Synthesis of the A-band polysaccharide sugar D-rhamnose requires Rmd and WbpW: identification of multiple AlgA homologues, WbpW and ORF488, in Pseudomonas aeruginosa

Identification and impact on Pseudomonas aeruginosa virulence of mutations conferring resistance to a phage cocktail for phage therapy

IMPORTANCE

In this work, we identified the putative receptors of 16 Pseudomonas phages and evaluated how resistance to phages recognizing different bacterial receptors may affect the virulence. Our findings are relevant for the implementation of phage therapy of Pseudomonas aeruginosa infections, which are difficult to treat with antibiotics. Overall, our results highlight the need to modify natural phages to enlarge the repertoire of receptors exploited by therapeutic phages and suggest that phages using the PAO1-type T4P as receptor may have limited value for the therapy of the cystic fibrosis infection.

Pyocin S5 Import into Pseudomonas aeruginosa Reveals a Generic Mode of Bacteriocin Transport

Pyocin S5 (PyoS5) is a potent protein bacteriocin that eradicates the human pathogen Pseudomonas aeruginosa in animal infection models, but its import mechanism is poorly understood. Here, using crystallography, biophysical and biochemical analyses, and live-cell imaging, we define the entry process of PyoS5 and reveal links to the transport mechanisms of other bacteriocins. In addition to its C-terminal pore-forming domain, elongated PyoS5 comprises two novel tandemly repeated kinked 3-helix bundle domains that structure-based alignments identify as key import domains in other pyocins. The central domain binds the lipid-bound common polysaccharide antigen, allowing the pyocin to accumulate on the cell surface. The N-terminal domain binds the ferric pyochelin transporter FptA while its associated disordered region binds the inner membrane protein TonB1, which together drive import of the bacteriocin across the outer membrane. Finally, we identify the minimal requirements for sensitizing Escherichia coli toward PyoS5, as well as other pyocins, and suggest that a generic pathway likely underpins the import of all TonB-dependent bacteriocins across the outer membrane of Gram-negative bacteria.IMPORTANCE Bacteriocins are toxic polypeptides made by bacteria to kill their competitors, making them interesting as potential antibiotics. Here, we reveal unsuspected commonalities in bacteriocin uptake pathways, through molecular and cellular dissection of the import pathway for the pore-forming bacteriocin pyocin S5 (PyoS5), which targets Pseudomonas aeruginosa In addition to its C-terminal pore-forming domain, PyoS5 is composed of two tandemly repeated helical domains that we also identify in other pyocins. Functional analyses demonstrate that they have distinct roles in the import process. One recognizes conserved sugars projected from the surface, while the other recognizes a specific outer membrane siderophore transporter, FptA, in the case of PyoS5. Through engineering of Escherichia coli cells, we show that pyocins can be readily repurposed to kill other species. This suggests basic ground rules for the outer membrane translocation step that likely apply to many bacteriocins targeting Gram-negative bacteria.

Pseudomonas putida Responds to the Toxin GraT by Inducing Ribosome Biogenesis Factors and Repressing TCA Cycle Enzymes

The potentially self-poisonous toxin-antitoxin modules are widespread in bacterial chromosomes, but despite extensive studies, their biological importance remains poorly understood. Here, we used whole-cell proteomics to study the cellular effects of the Pseudomonas putida toxin GraT that is known to inhibit growth and ribosome maturation in a cold-dependent manner when the graA antitoxin gene is deleted from the genome. Proteomic analysis of P. putida wild-type and ΔgraA strains at 30 °C and 25 °C, where the growth is differently affected by GraT, revealed two major responses to GraT at both temperatures. First, ribosome biogenesis factors, including the RNA helicase DeaD and RNase III, are upregulated in ΔgraA. This likely serves to alleviate the ribosome biogenesis defect of the ΔgraA strain. Secondly, proteome data indicated that GraT induces downregulation of central carbon metabolism, as suggested by the decreased levels of TCA cycle enzymes isocitrate dehydrogenase Idh, α-ketoglutarate dehydrogenase subunit SucA, and succinate-CoA ligase subunit SucD. Metabolomic analysis revealed remarkable GraT-dependent accumulation of oxaloacetate at 25 °C and a reduced amount of malate, another TCA intermediate. The accumulation of oxaloacetate is likely due to decreased flux through the TCA cycle but also indicates inhibition of anabolic pathways in GraT-affected bacteria. Thus, proteomic and metabolomic analysis of the ΔgraA strain revealed that GraT-mediated stress triggers several responses that reprogram the cell physiology to alleviate the GraT-caused damage.

Conjugative type IVb pilus recognizes lipopolysaccharide of recipient cells to initiate PAPI-1 pathogenicity island transfer in Pseudomonas aeruginosa

Background

Pseudomonas aeruginosa pathogenicity island 1 (PAPI-1) is one of the largest genomic islands of this important opportunistic human pathogen. Previous studies have shown that PAPI-1 encodes several putative virulence factors, including a major regulator of biofilm formation and antibiotic-resistance traits. PAPI-1 is horizontally transferable into recipient strains lacking this island via conjugation mediated by the specialized type IV pilus. The PAPI-1 encodes a cluster of ten genes associated with the synthesis and assembly of the type IV pilus. The PAPI-1 acquisition mechanism is currently not well understood.

Results

In this study, we performed a series of conjugation experiments and identified determinants of PAPI-1 acquisition by analyzing transfer efficiency between the donor and a series of mutant recipient strains. Our data show that common polysaccharide antigen (CPA) lipopolysaccharide (LPS), a homopolymer of D-rhamnose, is required for initiating PAPI-1 transfer, suggesting that this structure acts as a receptor for conjugative type IV pilus in recipient strains. These results were substantiated by experimental evidence from PAPI-1 transfer assay experiments, in which outer membrane or LPS preparations from well-defined LPS mutants were added to the transfer mix to assess the role of P. aeruginosa LPS in PAPI-1 transfer and in vitro binding experiments between pilin fusion protein GST-pilV2’ and immobilized LPS molecules were performed. Our data also showed that P. aeruginosa strains that had already acquired a copy of PAPI-1 were unable to import additional copies of the island, and that such strains produced proportionally lower amounts of CPA LPS compared to the strains lacking PAPI-1.

Conclusions

These results suggest that a PAPI-1 exclusion mechanism exists in P. aeruginosa that might serve to regulate the avoidance of uncontrolled expansions of the bacterial genome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0943-4) contains supplementary material, which is available to authorized users.

The Transmembrane Domain of a Bicomponent ABC Transporter Exhibits Channel-Forming Activity

Pseudomonas aeruginosa is an opportunistic pathogen that expresses two unique forms of lipopolysaccharides (LPSs) on its bacterial surface, the A- and B-bands. The A-band polysaccharides (A-band PSs) are thought to be exported into the periplasm via a bicomponent ATP-binding cassette (ABC) transporter located within the inner membrane. This ABC protein complex consists of the transmembrane (TMD) Wzm and nucleotide-binding (NBD) Wzt domain proteins. Here, we were able to probe ∼1.36 nS-average conductance openings of the Wzm-based protein complex when reconstituted into a lipid membrane buffered by a 200 mM KCl solution, demonstrating the large-conductance, channel-forming ability of the TMDs. In agreement with this finding, transmission electron microscopy (TEM) imaging revealed the ring-shaped structure of the transmembrane Wzm protein complex. As hypothesized, using liposomes, we demonstrated that Wzm interacts with Wzt. Further, the Wzt polypeptide indeed hydrolyzed ATP but exhibited a ∼75% reduction in the ATPase activity when its Walker A domain was deleted. The distribution and average unitary conductance of the TMD Wzm protein complex were altered by the presence of the NBD Wzt protein, confirming the regulatory role of the latter polypeptide. To our knowledge, the large-conductance, channel-like activity of the Wzm protein complex, although often hypothesized, has not previously been demonstrated. These results constitute a platform for future structural, biophysical, and functional explorations of this bicomponent ABC transporter.

