroups. The O antigen is important for the full function and virulence of many bacteria, and the considerable diversity of O antigens can confer selective advantage. Salmonella O antigens can be divided into two major groups: those which have N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) and those which have galactose (Gal) as the first sugar in the O unit. In recent years, we have determined 21 chemical structures and sequenced 28 gene clusters for GlcNAc-/GalNAc-initiated O antigens, thus completing the structure and DNA sequence data for the 46 Salmonella O antigens. The structures and gene clusters of the GlcNAc-/GalNAc-initiated O antigens were found to be highly diverse, and 24 of them were found to be identical or closely related to Escherichia coli O antigens. Sequence comparisons indicate that all or most of the shared gene clusters were probably present in the common ancestor, although alternative explanations are also possible. In contrast, the better-known eight Gal-initiated O antigens are closely related both in structures and gene cluster sequences.

Dam methylation participates in the regulation of PmrA/PmrB and RcsC/RcsD/RcsB two component regulatory systems in Salmonella enterica serovar Enteritidis

The absence of Dam in Salmonella enterica serovar Enteritidis causes a defect in lipopolysaccharide (LPS) pattern associated to a reduced expression of wzz gene. Wzz is the chain length regulator of the LPS O-antigen. Here we investigated whether Dam regulates wzz gene expression through its two known regulators, PmrA and RcsB. Thus, the expression of rcsB and pmrA was monitored by quantitative real-time RT-PCR and Western blotting using fusions with 3×FLAG tag in wild type (wt) and dam strains of S. Enteritidis. Dam regulated the expression of both rcsB and pmrA genes; nevertheless, the defect in LPS pattern was only related to a diminished expression of RcsB. Interestingly, regulation of wzz in serovar Enteritidis differed from that reported earlier for serovar Typhimurium; RcsB induces wzz expression in both serovars, whereas PmrA induces wzz in S. Typhimurium but represses it in serovar Enteritidis. Moreover, we found that in S. Enteritidis there is an interaction between both wzz regulators: RcsB stimulates the expression of pmrA and PmrA represses the expression of rcsB. Our results would be an example of differential regulation of orthologous genes expression, providing differences in phenotypic traits between closely related bacterial serovars.

The structural elucidation of the Salmonella enterica subsp. enterica, reveals that it contains both O-factors 4 and 5 on the LPS antigen

Spectroscopic investigation of the O-antigen from Salmonella enterica subsp. enterica revealed fine details on the acetylation pattern, the biological repeating unit and the polymerization degree. Acetylation at O-2 of the abequose residue, defined both O-factors 4 and 5 in the O-antigen chain of the lipopolysaccharide. NMR observation of the terminal non-reducing end of the polymer confirmed previous data regarding the biological repeating unit and showed an average polymerization degree of 5. The information about these structural elements might contribute to the understanding of key features of the biology of this pathogen, as phase variation and/or adaptation to the external environment.

In vitro O-antigen ligase assay

WaaL is a membrane enzyme that catalyzes the glycosidic bonding of a sugar at the proximal end of the undecaprenyl-diphosphate (Und-PP)-O-antigen with a terminal sugar of the lipid A-core oligosaccharide (OS). This is a critical step in lipopolysaccharide synthesis. We describe here an assay to perform the ligation reaction in vitro utilizing native substrates.

Synthase-dependent exopolysaccharide secretion in Gram-negative bacteria

The biosynthesis and export of bacterial cell-surface polysaccharides is known to occur through several distinct mechanisms. Recent advances in the biochemistry and structural biology of several proteins in synthase-dependent polysaccharide secretion systems have identified key conserved components of this pathway in Gram-negative bacteria. These components include an inner-membrane-embedded polysaccharide synthase, a periplasmic tetratricopeptide repeat (TPR)-containing scaffold protein, and an outer-membrane β-barrel porin. There is also increasing evidence that many synthase-dependent systems are post-translationally regulated by the bacterial second messenger bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we compare these core proteins in the context of the alginate, cellulose, and poly-β-D-N-acetylglucosamine (PNAG) secretion systems.

