Membrane topology of PssT, the transmembrane protein component of the type I exopolysaccharide transport system in Rhizobium leguminosarum bv. trifolii strain TA1

A New Face of the Old Gene: Deletion of the PssA, Encoding Monotopic Inner Membrane Phosphoglycosyl Transferase in Rhizobium leguminosarum, Leads to Diverse Phenotypes That Could Be Attributable to Downstream Effects of the Lack of Exopolysaccharide

The biosynthesis of subunits of rhizobial exopolysaccharides is dependent on glycosyltransferases, which are usually encoded by large gene clusters. PssA is a member of a large family of phosphoglycosyl transferases catalyzing the transfer of a phosphosugar moiety to polyprenol phosphate; thus, it can be considered as priming glycosyltransferase commencing synthesis of the EPS repeating units in Rhizobium leguminosarum. The comprehensive analysis of PssA protein features performed in this work confirmed its specificity for UDP-glucose and provided evidence that PssA is a monotopic inner membrane protein with a reentrant membrane helix rather than a transmembrane segment. The bacterial two-hybrid system screening revealed interactions of PssA with some GTs involved in the EPS octasaccharide synthesis. The distribution of differentially expressed genes in the transcriptome of the ΔpssA mutant into various functional categories indicated complexity of cell response to the deletion, which can mostly be attributed to the lack of exopolysaccharide and downstream effects caused by such deficiency. The block in the EPS biosynthesis at the pssA step, potentially leading to an increased pool of UDP-glucose, is likely to be filtered through to other pathways, and thus the absence of EPS may indirectly affect the expression of proteins involved in these pathways.

Putative functions of EpsK in teichuronic acid synthesis and phosphate starvation in Bacillus licheniformis

Extracellular polymeric substances (EPSs) are extracellular macromolecules in bacteria, which function in cell growth and show potential for mechanism study and biosynthesis application. However, the biosynthesis mechanism of EPS is still not clear. We herein chose Bacillus licheniformis CGMCC 2876 as a target strain to investigate the EPS biosynthesis. epsK, a member of eps cluster, the predicted polysaccharide synthesis cluster, was overexpressed and showed that the overexpression of epsK led to a 26.54% decrease in the production of EPS and resulted in slenderer cell shape. Transcriptome analysis combined with protein-protein interactions analysis and protein modeling revealed that epsK was likely responsible for the synthesis of teichuronic acid, a substitute cell wall component of teichoic acid when the strain was suffering phosphate limitation. Further cell cultivation showed that either phosphate limitation or the overexpression of teichuronic acid synthesis genes, tuaB and tuaE could similarly lead to EPS reduction. The enhanced production of teichuronic acid induced by epsK overexpression triggered the endogenous phosphate starvation, resulting in the decreased EPS synthesis and biomass, and the enhanced bacterial chemotaxis. This study presents an insight into the mechanism of EPS synthesis and offers the potential in controllable synthesis of target products.

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes that involves the rhizobial infection of roots and the bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role for EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia, and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Chromo-fluorogenic probes for β-galactosidase detection

β-Galactosidase (β-Gal) is a widely used enzyme as a reporter gene in the field of molecular biology which hydrolyzes the β-galactosides into monosaccharides. β-Gal is an essential enzyme in humans and its deficiency or its overexpression results in several rare diseases. Cellular senescence is probably one of the most relevant physiological disorders that involve β-Gal enzyme. In this review, we assess the progress made to date in the design of molecular-based probes for the detection of β-Gal both in vitro and in vivo. Most of the reported molecular probes for the detection of β-Gal consist of a galactopyranoside residue attached to a signalling unit through glycosidic bonds. The β-Gal-induced hydrolysis of the glycosidic bonds released the signalling unit with remarkable changes in color and/or emission. Additional examples based on other approaches are also described. The wide applicability of these probes for the rapid and in situ detection of de-regulation β-Gal-related diseases has boosted the research in this fertile field.