A Putative ABC Transporter Permease Is Necessary for Resistance to Acidified Nitrite and EDTA in Pseudomonas aeruginosa under Aerobic and Anaerobic Planktonic and Biofilm Conditions

Pseudomonas aeruginosa (PA) is an important airway pathogen of cystic fibrosis and chronic obstructive disease patients. Multiply drug resistant PA is becoming increasing prevalent and new strategies are needed to combat such insidious organisms. We have previously shown that a mucoid, mucA22 mutant PA is exquisitely sensitive to acidified nitrite (A-NO2−, pH 6.5) at concentrations that are well tolerated in humans. Here, we used a transposon mutagenesis approach to identify PA mutants that are hypersensitive to A-NO2−. Among greater than 10,000 mutants screened, we focused on PA4455, in which the transposon was found to disrupt the production of a putative cytoplasmic membrane-spanning ABC transporter permease. The PA4455 mutant was not only highly sensitive to A-NO2−, but also the membrane perturbing agent, EDTA and the antibiotics doxycycline, tigecycline, colistin, and chloramphenicol, respectively. Treatment of bacteria with A-NO2− plus EDTA, however, had the most dramatic and synergistic effect, with virtually all bacteria killed by 10 mM A-NO2−, and EDTA (1 mM, aerobic, anaerobic). Most importantly, the PA4455 mutant was also sensitive to A-NO2− in biofilms. A-NO2− sensitivity and an anaerobic growth defect was also noted in two mutants (rmlC and wbpM) that are defective in B-band LPS synthesis, potentially indicating a membrane defect in the PA4455 mutant. Finally, this study describes a gene, PA4455, that when mutated, allows for dramatic sensitivity to the potential therapeutic agent, A-NO2− as well as EDTA. Furthermore, the synergy between the two compounds could offer future benefits against antibiotic resistant PA strains.

The Widespread Multidrug-Resistant Serotype O12 Pseudomonas aeruginosa Clone Emerged through Concomitant Horizontal Transfer of Serotype Antigen and Antibiotic Resistance Gene Clusters

The O-specific antigen (OSA) in Pseudomonas aeruginosa lipopolysaccharide is highly varied by sugar identity, side chains, and bond between O-repeats. These differences classified P. aeruginosa into 20 distinct serotypes. In the past few decades, O12 has emerged as the predominant serotype in clinical settings and outbreaks. These serotype O12 isolates exhibit high levels of resistance to various classes of antibiotics. Here, we explore how the P. aeruginosa OSA biosynthesis gene clusters evolve in the population by investigating the association between the phylogenetic relationships among 83 P. aeruginosa strains and their serotypes. While most serotypes were closely linked to the core genome phylogeny, we observed horizontal exchange of OSA biosynthesis genes among phylogenetically distinct P. aeruginosa strains. Specifically, we identified a “serotype island” ranging from 62 kb to 185 kb containing the P. aeruginosa O12 OSA gene cluster, an antibiotic resistance determinant (gyrA^C248T), and other genes that have been transferred between P. aeruginosa strains with distinct core genome architectures. We showed that these genes were likely acquired from an O12 serotype strain that is closely related to P. aeruginosa PA7. Acquisition and recombination of the “serotype island” resulted in displacement of the native OSA gene cluster and expression of the O12 serotype in the recipients. Serotype switching by recombination has apparently occurred multiple times involving bacteria of various genomic backgrounds. In conclusion, serotype switching in combination with acquisition of an antibiotic resistance determinant most likely contributed to the dissemination of the O12 serotype in clinical settings.

Infection rates in hospital settings by multidrug-resistant (MDR) Pseudomonas aeruginosa clones have increased during the past decades, and serotype O12 is predominant among these epidemic strains. It is not known why the MDR phenotype is associated with serotype O12 and how this clone type has emerged. This study shows that evolution of MDR O12 strains involved a switch from an ancestral O4 serotype to O12. Serotype switching was the result of horizontal transfer and genetic recombination of lipopolysaccharide (LPS) biosynthesis genes originating from an MDR taxonomic outlier P. aeruginosa strain. Moreover, the recombination event also resulted in acquisition of antibiotic resistance genes. These results impact on our understanding of MDR outbreak strain and serotype evolution and can potentially assist in better monitoring and prevention.

Single-Nucleotide Polymorphisms Found in the migA and wbpX Glycosyltransferase Genes Account for the Intrinsic Lipopolysaccharide Defects Exhibited by Pseudomonas aeruginosa PA14

Pseudomonas aeruginosa PA14 is widely used by researchers in many laboratories because of its enhanced virulence over strain PAO1 in a wide range of hosts. Although lipopolysaccharide (LPS) is an important virulence factor of all P. aeruginosa strains, the LPS of PA14 has not been characterized fully. A recent study showed that the structure of its O-specific antigen (OSA) belongs to serotype O19. We found that the OSA gene cluster of PA14 shares ∼99% identity with those of the O10/O19 group. These two serotypes share the same O-unit structure, except for an O-acetyl substitution in one of the sugars in O10. Here we showed that both PA14 and O19 LPS cross-reacted with the O10-specific monoclonal antibody MF76-2 in Western blots. Analysis by SDS-PAGE and silver staining showed that PA14 LPS exhibited modal chain lengths that were different from those of O19 LPS, in that only "very long" and "short" chain lengths were observed, while "medium" and "long" chain lengths were not detected. Two other novel observations included the lack of the uncapped core oligosaccharide epitope and of common polysaccharide antigen (CPA) LPS. The lack of the uncapped core oligosaccharide was caused by point mutations in the glycosyltransferase gene migA, while the CPA-negative phenotype was correlated with a single amino acid substitution, G20R, in the glycosyltransferase WbpX. Additionally, we showed that restoring CPA biosynthesis in PA14 significantly stimulated mature biofilm formation after 72 h, while outer membrane vesicle production was not affected.

IMPORTANCE

P. aeruginosa PA14 is a clinical isolate that has become an important reference strain used by many researchers worldwide. LPS of PA14 has not been characterized fully, and hence, confusion about its phenotype exists in the literature. In the present study, we set out to characterize the O-specific antigen (OSA), the common polysaccharide antigen (CPA), and the core oligosaccharide produced by PA14. We present evidence that PA14 produces an LPS consisting of "very-long-chain" and some "short-chain" OSA belonging to the O19 serotype but is devoid of CPA and the uncapped core oligosaccharide epitope. These intrinsic defects in PA14 LPS were due to single-nucleotide polymorphisms (SNPs) in the genes that encode glycosyltransferases in the corresponding biosynthesis pathways. Since sugars in CPA and the uncapped core are receptors for different bacteriocins and pyocins, the lack of CPA and an intact core may contribute to the increased virulence of PA14. Restoring CPA production in PA14 was found to stimulate mature biofilm formation.