Roles of the structural symbiosis polysaccharide (syp) genes in host colonization, biofilm formation, and polysaccharide biosynthesis in Vibrio fischeri

The symbiosis polysaccharide locus, syp, is required for Vibrio fischeri to form a symbiotic association with the squid Euprymna scolopes. It is also required for biofilm formation induced by the unlinked regulator RscS. The syp locus includes 18 genes that can be classified into four groups based on putative function: 4 genes encode putative regulators, 6 encode glycosyltransferases, 2 encode export proteins, and the remaining 6 encode proteins with other functions, including polysaccharide modification. To understand the roles of each of the 14 structural syp genes in colonization and biofilm formation, we generated nonpolar in-frame deletions of each gene. All of the deletion mutants exhibited defects in their ability to colonize juvenile squid, although the impact of the loss of SypB or SypI was modest. Consistent with their requirement for colonization, most of the structural genes were also required for RscS-induced biofilm formation. In particular, the production of wrinkled colonies, pellicles, and the matrix on the colony surface was eliminated or severely decreased in all mutants except for the sypB and sypI mutants; in contrast, only a subset of genes appeared to play a role in attachment to glass. Finally, immunoblotting data suggested that the structural Syp proteins are involved in polysaccharide production and/or export. These results provide important insights into the requirements for the syp genes under different environmental conditions and thus lay the groundwork for a more complete understanding of the matrix produced by V. fischeri to enhance cell-cell interactions and promote symbiotic colonization.

Wzx flippase-mediated membrane translocation of sugar polymer precursors in bacteria

Bacterial cell surface polysaccharides confer resistance to external stress and promote survival in biotic and abiotic environments. Glycan assembly often occurs at the periplasmic leaflet of the inner membrane (IM) from undecaprenyl pyrophosphate (UndPP)-linked polysaccharide units via the Wzx/Wzy-dependent pathway. Wzx is an integral IM protein found in Gram-negative and Gram-positive bacteria that mediates IM translocation of UndPP-linked sugar repeats from the cytoplasmic to the periplasmic leaflet; interaction of Wzx with other assembly proteins is indirectly supported by genetic evidence. Topological mapping has indicated 12 α-helical transmembrane segments (TMS), with the number of charged TMS residues fluctuating based on the mapping method used. A novel Wzx tertiary structure model has been built, allowing for substrate-binding or energy-coupling roles to be proposed for functionally important charged and aromatic TMS residues. It has also led to a proposed antiport-like mechanism of Wzx function. Exquisite substrate specificity of Wzx proteins was recently revealed in distinguishing between UndPP-linked substrates with identical main-chain sugar repeats, but differing in the chemical composition of a terminal sugar side-branch cap. The objective of this review is to synthesize the most up-to-date knowledge concerning Wzx flippases and to provide perspective for future investigations in this burgeoning field.

The Wzx translocases for Salmonella enterica O-antigen processing have unexpected serotype specificity

Most Gram-negative bacteria have an O antigen, a polysaccharide with many repeats of a short oligosaccharide that is a part of the lipopolysaccharide, the major lipid in the outer leaflet of the outer membrane. Lipopolysaccharide is variable with 46 forms in Salmonella enterica that underpin the serotyping scheme. Repeat units are assembled on a lipid carrier that is embedded in the cell membrane, and are then translocated by the Wzx translocase from the cytoplasmic face to the outer face of the cell membrane, followed by polymerization. The O antigen is then incorporated into lipopolysaccharide and exported to the outer membrane. The Wzx translocase is widely thought to be specific only for the first sugar of the repeat unit, despite extensive variation in both O antigens and Wzx translocases. However, we found for S. enterica groups B, D2 and E that Wzx translocation exhibits significant specificity for the repeat-unit structure, as variants with single sugar differences are translocated with lower efficiency and little long-chain O antigen is produced. It appears that Wzx translocases are specific for their O antigen for normal levels of translocation.

Detection of O antigens in Escherichia coli

Lipopolysaccharide on the surface of Escherichia coli constitutes the O antigens which are important virulence factors that are targets of both the innate and adaptive immune systems and play a major role in host-pathogen interactions. O antigens are responsible for antigenic specificity of the strain and determine the O serogroup. The designation of O serogroups is important for classifying E. coli strains, for epidemiological studies, in tracing the source of outbreaks of gastrointestinal or other illness, and for linking the source to the infection. For conventional serogroup identification, serotyping by agglutination reactions against antisera developed for each of the O serogroups has been used. In the last decade, many O-antigen gene clusters that encode for the enzymes responsible for the synthesis of the variable oligosaccharide region on the surface of the bacteria have been sequenced and characterized. Unique gene sequences within the O-antigen gene clusters have been targeted for identification and detection of many O groups using the polymerase chain reaction and microarrays. This review summarizes current knowledge on the DNA sequences of the O-antigen gene clusters, genetic-based methods for O-group determination and detection of pathogenic E. coli based on O-antigen and virulence gene detection, and provides perspectives on future developments in the field.