PssJ Is a Terminal Galactosyltransferase Involved in the Assembly of the Exopolysaccharide Subunit in Rhizobium Leguminosarum bv. Trifolii

Rhizobium leguminosarum bv. trifolii produces exopolysaccharide (EPS) composed of glucose, glucuronic acid, and galactose residues at a molar ratio 5:2:1. A majority of genes involved in the synthesis, modification, and export of exopolysaccharide are located in the chromosomal Pss-I region. In the present study, a ΔpssJ deletion mutant was constructed and shown to produce EPS lacking terminal galactose in the side chain of the octasaccharide subunit. The lack of galactose did not block EPS subunit translocation and polymerization. The in trans delivery of the pssJ gene restored the production of galactose-containing exopolysaccharide. The mutant was compromised in several physiological traits, e.g., motility and biofilm production. An impact of the pssJ mutation and changed EPS structure on the symbiotic performance was observed as improper signaling at the stage of molecular recognition, leading to formation of a significant number of non-infected empty nodules. Terminal galactosyltransferase PssJ was shown to display a structure typical for the GT-A class of glycosyltransferases and interact with other GTs and Wzx/Wzy system proteins. The latter, together with PssJ presence in soluble and membrane protein fractions indicated that the protein plays its role at the inner membrane interface and as a component of a larger complex.

Mgl2 Is a Hypothetical Methyltransferase Involved in Exopolysaccharide Production, Biofilm Formation, and Motility in Rhizobium leguminosarum bv. trifolii

In this study, functional characterization of the mgl2 gene located near the Pss-I exopolysaccharide biosynthesis region in Rhizobium leguminosarum bv. trifolii TA1 is described. The hypothetical protein encoded by the mgl2 gene was found to be similar to methyltransferases (MTases). Protein homology and template-based modeling facilitated prediction of the Mgl2 structure, which greatly resembled class I MTases with a S-adenosyl-L-methionine-binding cleft. The Mgl2 protein was engaged in exopolysaccharide, but not lipopolysaccharide, synthesis. The mgl2 deletion mutant produced exopolysaccharide comprised of only low molecular weight fractions, while overexpression of mgl2 caused overproduction of exopolysaccharide with a normal low to high molecular weight ratio. The deletion of the mgl2 gene resulted in disturbances in biofilm formation and a slight increase in motility in minimal medium. Red clover (Trifolium pratense) inoculated with the mgl2 mutant formed effective nodules, and the appearance of the plants indicated active nitrogen fixation. The mgl2 gene was preceded by an active and strong promoter. Mgl2 was defined as an integral membrane protein and formed homodimers in vivo; however, it did not interact with Pss proteins encoded within the Pss-I region. The results are discussed in the context of the possible involvement of the newly described potential MTase in various metabolic traits, such as the exopolysaccharide synthesis and motility that are important for rhizobial saprophytic and symbiotic relationships.

Transcriptomic Studies Reveal that the Rhizobium leguminosarum Serine/Threonine Protein Phosphatase PssZ has a Role in the Synthesis of Cell-Surface Components, Nutrient Utilization, and Other Cellular Processes

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing symbiotic associations with clover plants (Trifolium spp.). Surface polysaccharides, transport systems, and extracellular components synthesized by this bacterium are required for both the adaptation to changing environmental conditions and successful infection of host plant roots. The pssZ gene located in the Pss-I region, which is involved in the synthesis of extracellular polysaccharide, encodes a protein belonging to the group of serine/threonine protein phosphatases. In this study, a comparative transcriptomic analysis of R. leguminosarum bv. trifolii wild-type strain Rt24.2 and its derivative Rt297 carrying a pssZ mutation was performed. RNA-Seq data identified a large number of genes differentially expressed in these two backgrounds. Transcriptome profiling of the pssZ mutant revealed a role of the PssZ protein in several cellular processes, including cell signalling, transcription regulation, synthesis of cell-surface polysaccharides and components, and bacterial metabolism. In addition, we show that inactivation of pssZ affects the rhizobial ability to grow in the presence of different sugars and at various temperatures, as well as the production of different surface polysaccharides. In conclusion, our results identified a set of genes whose expression was affected by PssZ and confirmed the important role of this protein in the rhizobial regulatory network.

Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium-legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

A Bacteriophage-Acquired O-Antigen Polymerase (Wzyβ) from P. aeruginosa Serotype O16 Performs a Varied Mechanism Compared to Its Cognate Wzyα