Influence of O polysaccharides on biofilm development and outer membrane vesicle biogenesis in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is a common opportunistic human pathogen known for its ability to adapt to changes in its environment during the course of infection. These adaptations include changes in the expression of cell surface lipopolysaccharide (LPS), biofilm development, and the production of a protective extracellular exopolysaccharide matrix. Outer membrane vesicles (OMVs) have been identified as an important component of the extracellular matrix of P. aeruginosa biofilms and are thought to contribute to the development and fitness of these bacterial communities. The goal of this study was to examine the relationships between changes in the cell surface expression of LPS O polysaccharides, biofilm development, and OMV biogenesis in P. aeruginosa. We compared wild-type P. aeruginosa PAO1 with three chromosomal knockouts. These knockouts have deletions in the rmd, wbpM, and wbpL genes that produce changes in the expression of common polysaccharide antigen (CPA), O-specific antigen (OSA), or both. Our results demonstrate that changes in O polysaccharide expression do not significantly influence OMV production but do affect the size and protein content of OMVs derived from both CPA(-) and OSA(-) cells; these mutant cells also exhibited different physical properties from wild-type cells. We further examined biofilm growth of the mutants and determined that CPA(-) cells could not develop into robust biofilms and exhibit changes in cell morphology and biofilm matrix production. Together these results demonstrate the importance of O polysaccharide expression on P. aeruginosa OMV composition and highlight the significance of CPA expression in biofilm development.

A novel baculovirus vector for the production of nonfucosylated recombinant glycoproteins in insect cells

Glycosylation is an important attribute of baculovirus-insect cell expression systems, but some insect cell lines produce core α1,3-fucosylated N-glycans, which are highly immunogenic and render recombinant glycoproteins unsuitable for human use. To address this problem, we exploited a bacterial enzyme, guanosine-5'-diphospho (GDP)-4-dehydro-6-deoxy-d-mannose reductase (Rmd), which consumes the GDP-l-fucose precursor. We expected this enzyme to block glycoprotein fucosylation by blocking the production of GDP-l-fucose, the donor substrate required for this process. Initially, we engineered two different insect cell lines to constitutively express Rmd and isolated subclones with fucosylation-negative phenotypes. However, we found the fucosylation-negative phenotypes induced by Rmd expression were unstable, indicating that this host cell engineering approach is ineffective in insect systems. Thus, we constructed a baculovirus vector designed to express Rmd immediately after infection and facilitate the insertion of genes encoding any glycoprotein of interest for expression later after infection. We used this vector to produce a daughter encoding rituximab and found, in contrast to an Rmd-negative control, that insect cells infected with this virus produced a nonfucosylated form of this therapeutic antibody. These results indicate that our Rmd(+) baculoviral vector can be used to solve the immunogenic core α1,3-fucosylation problem associated with the baculovirus-insect cell system. In conjunction with existing glycoengineered insect cell lines, this vector extends the utility of the baculovirus-insect cell system to include therapeutic glycoprotein production. This new vector also extends the utility of the baculovirus-insect cell system to include the production of recombinant antibodies with enhanced effector functions, due to its ability to block core α1,6-fucosylation.

Evaluation of the ability of C. albicans to form biofilm in the presence of phage-resistant phenotypes of P. aeruginosa

Pseudomonas aeruginosa and Candida albicans are disparate microbial species, but both are known to be opportunistic pathogens frequently associated with nosocomial infections. The aim of this study was to provide a better understanding of the interactions between these microorganisms in dual-species biofilms. Several bacteriophage-resistant P. aeruginosa phenotypes have been isolated and were used in dual-species mixed-biofilm studies. Twenty-four and 48 h mixed-biofilms were formed using the isolated phenotypes of phage-resistant P. aeruginosa and these were compared with similar experiments using other P. aeruginosa strains with a defined lipopolysaccharide (LPS) deficiency based on chromosomal knockout of specific LPS biosynthetic genes. Overall, the results showed that the variants of phage-resistant P. aeruginosa and LPS mutants were both less effective in inhibiting the growth of C. albicans in mixed-biofilms compared to the wild-type strains of P. aeruginosa. Conversely, the proliferation of P. aeruginosa was not influenced by the presence of C. albicans. In conclusion, the ability of strains of P. aeruginosa to inhibit the formation of a biofilm of C. albicans appears to be correlated with the LPS chain lengths of phenotypes of P. aeruginosa, suggesting that LPS has a suppressive effect on the growth of C. albicans.

Rhamnose biosynthesis in mycoplasmas requires precursor glycans larger than monosaccharide

Despite the apparent absence of genes coding for the known pathways for biosynthesis, the monosaccharide rhamnose was detected in the d configuration in Mycoplasma pneumoniae and Mycoplasma pulmonis, and in both the d and l configurations in Mycoplasma arthritidis. Surprisingly, the monosaccharide glucose was not a precursor for rhamnose biosynthesis and was not incorporated at detectable levels in glucose-containing polysaccharides or glycoconjugates. In contrast, carbon atoms from starch, a polymer of glucose, were incorporated into rhamnose in each of the three species examined. When grown in a serum-free medium supplemented with starch, M. arthritidis synthesized higher levels of rhamnose, with a shift in the relative amounts of the d and l configurations. Our findings suggest the presence of a novel pathway for rhamnose synthesis that is widespread in the genus Mycoplasma.

Five new genes are important for common polysaccharide antigen biosynthesis in Pseudomonas aeruginosa

Common polysaccharide antigen (CPA) is a conserved cell surface polysaccharide produced by Pseudomonas aeruginosa. It contains a rhamnan homopolymer and is one of the two forms of O polysaccharide attached to P. aeruginosa lipopolysaccharide (LPS). Our laboratory has previously characterized an eight-gene cluster (pa5447-pa5454 in P. aeruginosa PAO1) required for biosynthesis of CPA. Here we demonstrate that an adjacent five-gene cluster pa5455-pa5459 is also involved. Using reverse transcriptase PCR (RT-PCR), we showed that the original eight-gene cluster and the new five-gene cluster are both organized as operons. We have analyzed the LPS phenotypes of in-frame deletion mutants made in each of the five genes, and the results verified that these five genes are indeed required for CPA biosynthesis, extending the CPA biosynthesis locus to contain 13 contiguous genes. By performing overexpression experiments of different sets of these biosynthesis genes, we were able to obtain information about their possible functions in CPA biosynthesis.

Lipopolysaccharide (LPS) is an important cell surface structure of Gram-negative bacteria. The human opportunistic pathogen Pseudomonas aeruginosa simultaneously produces an O-antigen-specific (OSA) form and a common polysaccharide antigen (CPA) form of LPS. CPA, the focus of this study, is composed of α-1-2, α1-3-linked d-rhamnose sugars and has been shown to be important for attachment of the bacteria to human airway epithelial cells. Genome sequencing of this species revealed a new five-gene cluster that we predicted to be involved in CPA biosynthesis and modification. In this study, we have generated chromosomal knockouts by performing in-frame deletions and allelic replacements. Characterizing the function of each of the five genes is important for us to better understand CPA biosynthesis and the mechanisms of chain length termination and regulation of this unique D-rhamnan polysaccharide.

Pseudomonas aeruginosa exopolysaccharide Psl promotes resistance to the biofilm inhibitor polysorbate 80

Polysorbate 80 (PS80) is a nonionic surfactant and detergent that inhibits biofilm formation by Pseudomonas aeruginosa at concentrations as low as 0.001% and is well tolerated in human tissues. However, certain clinical and laboratory strains (PAO1) of P. aeruginosa are able to form biofilms in the presence of PS80. To better understand this resistance, we performed transposon mutagenesis with a PS80-resistant clinical isolate, PA738. This revealed that mutation of algC rendered PA738 sensitive to PS80 biofilm inhibition. AlgC contributes to the biosynthesis of the exopolysaccharides Psl and alginate, as well as lipopolysaccharide and rhamnolipid. Analysis of mutations downstream of AlgC in these biosynthetic pathways established that disruption of the psl operon was sufficient to render the PA738 and PAO1 strains sensitive to PS80-mediated biofilm inhibition. Increased levels of Psl production in the presence of arabinose in a strain with an arabinose-inducible psl promoter were correlated with increased biofilm formation in PS80. In P. aeruginosa strains MJK8 and ZK2870, known to produce both Pel and Psl, disruption of genes in the psl but not the pel operon conferred susceptibility to PS80-mediated biofilm inhibition. The laboratory strain PA14 does not produce Psl and does not form biofilms in PS80. However, when PA14 was transformed with a cosmid containing the psl operon, it formed biofilms in the presence of PS80. Taken together, these data suggest that production of the exopolysaccharide Psl by P. aeruginosa promotes resistance to the biofilm inhibitor PS80.