Synthesis of lipopolysaccharide O-antigens by ABC transporter-dependent pathways

The O-polysaccharide (O-PS; O-antigen) of bacterial lipopolysaccharides is made up of repeating units of one or more sugar residues and displays remarkable structural diversity. Despite the structural variations, there are only three strategies for O-PS assembly. The ATP-binding cassette (ABC)-transporter-dependent mechanism of O-PS biosynthesis is widespread. The Escherichia coli O9a and Klebsiella pneumoniae O2a antigens provide prototypes, which are distinguished by the fine details that link glycan polymerization and chain termination at the cytoplasmic face of the inner membrane to its export via the ABC transporter. Here, we describe the current understanding of these processes. Since glycoconjugate assembly complexes that utilize an ABC transporter-dependent pathway are widespread among the bacterial kingdom, the models described here are expected to extend beyond O-PS biosynthesis systems.

The WaaL O-antigen lipopolysaccharide ligase has features in common with metal ion-independent inverting glycosyltransferases

WaaL is a membrane enzyme that catalyzes a key step in lipopolysaccharide (LPS) synthesis: the glycosidic bonding of a sugar at the proximal end of the undecaprenyl-diphosphate (Und-PP) O-antigen with a terminal sugar of the lipid A-core oligosaccharide (OS). Utilizing an in vitro assay, we demonstrate here that ligation with purified Escherichia coli WaaL occurs without adenosine-5'-triphosphate (ATP) and magnesium ions. Furthermore, E. coli and Pseudomonas aeruginosa WaaL proteins cannot catalyze ATP hydrolysis in vitro. We also show that a lysine substitution of the arginine (Arg)-215 residue renders an active protein, whereas WaaL mutants with alanine replacements in the periplasmic-exposed residues Arg-215, Arg-288 and histidine (His)-338 and also the membrane-embedded aspartic acid-389 are nonfunctional. An in silico approach, combining predicted topological information with the analysis of sequence conservation, confirms the importance of a positive charge at the small periplasmic loop of WaaL, since an Arg corresponding to Arg-215 was found at a similar position in all the WaaL homologs. Also, a universally conserved H[NSQ]X(9)GXX[GTY] motif spanning the C-terminal end of the predicted large periplasmic loop and the membrane boundary of the transmembrane helix was identified. The His residue in this motif corresponds to His-338. A survey of LPS structures in which the linkage between O-antigen and lipid A-core OS was elucidated reveals that it is always in the β-configuration, whereas the sugars bound to Und-PP are in the α-configuration. Together, our biochemical and in silico data argue that WaaL proteins use a common reaction mechanism and share features of metal ion-independent inverting glycosyltransferases.

Characterization of the O-antigen polymerase (Wzy) of Francisella tularensis

The O-antigen polymerase of gram-negative bacteria has been difficult to characterize. Herein we report the biochemical and functional characterization of the protein product (Wzy) of the gene annotated as the putative O-antigen polymerase, which is located in the O-antigen biosynthetic locus of Francisella tularensis. In silico analysis (homology searching, hydropathy plotting, and codon usage assessment) strongly suggested that Wzy is an O-antigen polymerase whose function is to catalyze the addition of newly synthesized O-antigen repeating units to a glycolipid consisting of lipid A, inner core polysaccharide, and one repeating unit of the O-polysaccharide (O-PS). To characterize the function of the Wzy protein, a non-polar deletion mutant of wzy was generated by allelic replacement, and the banding pattern of O-PS was observed by immunoblot analysis of whole-cell lysates obtained by SDS-PAGE and stained with an O-PS-specific monoclonal antibody. These immunoblot analyses showed that O-PS of the wzy mutant expresses only one repeating unit of O-antigen. Further biochemical characterization of the subcellular fractions of the wzy mutant demonstrated that (as is characteristic of O-antigen polymerase mutants) the low molecular weight O-antigen accumulates in the periplasm of the mutant. Site-directed mutagenesis based on protein homology and topology, which was carried out to locate a catalytic residue of the protein, showed that modification of specific residues (Gly(176), Asp(177), Gly(323), and Tyr(324)) leads to a loss of O-PS polymerization. Topology models indicate that these amino acids most likely lie in close proximity on the bacterial surface.