Pseudomonas aeruginosa is a Gram-negative bacterium that produces highly varied lipopolysaccharide (LPS) structures. The O antigen (O-Ag) in the LPS is synthesized through the Wzx/Wzy-dependent pathway where lipid-linked O-Ag repeats are polymerized by Wzy. Horizontal-gene transfer has been associated with O-Ag diversity. The O-Ag present on the surface of serotypes O5 and O16, differ in the intra-molecular bonds, α and β, respectively; the latter arose from the action of three genes in a serotype converting unit acquired from bacteriophage D3, including a β-polymerase (Wzy_β). To further our understanding of O-polymerases, the inner membrane (IM) topology of Wzy_β was determined using a dual phoA-lacZα reporter system wherein random 3′ gene truncations were localized to specific loci with respect to the IM by normalized reporter activities as determined through the ratio of alkaline phosphatase activity to β-galactosidase activity. The topology of Wzy_β developed through this approach was shown to contain two predominant periplasmic loops, PL3 (containing an RX₁₀G motif) and PL4 (having an O-Ag ligase superfamily motif), associated with inverting glycosyltransferase reaction. Through site-directed mutagenesis and complementation assays, residues Arg²⁵⁴, Arg²⁷⁰, Arg²⁷², and His³⁰⁰ were found to be essential for Wzy_β function. Additionally, like-charge substitutions, R254K and R270K, could not complement the wzy_β knockout, highlighting the essential guanidium side group of Arg residues. The O-Ag ligase domain is conserved among heterologous Wzy proteins that produce β-linked O-Ag repeat units. Taking advantage of the recently obtained whole-genome sequence of serotype O16 a candidate promoter was identified. Wzy_β under its native promoter was integrated in the PAO1 genome, which resulted in simultaneous production of α- and β-linked O-Ag. These observations established that members of Wzy-like family consistently exhibit a dual-periplasmic loops topology, and identifies motifs that are plausible to be involved in enzymatic activities. Based on these results, the phage-derived Wzy_β utilizes a different reaction mechanism in the P. aeruginosa host to avoid self-inhibition during serotype conversion.

Challenges and perspectives in combinatorial assembly of novel exopolysaccharide biosynthesis pathways

Because of their rheological properties various microbial polysaccharides are applied as thickeners and viscosifiers both in food and non-food industries. A broad variety of microorganisms secrete structurally diverse exopolysaccharides (EPS) that contribute to their surface attachment, protection against abiotic or biotic stress factors, and nutrient gathering. Theoretically, a massive number of EPS structures are possible through variations in monosaccharide sequences, condensation linkages and non-sugar decorations. Given the already-high diversity of EPS structures, taken together with the principal of combinatorial biosynthetic pathways, microbial polysaccharides are an attractive class of macromolecules with which to generate novel structures via synthetic biology approaches. However, previous manipulations primarily focused on increasing polysaccharide yield, with structural modifications restricted to removal of side chains or non-sugar decorations. This article outlines the biosynthetic pathways of the bacterial heteroexopolysaccharides xanthan and succinoglycan, which are used as thickening and stabilizing agents in food and non-food industries. Challenges and perspectives of combining synthetic biology approaches with directed evolution to overcome obstacles in assembly of novel EPS biosynthesis pathways are discussed.

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.

PssP2 is a polysaccharide co-polymerase involved in exopolysaccharide chain-length determination in Rhizobium leguminosarum

Production of extracellular polysaccharides is a complex process engaging proteins localized in different subcellular compartments, yet communicating with each other or even directly interacting in multicomponent complexes. Proteins involved in polymerization and transport of exopolysaccharide (EPS) in Rhizobium leguminosarum are encoded within the chromosomal Pss-I cluster. However, genes implicated in polysaccharide synthesis are common in rhizobia, with several homologues of pss genes identified in other regions of the R. leguminosarum genome. One such region is chromosomally located Pss-II encoding proteins homologous to known components of the Wzx/Wzy-dependent polysaccharide synthesis and transport systems. The pssP2 gene encodes a protein similar to polysaccharide co-polymerases involved in determination of the length of polysaccharide chains in capsule and O-antigen biosynthesis. In this work, a mutant with a disrupted pssP2 gene was constructed and its capabilities to produce EPS and enter into a symbiotic relationship with clover were studied. The pssP2 mutant, while not altered in lipopolysaccharide (LPS), displayed changes in molecular mass distribution profile of EPS. Lack of the full-length PssP2 protein resulted in a reduction of high molecular weight EPS, yet polymerized to a longer length than in the RtTA1 wild type. The mutant strain was also more efficient in symbiotic performance. The functional interrelation between PssP2 and proteins encoded within the Pss-I region was further supported by data from bacterial two-hybrid assays providing evidence for PssP2 interactions with PssT polymerase, as well as glycosyltransferase PssC. A possible role for PssP2 in a complex involved in EPS chain-length determination is discussed.