Synthesis of multiple Pseudomonas aeruginosa biofilm matrix exopolysaccharides is post-transcriptionally regulated

Exopolysaccharide is a critical biofilm matrix component, yet little is known about how the synthesis of multiple exopolysaccharides is regulated. Pseudomonas aeruginosa can produce several biofilm matrix exopolysaccharides that include alginate, Psl and Pel. Here we demonstrated that AlgC, a key enzyme that provides sugar precursors for the synthesis of alginate and lipopolysaccharides (LPS) is also required for both Psl and Pel production. We showed that forced-synthesis of Psl in alginate-producing mucoid bacteria reduced alginate production but this was not due to transcription of the alginate biosynthesis-operon. Likewise, when either alginate or Psl were overproduced, levels of B-band LPS decreased. Induction of Pel resulted in a reduction of Psl levels. Because the effects of reduced exopolysaccharide synthesis when another is overproduced didn't appear to be regulated at the transcriptional level, this suggests that the biosynthesis pathways of Psl, Pel, alginate, and LPS compete for common sugar precursors. As AlgC is the only enzyme that provides precursors for each of these exopolysaccharides, we propose that AlgC is a key checkpoint enzyme that coordinates the total amount of exopolysaccharide biosynthesis by controlling sugar precursor pool. Our data also provide a plausible strategy that P.aeruginosa utilizes to modulate its biofilm matrix exopolysaccharides.

Solution NMR of a 463-residue phosphohexomutase: domain 4 mobility, substates, and phosphoryl transfer defect

Phosphomannomutase/phosphoglucomutase contributes to the infectivity of Pseudomonas aeruginosa, retains and reorients its intermediate by 180°, and rotates domain 4 to close the deep catalytic cleft. Nuclear magnetic resonance (NMR) spectra of the backbone of wild-type and S108C-inactivated enzymes were assigned to at least 90%. (13)C secondary chemical shifts report excellent agreement of solution and crystallographic structure over the 14 α-helices, C-capping motifs, and 20 of the 22 β-strands. Major and minor NMR peaks implicate substates affecting 28% of assigned residues. These can be attributed to the phosphorylation state and possibly to conformational interconversions. The S108C substitution of the phosphoryl donor and acceptor slowed transformation of the glucose 1-phosphate substrate by impairing k(cat). Addition of the glucose 1,6-bisphosphate intermediate accelerated this reaction by 2-3 orders of magnitude, somewhat bypassing the defect and apparently relieving substrate inhibition. The S108C mutation perturbs the NMR spectra and electron density map around the catalytic cleft while preserving the secondary structure in solution. Diminished peak heights and faster (15)N relaxation suggest line broadening and millisecond fluctuations within four loops that can contact phosphosugars. (15)N NMR relaxation and peak heights suggest that domain 4 reorients slightly faster in solution than domains 1-3, and with a different principal axis of diffusion. This adds to the crystallographic evidence of domain 4 rotations in the enzyme, which were previously suggested to couple to reorientation of the intermediate, substrate binding, and product release.

Genetic and Functional Diversity of Pseudomonas aeruginosa Lipopolysaccharide

Lipopolysccharide (LPS) is an integral component of the Pseudomonas aeruginosa cell envelope, occupying the outer leaflet of the outer membrane in this Gram-negative opportunistic pathogen. It is important for bacterium-host interactions and has been shown to be a major virulence factor for this organism. Structurally, P. aeruginosa LPS is composed of three domains, namely, lipid A, core oligosaccharide, and the distal O antigen (O-Ag). Most P. aeruginosa strains produce two distinct forms of O-Ag, one a homopolymer of D-rhamnose that is a common polysaccharide antigen (CPA, formerly termed A band), and the other a heteropolymer of three to five distinct (and often unique dideoxy) sugars in its repeat units, known as O-specific antigen (OSA, formerly termed B band). Compositional differences in the O units among the OSA from different strains form the basis of the International Antigenic Typing Scheme for classification via serotyping of different strains of P. aeruginosa. The focus of this review is to provide state-of-the-art knowledge on the genetic and resultant functional diversity of LPS produced by P. aeruginosa. The underlying factors contributing to this diversity will be thoroughly discussed and presented in the context of its contributions to host-pathogen interactions and the control/prevention of infection.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Genetic and biochemical analyses of the Pseudomonas aeruginosa Psl exopolysaccharide reveal overlapping roles for polysaccharide synthesis enzymes in Psl and LPS production

Exopolysaccharides contribute significantly to attachment and biofilm formation in the opportunisitc pathogen Pseudomonas aeruginosa. The Psl polysaccharide, which is synthesized by the polysaccharide synthesis locus (psl), is required for biofilm formation in non-mucoid strains that do not rely on alginate as the principal biofilm polysaccharide. In-frame deletion and complementation studies of individual psl genes revealed that 11 psl genes, pslACDEFGHIJKL, are required for Psl production and surface attachment. We also present the first structural analysis of the psl-dependent polysaccharide, which consists of a repeating pentasaccharide containing d-mannose, d-glucose and l-rhamnose: [See text]. In addition, we identified the sugar nucleotide precursors involved in Psl generation and demonstrated the requirement for GDP-d-mannose, UDP-d-glucose and dTDP-l-rhamnose in Psl production and surface attachment. Finally, genetic analyses revealed that wbpW restored Psl production in a pslB mutant and pslB promoted A-band LPS synthesis in a wbpW mutant, indicating functional redundancy and overlapping roles for these two enzymes. The structural and genetic data presented here provide a basis for further investigation of the Psl proteins and potential roles for Psl in the biology and pathogenesis of P. aeruginosa.

Truncation in the core oligosaccharide of lipopolysaccharide affects flagella-mediated motility in Pseudomonas aeruginosa PAO1 via modulation of cell surface attachment

In many Gram-negative bacterial species, rough strains producing truncated lipopolysaccharide (LPS) generally exhibit defects in motility compared with smooth strains. However, the role that LPS plays in bacterial motility is not well understood. The goal of this study was to examine the relationship between LPS defects and motility of Pseudomonas aeruginosa. P. aeruginosa wild-type strain PAO1 and three isogenic mutants with defects in the rmlC, migA and wapR genes and producing truncated core oligosaccharide were investigated in terms of motility, attachment to glass and flagella expression. Compared with the wild-type, the three mutants showed significant retardation in both swarming motility on 0.5 % soft-agar plates and swimming motility on 0.3 % soft-agar plates. Moreover, attachment to abiotic surfaces was observed to be stronger in these mutants. The assembly of flagella appeared to be intact in these strains and the ability of individual cells to swim was unaffected. Flagellin proteins prepared from mutants rmlC and rmd, defective in the production of TDP-l-rhamnose and GDP-d-rhamnose, respectively, were compared and a change in molecular mass was observed only in the rmlC mutant. These data indicated that l-rhamnose, and not its enantiomer, d-rhamnose, is incorporated into the flagellin glycan of P. aeruginosa PAO1. The nucleotide-activated sugar precursor TDP-l-rhamnose is therefore shared between LPS biosynthesis and flagellin glycosylation in P. aeruginosa PAO1. Our results suggest that although biochemical precursors are shared by LPS and flagellin glycan biosynthesis, LPS truncations probably alter flagella-mediated motility in P. aeruginosa by modulating cell-surface attachment but not flagella synthesis.