Loss of the waaL O-antigen ligase prevents surface activation of the flagellar gene cascade in Proteus mirabilis

Proteus mirabilis is a Gram-negative bacterium that undergoes a physical and biochemical change from a vegetative swimmer cell (a typical Gram-negative rod) to an elongated swarmer cell when grown on a solid surface. In this study, we report that a transposon insertion in the waaL gene, encoding O-antigen ligase, blocked swarming motility on solid surfaces but had little effect on swimming motility in soft agar. The waaL mutant was unable to differentiate into a swarmer cell. Differentiation was also prevented by a mutation in wzz, encoding a chain length determinant for O antigen, but not by a mutation in wzyE, encoding an enzyme that polymerizes enterobacterial common antigen, a surface polysaccharide different from the lipid A::core. In wild-type P. mirabilis, increased expression of the flhDC operon occurs after growth on solid surfaces and is required for the high-level expression of flagellin that is characteristic of swarmer cells. However, in both the waaL and the wzz mutants, the flhDC operon was not activated during growth on agar. A loss-of-function mutation in the rcsB response regulator or overexpression of flhDC restored swarming to the waaL mutant, despite the absence of O antigen. Therefore, although O antigen may serve a role in swarming by promoting wettability, the loss of O antigen blocks a regulatory pathway that links surface contact with the upregulation of flhDC expression.

Dam methylation controls O-antigen chain length in Salmonella enterica serovar enteritidis by regulating the expression of Wzz protein

We reported previously that a Salmonella enterica serovar Enteritidis dam mutant expressing a truncated Dam protein does not agglutinate in the presence of specific antibodies against O9 polysaccharide. Here we investigate the participation of Dam in lipopolysaccharide (LPS) synthesis in Salmonella. The LPS O-antigen profiles of a dam null mutant (SEDeltadam) and the Salmonella serovar Enteritidis parental strain were examined by using electrophoresis and silver staining. Compared to the parental strain, SEDeltadam produced LPS with shorter O-antigen polysaccharide chains. Since Wzz is responsible for the chain length distribution of the O antigen, we investigated whether Dam methylation is involved in regulating wzz expression. Densitometry analysis showed that the amount of Wzz produced by SEDeltadam is threefold lower than the amount of Wzz produced by the parental strain. Concomitantly, the activity of the wzz promoter in SEDeltadam was reduced nearly 50% in logarithmic phase and 25% in stationary phase. These results were further confirmed by reverse transcription-PCR showing that wzz gene expression was threefold lower in the dam mutant than in the parental strain. Our results demonstrate that wzz gene expression is downregulated in a dam mutant, indicating that Dam methylation activates expression of this gene. This work indicates that wzz is a new target regulated by Dam methylation and demonstrates that DNA methylation not only affects the production of bacterial surface proteins but also the production of surface polysaccharides.

Structure and Assembly of Escherichia coli Capsules

The capsule is a cell surface structure composed of long-chain polysaccharides that envelops many isolates of Escherichia coli. It protects the cell against host defenses or physical environmental stresses, such as desiccation. The component capsular polysaccharides (CPSs) are major surface antigens in E. coli. They are named K antigens (after the German word Kapsel). Due to variations in CPS structures, more than 80 serologically unique K antigens exist in E. coli. Despite the hypervariability in CPS structures, only two capsule-assembly strategies exist in E. coli. These have led to the assignment of group 1 and group 2 capsules, and many of the key elements of the corresponding assembly pathways have been resolved. Structural features, as well as genetic and regulatory variations, give rise to additional groups 3 and 4. These employ the same biosynthesis processes described in groups 2 and 1, respectively. Each isolate possesses a distinctive set of cytosolic and inner-membrane enzymes, which generate a precise CPS structure, defining a given K serotype. Once synthesized, a multiprotein complex is needed to translocate the nascent CPS across the Gram-negative cell envelope to the outer surface of the outer membrane, where the capsule structure is assembled. While the translocation machineries for group 1 and group 2 CPSs are fundamentally different from one another, they possess no specificity for a given CPS structure. Each is conserved in all isolates producing capsules belonging to a particular group.