Conserved-residue mutations in Wzy affect O-antigen polymerization and Wzz-mediated chain-length regulation in Pseudomonas aeruginosa PAO1

O antigen (O-Ag) in many bacteria is synthesized via the Wzx/Wzy-dependent pathway in which Wzy polymerizes lipid-linked O-Ag subunits to modal lengths regulated by Wzz. Characterization of 83 site-directed mutants of Wzy from Pseudomonas aeruginosa PAO1 (Wzy_Pa) in topologically-mapped periplasmic (PL) and cytoplasmic loops (CL) verified the functional importance of PL3 and PL5, with the former shown to require overall cationic properties. Essential Arg residues in the RX₁₀G motifs of PL3 and PL5 were found to be conserved in putative homologues of Wzy_Pa, as was the overall sequence homology between these two periplasmic loops in each protein. Amino acid substitutions in CL6 were found to alter Wzz-mediated O-antigen modality, with evidence suggesting that these changes may perturb the C-terminal Wzy_Pa tertiary structure. Together, these data suggest that the catch-and-release mechanism of O-Ag polymerization is widespread among bacteria and that regulation of polymer length is affected by interaction of Wzz with Wzy.

Environmental signals and regulatory pathways that influence exopolysaccharide production in rhizobia

Rhizobia are Gram-negative bacteria that can exist either as free-living bacteria or as nitrogen-fixing symbionts inside root nodules of leguminous plants. The composition of the rhizobial outer surface, containing a variety of polysaccharides, plays a significant role in the adaptation of these bacteria in both habitats. Among rhizobial polymers, exopolysaccharide (EPS) is indispensable for the invasion of a great majority of host plants which form indeterminate-type nodules. Various functions are ascribed to this heteropolymer, including protection against environmental stress and host defense, attachment to abiotic and biotic surfaces, and in signaling. The synthesis of EPS in rhizobia is a multi-step process regulated by several proteins at both transcriptional and post-transcriptional levels. Also, some environmental factors (carbon source, nitrogen and phosphate starvation, flavonoids) and stress conditions (osmolarity, ionic strength) affect EPS production. This paper discusses the recent data concerning the function of the genes required for EPS synthesis and the regulation of this process by several environmental signals. Up till now, the synthesis of rhizobial EPS has been best studied in two species, Sinorhizobium meliloti and Rhizobium leguminosarum. The latest data indicate that EPS synthesis in rhizobia undergoes very complex hierarchical regulation, in which proteins engaged in quorum sensing and the regulation of motility genes also participate. This finding enables a better understanding of the complex processes occurring in the rhizosphere which are crucial for successful colonization and infection of host plant roots.

Dual conserved periplasmic loops possess essential charge characteristics that support a catch-and-release mechanism of O-antigen polymerization by Wzy in Pseudomonas aeruginosa PAO1

Heteropolymeric B-band lipopolysaccharide in Pseudomonas aeruginosa PAO1 is synthesized via the so-called Wzy-dependent pathway, requiring a functional Wzy for polymerization of O-antigen repeat units in the periplasm. Wzy is an integral inner membrane protein for which the detailed topology has been mapped in a recent investigation (Islam, S. T., Taylor, V. L., Qi, M., and Lam, J. S. (2010) mBio 1, e00189-10), revealing two principal periplasmic loops (PL), PL3 and PL5, each containing an RX₁₀G motif. Despite considerable sequence conservation between the two loops, the isoelectric point for each peptide displayed marked differences, with PL3 exhibiting a net-positive charge and PL5 showing a net-negative charge. Data from site-directed mutagenesis of amino acids in each PL have led to the identification of several key Arg residues within the two RX₁₀G motifs that are important for Wzy function, of which Arg¹⁷⁶, Arg²⁹⁰, and Arg²⁹¹ could not be functionally substituted with Lys. These observations support the proposed role of each PL in a catch-and-release mechanism for Wzy-mediated O-antigen polymerization.

Membrane topology mapping of the O-antigen flippase (Wzx), polymerase (Wzy), and ligase (WaaL) from Pseudomonas aeruginosa PAO1 reveals novel domain architectures

Biosynthesis of B-band lipopolysaccharide (LPS) in Pseudomonas aeruginosa follows the Wzy-dependent pathway, requiring the integral inner membrane proteins Wzx (O-antigen [O-Ag] flippase), Wzy (O-Ag polymerase), and WaaL (O-Ag ligase). For an important first step in deciphering the mechanisms of LPS assembly, we set out to map the membrane topology of these proteins. Random and targeted 3′wzx, wzy, and waaL truncations were fused to a phoA-lacZα dual reporter capable of displaying both alkaline phosphatase and β-galactosidase activity. The results from truncation fusion expression and the corresponding differential enzyme activity ratios allowed for the assignment of specific regions of the proteins to cytoplasmic, transmembrane (TM), or periplasmic loci. Protein orientation in the inner membrane was confirmed via C-terminal fusion to green fluorescent protein. Our data revealed unique TM domain properties in these proteins, particularly for Wzx, indicating the potential for a charged pore. Novel periplasmic and cytoplasmic loop domains were also uncovered, with the latter in Wzy and WaaL revealing tracts consistent with potential Walker A/B motifs.