The structural basis for catalytic function of GMD and RMD, two closely related enzymes from the GDP-D-rhamnose biosynthesis pathway

The rare 6-deoxysugar D-rhamnose is a component of bacterial cell surface glycans, including the D-rhamnose homopolymer produced by Pseudomonas aeruginosa, called A-band O polysaccharide. GDP-D-rhamnose synthesis from GDP-D-mannose is catalyzed by two enzymes. The first is a GDP-D-mannose-4,6-dehydratase (GMD). The second enzyme, RMD, reduces the GMD product (GDP-6-deoxy-D-lyxo-hexos-4-ulose) to GDP-d-rhamnose. Genes encoding GMD and RMD are present in P. aeruginosa, and genetic evidence indicates they act in A-band O-polysaccharide biosynthesis. Details of their enzyme functions have not, however, been previously elucidated. We aimed to characterize these enzymes biochemically, and to determine the structure of RMD to better understand what determines substrate specificity and catalytic activity in these enzymes. We used capillary electrophoresis and NMR analysis of reaction products to precisely define P. aeruginosa GMD and RMD functions. P. aeruginosa GMD is bifunctional, and can catalyze both GDP-d-mannose 4,6-dehydration and the subsequent reduction reaction to produce GDP-D-rhamnose. RMD catalyzes the stereospecific reduction of GDP-6-deoxy-D-lyxo-hexos-4-ulose, as predicted. Reconstitution of GDP-D-rhamnose biosynthesis in vitro revealed that the P. aeruginosa pathway may be regulated by feedback inhibition in the cell. We determined the structure of RMD from Aneurinibacillus thermoaerophilus at 1.8 A resolution. The structure of A. thermoaerophilus RMD is remarkably similar to that of P. aeruginosa GMD, which explains why P. aeruginosa GMD is also able to catalyze the RMD reaction. Comparison of the active sites and amino acid sequences suggests that a conserved amino acid side chain (Arg185 in P. aeruginosa GMD) may be crucial for orienting substrate and cofactor in GMD enzymes.

Newly introduced genomic prophage islands are critical determinants of in vivo competitiveness in the Liverpool Epidemic Strain of Pseudomonas aeruginosa

Pseudomonas aeruginosa isolates have a highly conserved core genome representing up to 90% of the total genomic sequence with additional variable accessory genes, many of which are found in genomic islands or islets. The identification of the Liverpool Epidemic Strain (LES) in a children's cystic fibrosis (CF) unit in 1996 and its subsequent observation in several centers in the United Kingdom challenged the previous widespread assumption that CF patients acquire only unique strains of P. aeruginosa from the environment. To learn about the forces that shaped the development of this important epidemic strain, the genome of the earliest archived LES isolate, LESB58, was sequenced. The sequence revealed the presence of many large genomic islands, including five prophage clusters, one defective (pyocin) prophage cluster, and five non-phage islands. To determine the role of these clusters, an unbiased signature tagged mutagenesis study was performed, followed by selection in the chronic rat lung infection model. Forty-seven mutants were identified by sequencing, including mutants in several genes known to be involved in Pseudomonas infection. Furthermore, genes from four prophage clusters and one genomic island were identified and in direct competition studies with the parent isolate; four were demonstrated to strongly impact on competitiveness in the chronic rat lung infection model. This strongly indicates that enhanced in vivo competitiveness is a major driver for maintenance and diversifying selection of these genomic prophage genes.

Identification of amino acid residues important for the phosphomannose isomerase activity of PslB in Pseudomonas aeruginosa PAO1

Phosphomannose isomerase (PMI) plays a pivotal role in biosynthesis of GDP-mannose, an important precursor of many polysaccharides. We demonstrate in this study that Pseudomonas aeruginosa pslB encodes a protein with GDP-mannose pyrophosphorylase/PMI dual activities. The PMI activity is Co2+-dependent and could be inhibited by GDP-mannose in a competitive manner. Furthermore, the activity could be inactivated by 2,3-butanedione suggesting the presence of a catalytic Arg residue. Site-specific mutations at R373, R472, R479, E410, H411, N433 and E458 increase the KM approximately 8-20-fold. The PMI activity of PslB was completely diminished with a R408K or R408A, reflecting the importance of this residue in catalysis. Overall, these results provide a basis for understanding the catalytic mechanism of PMI.

Functional characterization of MigA and WapR: putative rhamnosyltransferases involved in outer core oligosaccharide biosynthesis of Pseudomonas aeruginosa

Pseudomonas aeruginosa lipopolysaccharide (LPS) contains two glycoforms of core oligosaccharide (OS); one form is capped with O antigen through an alpha-1,3-linked L-rhamnose (L-Rha), while the other is uncapped and contains an alpha-1,6-linked L-Rha. Two genes in strain PAO1, wapR (PA5000) and migA (PA0705), encode putative glycosyltransferases associated with core biosynthesis. We propose that WapR and MigA are the rhamnosyltransferases responsible for the two linkages of L-Rha to the core. Knockout mutants with mutations in both genes were generated. The wapR mutant produced LPS lacking O antigen, and addition of wapR in trans complemented this defect. The migA mutant produced LPS with a truncated outer core and showed no reactivity to outer core-specific monoclonal antibody (MAb) 5C101. Complementation of this mutant with migA restored reactivity of the LPS to MAb 5C101. Interestingly, LPS from the complemented migA strain was not reactive to MAb 18-19 (specific for the core-plus-one O repeat). This was due to overexpression of MigA in the complemented strain that caused an increase in the proportion of the uncapped core OS, thereby decreasing the amount of the core-plus-one O repeat, indicating that MigA has a regulatory role. The structures of LPS from both mutants were elucidated using nuclear magnetic resonance spectroscopy and mass spectrometry. The capped core of the wapR mutant was found to be truncated and lacked alpha-1,3-L-Rha. In contrast, uncapped core OS from the migA mutant lacked alpha-1,6-L-Rha. These results provide evidence that WapR is the alpha-1,3-rhamnosyltransferase, while MigA is the alpha-1,6-rhamnosyltransferase.

Pseudomonas aeruginosa lipopolysaccharide: a major virulence factor, initiator of inflammation and target for effective immunity

Pseudomonas aeruginosa is one of the most important bacterial pathogens encountered by immunocompromised hosts and patients with cystic fibrosis (CF), and the lipopolysaccharide (LPS) elaborated by this organism is a key factor in virulence as well as both innate and acquired host responses to infection. The molecule has a fair degree of heterogeneity in its lipid A and O-antigen structure, and elaborates two different outer-core glycoforms, of which only one is ligated to the O-antigen. A close relatedness between the chemical structures and genes encoding biosynthetic enzymes has been established, with 11 major O-antigen groups identified. The lipid A can be variably penta-, hexa- or hepta-acylated, and these isoforms have differing potencies when activating host innate immunity via binding to Toll-like receptor 4 (TLR4). The O-antigen is a major target for protective immunity as evidenced by numerous animal studies, but attempts, to date, to produce a human vaccine targeting these epitopes have not been successful. Newer strategies employing live attenuated P. aeruginosa, or heterologous attenuated bacteria expressing P. aeruginosa O-antigens are potential means to solve some of the existing problems related to making a P. aeruginosa LPS-specific vaccine. Overall, there is now a large amount of information available about the genes and enzymes needed to produce the P. aeruginosa LPS, detailed chemical structures have been determined for the major O-antigens, and significant biologic and immunologic studies have been conducted to define the role of this molecule in virulence and immunity to P. aeruginosa infection.