Post-assembly modification of Bordetella bronchiseptica O polysaccharide by a novel periplasmic enzyme encoded by wbmE

Bordetella bronchiseptica is a pathogen of humans and animals that colonizes the respiratory tract. It produces a lipopolysaccharide O antigen that contains a homopolymer of 2,3-dideoxy-2,3-diacetamido-l-galacturonic acid (l-GalNAc3NAcA). Some of these sugars are found in the uronamide form (l-GalNAc3NAcAN), and there is no discernible pattern in the distribution of amides along the chain. A B. bronchiseptica wbmE mutant expresses an O polysaccharide unusually rich in uronamides. The WbmE protein localizes to the periplasm and catalyzes the deamidation of uronamide-rich O chains in lipopolysaccharide purified from the mutant, to attain a wild-type uronamide/uronic acid ratio. WbmE is a member of the papain-like transglutaminase superfamily, and this categorization is consistent with a deamidase role. The periplasmic location of WbmE and its acceptance of complete lipopolysaccharide as substrate indicate that it operates at a late stage in lipopolysaccharide biosynthesis, after polymerization and export of the O chain from the cytoplasm. This is the first report of such a modification of O antigen after assembly. The expression of wbmE is controlled by the Bordetella virulence gene two-component regulatory system, BvgAS, suggesting that this deamidation is a novel mechanism by which these bacteria modify their cell surface charge in response to environmental stimuli.

Functional analysis of predicted coiled-coil regions in the Escherichia coli K-12 O-antigen polysaccharide chain length determinant Wzz

Wzz is a membrane protein that determines the chain length distribution of the O-antigen lipopolysaccharide by an unknown mechanism. Wzz proteins consist of two transmembrane helices separated by a large periplasmic loop. The periplasmic loop of Escherichia coli K-12 Wzz (244 amino acids from K65 to A308) was purified and found to be a monomer with an extended conformation, as determined by gel filtration chromatography and analytical ultracentrifugation. Circular dichroism showed that the loop has a 60% helical content. The Wzz periplasmic loop also contains three regions with predicted coiled coils. To probe the function of the predicted coiled coils, we constructed amino acid replacement mutants of the E. coli K-12 Wzz protein, which were designed so that the coiled coils could be separate without compromising the helicity of the individual molecules. Mutations in one of the regions, spanning amino acids 108 to 130 (region I), were associated with a partial defect in O-antigen chain length distribution, while mutants with mutations in the region spanning amino acids 209 to 223 (region III) did not have an apparent functional defect. In contrast, mutations in the region spanning amino acids 153 to 173 (region II) eliminated the Wzz function. This phenotype was associated with protein instability, most likely due to conformational changes caused by the amino acid replacements, which was confirmed by limited trypsin proteolysis. Additional mutagenesis based on a three-dimensional model of region I demonstrated that the amino acids implicated in function are all located at the same face of a predicted alpha-helix, suggesting that a coiled coil actually does not exist in this region. Together, our results suggest that the regions predicted to be coiled coils are important for Wzz function because they maintain the native conformation of the protein, although the existence of coiled coils could not be demonstrated experimentally.

PelC is a Pseudomonas aeruginosa outer membrane lipoprotein of the OMA family of proteins involved in exopolysaccharide transport

Pseudomonas aeruginosa is a gram-negative bacterium, opportunistic pathogen, which causes severe acute or chronic infections, as is the case with cystic fibrosis patients. Chronic infections are frequently accompanied by the development of the bacterial population into a specialized community called biofilm. The pelA-G gene cluster of P. aeruginosa has been shown to be involved in pellicle production and biofilm formation. The pel genes have been proposed to contribute to the formation of the exopolysaccharide-containing pellicle. However, the function and the subcellular localization of the seven different Pel proteins are poorly understood. Based on bioinformatics analysis, we have previously considered that PelF is a putative glycosyltransferase (GT4 family), whereas PelG is a Wzx-like polysaccharide transporter from the PST family. In this study we have further characterized the PelC protein. We have shown that PelC is an outer membrane lipoprotein. The N-terminal signal peptide of the PelC lipoprotein is sufficient to target the protein into the membranes. However, by constructing various PelC hybrid proteins we also proposed that efficient and functional outer membrane insertion of PelC requires not only the signal peptide and the lipid modification, but also requires the C-terminal domain of PelC. Because the gene encoding the outer membrane lipoprotein PelC is part of a putative gene cluster involved in exopolysaccharide biogenesis, we suggest that PelC is a new member of the outer membrane auxiliary (OMA) family of lipoprotein whose Wza, involved in Escherichia coli capsular polysaccharide transport, is an archetype.