The Burkholderia cepacia bceA gene encodes a protein with phosphomannose isomerase and GDP-D-mannose pyrophosphorylase activities

The bceA gene is part of the Burkholderia cepacia IST408 exopolysaccharide (EPS) biosynthetic cluster. It encodes a 55.3-kDa bifunctional protein (type II PMI family) with phosphomannose isomerase (PMI) and GDP-mannose pyrophosphorylase (GMP) activities. GMP activity is strongly dependent on the presence of Ca(2+) or Mn(2+), while PMI activity can use a broader variety of divalent cations (Ca(2+)>Mn(2+)>Mg(2+)>Co(2+)>Ni(2+)). The lack of a functional bceA gene does not affect EPS production yield in a non-polar insertion bceA mutant. The in silico search for putative bceA homologues revealed the presence of 2-5 bceA orthologues in the Burkholderia genomes available. This suggests that in B. cepacia IST408 putative bceA functional homologues may compensate the bceA mutation. However, the viscosity of aqueous solutions prepared with the EPS produced by the bceA mutant was significantly reduced compared with wild-type biopolymer and the mutant forms biofilms with a size reduced by 6-fold.

Differential biofilm formation and motility associated with lipopolysaccharide/exopolysaccharide-coupled biosynthetic genes in Stenotrophomonas maltophilia

Stenotrophomonas maltophilia WR-C is capable of forming biofilm on polystyrene and glass. The lipopolysaccharide/exopolysaccharide-coupled biosynthetic genes rmlA, rmlC, and xanB are necessary for biofilm formation and twitching motility. Mutants with mutations in rmlAC and xanB display contrasting biofilm phenotypes on polystyrene and glass and differ in swimming motility.

Putative exopolysaccharide synthesis genes influence Pseudomonas aeruginosa biofilm development

An analysis of the Pseudomonas aeruginosa genomic sequence revealed three gene clusters, PA1381-1393, PA2231-2240, and PA3552-3558, in addition to the alginate biosynthesis gene cluster, which appeared to encode functions for exopolysaccharide (EPS) biosynthesis. Recent evidence indicates that alginate is not a significant component of the extracellular matrix in biofilms of the sequenced P. aeruginosa strain PAO1. We hypothesized that at least one of the three potential EPS gene clusters revealed by genomic sequencing is an important component of P. aeruginosa PAO1 biofilms. Thus, we constructed mutants with chromosomal insertions in PA1383, PA2231, and PA3552. The mutant with a PA2231 defect formed thin unstructured abnormal biofilms. The PA3552 mutant formed structured biofilms that appeared different from those formed by the parent, and the PA1383 mutant formed structured biofilms that were indistinguishable from those formed by the parent. Consistent with a previous report, we found that polysaccharides were one component of the extracellular matrix, which also contained DNA. We suggest that the genes that were inactivated in our PA2231 mutant are required for the production of an EPS, which, although it may be a minor constituent of the matrix, is critical for the formation of P. aeruginosa PAO1 biofilms.

Two genetic loci produce distinct carbohydrate-rich structural components of the Pseudomonas aeruginosa biofilm matrix

Pseudomonas aeruginosa forms biofilms, which are cellular aggregates encased in an extracellular matrix. Molecular genetics studies of three common autoaggregative phenotypes, namely wrinkled colonies, pellicles, and solid-surface-associated biofilms, led to the identification of two loci, pel and psl, that are involved in the production of carbohydrate-rich components of the biofilm matrix. The pel gene cluster is involved in the production of a glucose-rich matrix material in P. aeruginosa strain PA14 (L. Friedman and R. Kolter, Mol. Microbiol. 51:675-690, 2004). Here we investigate the role of the pel gene cluster in P. aeruginosa strain ZK2870 and identify a second genetic locus, termed psl, involved in the production of a mannose-rich matrix material. The 11 predicted protein products of the psl genes are homologous to proteins involved in carbohydrate processing. P. aeruginosa is thus able to produce two distinct carbohydrate-rich matrix materials. Either carbohydrate-rich matrix component appears to be sufficient for mature biofilm formation, and at least one of them is required for mature biofilm formation in P. aeruginosa strains PA14 and ZK2870.

Identification of psl, a locus encoding a potential exopolysaccharide that is essential for Pseudomonas aeruginosa PAO1 biofilm formation

Bacteria inhabiting biofilms usually produce one or more polysaccharides that provide a hydrated scaffolding to stabilize and reinforce the structure of the biofilm, mediate cell-cell and cell-surface interactions, and provide protection from biocides and antimicrobial agents. Historically, alginate has been considered the major exopolysaccharide of the Pseudomonas aeruginosa biofilm matrix, with minimal regard to the different functions polysaccharides execute. Recent chemical and genetic studies have demonstrated that alginate is not involved in the initiation of biofilm formation in P. aeruginosa strains PAO1 and PA14. We hypothesized that there is at least one other polysaccharide gene cluster involved in biofilm development. Two separate clusters of genes with homology to exopolysaccharide biosynthetic functions were identified from the annotated PAO1 genome. Reverse genetics was employed to generate mutations in genes from these clusters. We discovered that one group of genes, designated psl, are important for biofilm initiation. A PAO1 strain with a disruption of the first two genes of the psl cluster (PA2231 and PA2232) was severely compromised in biofilm initiation, as confirmed by static microtiter and continuous culture flow cell and tubing biofilm assays. This impaired biofilm phenotype could be complemented with the wild-type psl sequences and was not due to defects in motility or lipopolysaccharide biosynthesis. These results implicate an as yet unknown exopolysaccharide as being required for the formation of the biofilm matrix. Understanding psl-encoded exopolysaccharide expression and protection in biofilms will provide insight into the pathogenesis of P. aeruginosa in cystic fibrosis and other infections involving biofilms.

Guanosine diphosphate-4-keto-6-deoxy-d-mannose reductase in the pathway for the synthesis of GDP-6-deoxy-d-talose in Actinobacillus actinomycetemcomitans

The serotype a-specific polysaccharide antigen of Actinobacillus actinomycetemcomitans is an unusual sugar, 6-deoxy-d-talose. Guanosine diphosphate (GDP)-6-deoxy-d-talose is the activated sugar nucleotide form of 6-deoxy-d-talose, which has been identified as a constituent of only a few microbial polysaccharides. In this paper, we identify two genes encoding GDP-6-deoxy-d-talose synthetic enzymes, GDP-alpha-d-mannose 4,6-dehydratase and GDP-4-keto-6-deoxy-d-mannose reductase, in the gene cluster required for the biosynthesis of serotype a-specific polysaccharide antigen from A. actinomycetemcomitans SUNYaB 75. Both gene products were produced and purified from Escherichia coli transformed with plasmids containing these genes. Their enzymatic reactants were analysed by reversed-phase HPLC (RP-HPLC). The sugar nucleotide produced from GDP-alpha-d-mannose by these enzymes was purified by RP-HPLC and identified by electrospray ionization-MS, 1H nuclear magnetic resonance, and GC/MS. The results indicated that GDP-6-deoxy-d-talose is produced from GDP-alpha-d-mannose. This paper is the first report on the GDP-6-deoxy-d-talose biosynthetic pathway and the role of GDP-4-keto-6-deoxy-d-mannose reductase in the synthesis of GDP-6-deoxy-d-talose.

Crystal structure of PMM/PGM: an enzyme in the biosynthetic pathway of P. aeruginosa virulence factors

The enzyme phosphomannomutase/phosphoglucomutase (PMM/PGM) from P. aeruginosa is required for the biosynthesis of two bacterial exopolysaccharides: alginate and lipopolysaccharide (LPS). Both of these molecules play a role in the virulence of P. aeruginosa, an important human pathogen known for its ability to develop antibiotic resistance and cause chronic lung infections in cystic fibrosis patients. The crystal structure of PMM/PGM shows that the enzyme has four domains, three of which have a similar three-dimensional fold. Residues from all four domains of the protein contribute to the formation of a large active site cleft in the center of the molecule. Detailed information on the active site of PMM/PGM lays the foundation for structure-based inhibitor design. Inhibitors of sufficient potency and specificity should impair the biosynthesis of alginate and LPS, and may facilitate clearance of the bacteria by the host immune system and increase the efficacy of conventional antibiotic treatment against chronic P. aeruginosa infections.

Functional expression of Pseudomonas aeruginosa GDP-4-keto-6-deoxy-D-mannose reductase which synthesizes GDP-rhamnose

Pseudomonas aeruginosa is an opportunistic Gram-negative bacterium that causes severe infections in a number of hosts from plants to mammals. A-band lipopolysaccharide of P. aeruginosa contains d-rhamnosylated O-antigen. The synthesis of GDP-D-rhamnose, the d-rhamnose donor in d-rhamnosylation, starts from GDP-D-mannose. It is first converted by the GDP-mannose-4,6-dehydratase (GMD) into GDP-4-keto-6-deoxy-D-mannose, and then reduced to GDP-D-rhamnose by GDP-4-keto-6-deoxy-D-mannose reductase (RMD). Here, we describe the enzymatic characterization of P. aeruginosa RMD expressed in Saccharomyces cerevisiae. Previous success in functional expression of bacterial gmd genes in S. cerevisiae allowed us to convert GDP-D-mannose into GDP-4-keto-6-deoxy-D-mannose. Thus, coexpression of the Helicobacter pylori gmd and P. aeruginosa rmd genes resulted in conversion of the 4-keto-6-deoxy intermediate into GDP-deoxyhexose. This synthesized GDP-deoxyhexose was confirmed to be GDP-rhamnose by HPLC, matrix-assisted laser desorption/ionization time-of-flight MS, and finally NMR spectroscopy. The functional expression of P. aeruginosa RMD in S. cerevisiae will provide a tool for generating GDP-rhamnose for in vitro rhamnosylation of glycoprotein and glycopeptides.

Topological and functional characterization of WbpM, an inner membrane UDP-GlcNAc C6 dehydratase essential for lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

WbpM is essential for the biosynthesis of B-band lipopolysaccharide (LPS) in many serotypes of Pseudomonas aeruginosa. Homologues that can functionally complement a wbpM null mutant and that are also necessary for virulence have been identified in numerous pathogenic bacteria. WbpM and most of its homologues are large membrane proteins, which has long hampered the elucidation of their biochemical function. This paper describes the detailed characterization of WbpM using both in vivo and in vitro approaches. LacZ and PhoA fusion experiments showed that WbpM was anchored to the inner membrane via four N-terminal transmembrane domains, whereas the C-terminal catalytic domain resided in the cytoplasm. Although the membrane domains did not have any catalytic activity, complementation experiments suggested that they were important for the polymerization of high-molecular-weight B-band LPS. The biochemical characterization of a soluble truncated form of WbpM, His-S262, showed that WbpM was a C6 dehydratase specific for UDP-GlcNAc. It exhibited unusual low temperature (25-30 degrees C) and high pH (pH 10) optima. Although WbpM possessed an altered catalytic triad composed of SMK as opposed to SYK commonly found in other dehydratases, its catalysis was very efficient, with a kcat of 168 min(-1) and a kcat/Km of 58 mM(-1) min(-1). These unusual physico-kinetic properties suggested a potentially different mechanism of C6 dehydration for WbpM and its large homologues. His-S262 is now a precious tool for further structure-function studies.

Identification of two GDP-6-deoxy-D-lyxo-4-hexulose reductases synthesizing GDP-D-rhamnose in Aneurinibacillus thermoaerophilus L420-91T

The glycan repeats of the surface layer glycoprotein of Aneurinibacillus thermoaerophilus L420-91T contain d-rhamnose and 3-acetamido-3,6-dideoxy-d-galactose, both of which are also constituents of lipopolysaccharides of Gram-negative plant and human pathogenic bacteria. The two genes required for biosynthesis of the nucleotide-activated precursor GDP-d-rhamnose, gmd and rmd, were cloned, sequenced, and overexpressed in Escherichia coli. The corresponding enzymes Gmd and Rmd were purified to homogeneity, and functional studies were performed. GDP-d-mannose dehydratase (Gmd) converted GDP-d-mannose to GDP-6-deoxy-d-lyxo-4-hexulose, with NADP+ as cofactor. The reductase Rmd catalyzed the second step in the pathway, namely the reduction of the keto-intermediate to the final product GDP-d-rhamnose using both NADH and NADPH as hydride donor. The elution behavior of the intermediate and end product was analyzed by high performance liquid chromatography. Nuclear magnetic resonance spectroscopy was used to identify the structure of the final product of the reaction sequence as GDP-alpha-d-rhamnose. This is the first characterization of a GDP-6-deoxy-d-lyxo-4-hexulose reductase. In addition, Gmd has been shown to be a bifunctional enzyme with both dehydratase and reductase activities. So far, no enzyme catalyzing these two types of reactions has been identified. Both Gmd and Rmd are members of the SDR (short chain dehydrogenase/reductase) protein family.

Genetic analysis of the gene cluster for the synthesis of serotype a-specific polysaccharide antigen in Aactinobacillus actinomycetemcomitans

The serotype a-specific polysaccharide antigen (SPA) of Actinobacillus actinomycetemcomitans consists of 6-deoxy-D-talose. A gene cluster associated with the biosynthesis of SPA was cloned and sequenced from the chromosomal DNA of A. actinomycetemcomitans SUNYaB 75 (serotype a). This cluster consisted of 14 open reading frames. Insertional inactivation of eight genes in this cluster resulted in loss of the ability of A. actinomycetemcomitans SUNYaB 75 cells to produce the polysaccharide. A protein database search revealed that the 11 sequential genes containing these eight genes were not found in SPA-associated gene clusters of the other serotypes of A. actinomycetemcomitans. These results suggest that the gene cluster is unique to serotype a and is essential to the synthesis of the SPA.

The rhamnose pathway

L-Rhamnose is a deoxy sugar found widely in bacteria and plants. Evidence continues to emerge about its essential role in many pathogenic bacteria. The crystal structures of two of the four enzymes involved in its biosynthetic pathway have been reported and the other two have been submitted for publication. This pathway does not exist in humans, making enzymes of this pathway very attractive targets for therapeutic intervention.

Involvement of the rml locus in core oligosaccharide and O polysaccharide assembly in Pseudomonas aeruginosa

L-Rhamnose (L-Rha) is a component of the lipopolysaccharide (LPS) core, several O antigen polysaccharides, and the cell surface surfactant rhamnolipid of Pseudomonas aeruginosa. In this study, four contiguous genes (rmlBDAC) responsible for the synthesis of dTDP-L-Rha in P. aeruginosa have been cloned and characterized. Non-polar chromosomal rmlC mutants were generated in P. aeruginosa strains PAO1 (serotype O5) and PAK (serotype O6) and LPS extracted from the mutants was analysed by SDS-PAGE and Western immunoblotting. rmlC mutants of both serotype O5 and serotype O6 synthesized a truncated core region which was unable to act as an attachment point for either A-band or B-band O antigen. A rmd rmlC PAO1 double mutant (deficient in biosynthesis of both D-Rha and L-Rha) was constructed to facilitate structural analysis of the mutant core region. This strain has an incomplete core oligosaccharide region and does not produce A-band O antigen. These results provide the genetic and structural evidence that L-Rha is the receptor on the P. aeruginosa LPS core for the attachment of O polysaccharides. This is the first report of a genetically defined mutation that affects the synthesis of a single sugar in the core oligosaccharide region of P. aeruginosa LPS, and provides further insight into the mechanisms of LPS biosynthesis and assembly in this bacterium.