Functional characterization and membrane topology of Escherichia coli WecA, a sugar-phosphate transferase initiating the biosynthesis of enterobacterial common antigen and O-antigen lipopolysaccharide

WecA is an integral membrane protein that initiates the biosynthesis of enterobacterial common antigen and O-antigen lipopolysaccharide (LPS) by catalyzing the transfer of N-acetylglucosamine (GlcNAc)-1-phosphate onto undecaprenyl phosphate (Und-P) to form Und-P-P-GlcNAc. WecA belongs to a large family of eukaryotic and prokaryotic prenyl sugar transferases. Conserved aspartic acids in putative cytoplasmic loops 2 (Asp90 and Asp91) and 3 (Asp156 and Asp159) were targeted for replacement mutagenesis with either glutamic acid or asparagine. We examined the ability of each mutant protein to complement O-antigen LPS synthesis in a wecA-deficient strain and also determined the steady-state kinetic parameters of the mutant proteins in an in vitro transfer assay. Apparent K(m) and V(max) values for UDP-GlcNAc, Mg(2+), and Mn(2+) suggest that Asp156 is required for catalysis, while Asp91 appears to interact preferentially with Mg(2+), possibly playing a role in orienting the substrates. Topological analysis using the substituted cysteine accessibility method demonstrated the cytosolic location of Asp90, Asp91, and Asp156 and provided a more refined overall topological map of WecA. Also, we show that cells expressing a WecA derivative C terminally fused with the green fluorescent protein exhibited a punctate distribution of fluorescence on the bacterial surface, suggesting that WecA localizes to discrete regions in the bacterial plasma membrane.

Pseudomonas aeruginosa 1244 pilin glycosylation: glycan substrate recognition

The pilin of Pseudomonas aeruginosa 1244 is glycosylated with an oligosaccharide that is structurally identical to the O-antigen repeating unit of this organism. Concordantly, the metabolic source of the pilin glycan is the O-antigen biosynthetic pathway. The present study was conducted to investigate glycan substrate recognition in the 1244 pilin glycosylation reaction. Comparative structural analysis of O subunits that had been previously shown to be compatible with the 1244 glycosylation machinery revealed similarities among sugars at the presumed reducing termini of these oligosaccharides. We therefore hypothesized that the glycosylation substrate was within the sugar at the reducing end of the glycan precursor. Since much is known of PA103 O-antigen genetics and because the sugars at the reducing termini of the O7 (strain 1244) and O11 (strain PA103) are identical (beta-N-acetyl fucosamine), we utilized PA103 and strains that express lipopolysaccharide (LPS) with a truncated O-antigen subunit to test our hypothesis. LPS from a strain mutated in the wbjE gene produced an incomplete O subunit, consisting only of the monosaccharide at the reducing end (beta-d-N-acetyl fucosamine), indicating that this moiety contained substrate recognition elements for WaaL. Expression of pilAO(1244) in PA103 wbjE::aacC1, followed by Western blotting of extracts of these cells, indicated that pilin produced has been modified by the addition of material consistent with a single N-acetyl fucosamine. This was confirmed by analyzing endopeptidase-treated pilin by mass spectrometry. These data suggest that the pilin glycosylation substrate recognition features lie within the reducing-end moiety of the O repeat and that structures of the remaining sugars are irrelevant.

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.

Biosynthesis and Assembly of Capsular Polysaccharides inEscherichia coli

Capsules are protective structures on the surfaces of many bacteria. The remarkable structural diversity in capsular polysaccharides is illustrated by almost 80 capsular serotypes in Escherichia coli. Despite this variation, the range of strategies used for capsule biosynthesis and assembly is limited, and E. coli isolates provide critical prototypes for other bacterial species. Related pathways are also used for synthesis and export of other bacterial glycoconjugates and some enzymes/processes have counterparts in eukaryotes. In gram-negative bacteria, it is proposed that biosynthesis and translocation of capsular polysaccharides to the cell surface are temporally and spatially coupled by multiprotein complexes that span the cell envelope. These systems have an impact on both a general understanding of membrane trafficking in bacteria and on bacterial pathogenesis.