Identification of the Pseudomonas aeruginosa glmM gene, encoding phosphoglucosamine mutase

A search for a potential algC homologue within the Pseudomonas aeruginosa PAO1 genome database has revealed an open reading frame (ORF) of unknown function, ORF540 in contig 54 (July 1999 Pseudomonas genome release), that theoretically coded for a 445-amino-acid-residue polypeptide (I. M. Tavares, J. H. Leitão, A. M. Fialho, and I. Sá-Correia, Res. Microbiol. 150:105-116, 1999). The product of this gene is here identified as the phosphoglucosamine mutase (GlmM) which catalyzes the conversion of glucosamine-6-phosphate to glucosamine-1-phosphate, an essential step in the formation of the cell wall precursor UDP-N-acetylglucosamine. The P. aeruginosa gene has been cloned into expression vectors and shown to restore normal peptidoglycan biosynthesis and cell growth of a glmM Escherichia coli mutant strain. The GlmM enzyme from P. aeruginosa has been overproduced to high levels and purified to homogeneity in a six-histidine-tagged form. Beside its phosphoglucosamine mutase activity, the P. aeruginosa enzyme is shown to exhibit phosphomannomutase and phosphoglucomutase activities, which represent about 20 and 2% of its GlmM activity, respectively.

Lipopolysaccharide core phosphates are required for viability and intrinsic drug resistance in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that is notorious for its intrinsic drug resistance. We have used chemical and genetic techniques to characterize three putative kinase genes that are involved in the addition of phosphate to the inner core region of P. aeruginosa lipopolysaccharide. The first gene is a waaP homologue, whereas the other two (wapP and wapQ) are unique to P. aeruginosa. Repeated attempts using a variety of membrane-stabilizing conditions to generate waaP:Gm (Gm, gentamicin) or wapP:Gm mutants were unsuccessful. We were able to generate a chromosomal waaP mutant that had a wild-type copy of either waaPPa or waaPEc in trans, but were unable to cure this plasmid-borne copy of the gene. These results are consistent with the fact that P. aeruginosa mutants lacking inner core heptose (Hep) or phosphate have never been isolated and demonstrate the requirement of Hep-linked phosphate for P. aeruginosa viability. A wapQ:Gm mutant was isolated and it had an unaltered minimum inhibitory concentration (MIC) for novobiocin and only a small decrease in the MIC for sodium dodecyl sulphate (SDS), suggesting that the loss of a phosphate group transferred by WapQ may only be having a small impact on outer-membrane permeability. Nuclear magnetic resonance and methylation linkage analysis showed that WaaPPa could add one phosphate to O4 of HepI in a Salmonella typhimurium waaP mutant. The expression of WaaPPa increased the outer-membrane integrity of these complemented mutants, as evidenced by 35-fold and 75-fold increases in the MIC for novobiocin and SDS respectively. The S. typhimurium waaP mutant transformed with both waaP and wapP had over 250-fold and 1000-fold increases, respectively, in these MICs. The inner core phosphates of P. aeruginosa appear to be playing a key role in the intrinsic drug resistance of this bacterium.

The Pseudomonas aeruginosa algC gene product participates in rhamnolipid biosynthesis

Pseudomonas aeruginosa produces exoproducts correlated with its pathogenicity. One of these virulence-associated traits is the surfactant rhamnolipid. The production of alginate and lipopolysaccharide (LPS) are also of importance for P. aeruginosa virulence. The product of the algC gene (which is involved in alginate production through its phosphomannomutase activity and in LPS synthesis through its phosphoglucomutase activity) participates in rhamnolipid production, presumably catalyzing the first step in the deoxy-thymidine-diphospho-L-rhamnose (dTDP-L-rhamnose) pathway, the conversion of glucose-6-phosphate to glucose-1-phosphate. Other structural alg genes, encoded in the alg operon, are not involved in rhamnolipid nor LPS production. These results show that the AlgC protein plays a central role in the production of the three P. aeruginosa virulence-associated saccharides: alginate, LPS and rhamnolipid.

Genetics of O-antigen biosynthesis in Pseudomonas aeruginosa

Pathogenic bacteria produce an elaborate assortment of extracellular and cell-associated bacterial products that enable colonization and establishment of infection within a host. Lipopolysaccharide (LPS) molecules are cell surface factors that are typically known for their protective role against serum-mediated lysis and their endotoxic properties. The most heterogeneous portion of LPS is the O antigen or O polysaccharide, and it is this region which confers serum resistance to the organism. Pseudomonas aeruginosa is capable of concomitantly synthesizing two types of LPS referred to as A band and B band. The A-band LPS contains a conserved O polysaccharide region composed of D-rhamnose (homopolymer), while the B-band O-antigen (heteropolymer) structure varies among the 20 O serotypes of P. aeruginosa. The genes coding for the enzymes that direct the synthesis of these two O antigens are organized into two separate clusters situated at different chromosomal locations. In this review, we summarize the organization of these two gene clusters to discuss how A-band and B-band O antigens are synthesized and assembled by dedicated enzymes. Examples of unique proteins required for both A-band and B-band O-antigen synthesis and for the synthesis of both LPS and alginate are discussed. The recent identification of additional genes within the P. aeruginosa genome that are homologous to those in the A-band and B-band gene clusters are intriguing since some are able to influence O-antigen synthesis. These studies demonstrate that P. aeruginosa represents a unique model system, allowing studies of heteropolymeric and homopolymeric O-antigen synthesis, as well as permitting an examination of the interrelationship of the synthesis of LPS molecules and other virulence determinants.

Pattern of changes in the activity of enzymes of GDP-D-mannuronic acid synthesis and in the level of transcription of algA, algC and algD genes accompanying the loss and emergence of mucoidy in Pseudomonas aeruginosa

The low activity levels of the four GDP-D-mannuronic acid-forming enzymes, even in highly alginate-producing strains of Pseudomonas aeruginosa, have made it difficult to compare enzyme activities accompanying the loss/acquisition of mucoidy. Using optimized conditions, we compared the specific activity of these enzymes in three different mucoid P. aeruginosa cystic fibrosis isolates, in their nonmucoid spontaneous variants, and in mucoid variants that emerged during extended incubation of these nonmucoid forms in acetamide broth. A correlation was established between the promptness of emergence of the mucoid forms and the differing sensitivity to nutrient-limitation-induced death of the nonmucoid compared with the isogenic mucoid population. Consistent with the undetectable levels of algD mRNA in nonmucoid forms and with the concept that the step catalyzed by the algD-encoded GDP-mannose dehydrogenase (GMD) is a key step in control of the alginate pathway, GMD activity was undetectable or showed negligible values in nonmucoid variants and correlated with alginate production. However, phosphomannose isomerase (PMI), phosphomannomutase (PMM), and GDP-mannose pyrophosphorylase (GMP) activities in the nonmucoid forms were only slightly (40-70%) below the values in the mucoid forms. Nevertheless, no transcripts homologous to algA (encoding a bifunctional enzyme that possesses both PMI and GMP activities) were detected in the nonmucoid form, and the levels of algC (encoding PMM) transcripts, although detectable in the nonmucoid variants, were, in general, much higher in the mucoid forms. These apparently intriguing observations were cleared up by the identification of two algA functional homologues in P. aeruginosa, recently reported by others, and by the identification of one algC homologue, in contig225 of the PAO1 genome sequence, defining a polypeptide with a deduced amino acid sequence that showed significant homology with that of enzymes of the phosphohexomutase family found in databases. Results are also consistent with the requirement of PMI, GMP and PMM activities for the supply of GDP-D-mannose to (at least) A-band lipopolysaccharide synthesis, while GMD channels this precursor into the alginate pathway.