The opportunistic pathogen Pseudomonas aeruginosa synthesizes its virulence factor lipopolysaccharide via the Wzy-dependent pathway, requiring translocation, polymerization, and ligation of lipid-linked polysaccharide repeat units by the integral inner membrane proteins Wzx, Wzy, and WaaL, respectively. However, structural evidence to help explain the function of these proteins is lacking. Since membrane proteins are difficult to crystallize, topological mapping is an important first step in identifying exposed and membrane-embedded domains. We mapped the topologies of Wzx, Wzy, and WaaL from P. aeruginosa PAO1 by use of truncation libraries of a randomly fused C-terminal reporter capable of different enzyme activities in the periplasm and cytoplasm. Topology maps were created based directly on residue localization data, eliminating the bias associated with reliance on multiple topology prediction algorithms for initial generation of consensus transmembrane domain localizations. Consequently, we have identified novel periplasmic, cytoplasmic, and transmembrane domain properties that would help to explain the proposed functions of Wzx, Wzy, and WaaL.

Toxicogenomic response of Rhodospirillum rubrum S1H to the micropollutant triclosan

In the framework of the Micro-Ecological Life Support System Alternative (MELiSSA) project, a pilot study was performed to identify the effects of triclosan on the MELiSSA carbon-mineralizing microorganism Rhodospirillum rubrum S1H. Triclosan is a biocide that is commonly found in human excrement and is considered an emerging pollutant in wastewater and the environment. Chronic exposure to MELiSSA-relevant concentrations (> or =25 microg liter(-1)) of triclosan resulted in a significant extension of the lag phase of this organism but hardly affected the growth rate. Analytical determinations gave no indication of triclosan biodegradation during the growth experiment, and flow cytometric viability analyses revealed that triclosan is bacteriostatic and only slightly toxic to R. rubrum S1H. Using microarray analyses, the genetic mechanisms supporting the reversibility of triclosan-induced inhibition were scrutinized. An extremely triclosan-responsive cluster of four small adjacent genes was identified, for which there was up to 34-fold induction with 25 microg liter(-1) triclosan. These four genes, for which the designation microf (micropollutant-upregulated factor) is proposed, appear to be unique to R. rubrum and are shown here for the first time to be involved in the response to stress. Moreover, numerous other systems that are associated with the proton motive force were shown to be responsive to triclosan, but they were never as highly upregulated as the microf genes. In response to triclosan, R. rubrum S1H induced transcription of the phage shock protein operon (pspABC), numerous efflux systems, cell envelope consolidation mechanisms, the oxidative stress response, beta-oxidation, and carbonic anhydrase, while there was downregulation of bacterial conjugation and carboxysome synthesis genes. The microf genes and three efflux-related genes showed the most potential to be low-dose biomarkers.

PssO, a unique extracellular protein important for exopolysaccharide synthesis in Rhizobium leguminosarum bv. trifolii

Synthesis and secretion of polysaccharides by Gram-negative bacteria are a result of a concerted action of enzymatic and channel-forming proteins localized in different compartments of the cell. The presented work comprises functional characterization of PssO protein encoded within the previously identified, chromosomal exopolysaccharide (EPS) biosynthesis region (Pss-I) of symbiotic bacterium Rhizobium leguminosarum bv. trifolii TA1 (RtTA1). pssO gene localization between pssN and pssP genes encoding proteins engaged in exopolysaccharide synthesis and transport, suggested its role in EPS synthesis and/or secretion. RtTA1 pssO deletion mutant and the PssO protein overproducing strains were constructed. The mutant strain was EPS-deficient, however, this mutation was not complemented. The PssO-overproducing strain was characterized by increase in EPS secretion. Subcellular fractionation, pssO-phoA/lacZ translational fusion analyses and immunolocalisation of PssO on RtTA1 cell surface by electron microscopy demonstrated that PssO is secreted to the extracellular medium and remains attached to the cell. Western blotting analysis revealed the presence of immunologically related proteins within the species R. leguminosarum bv. trifolii, bv. viciae and Rhizobium etli. The secondary structure of PssO-His(6), as determined by FTIR spectroscopy, consists of at least 32% alpha-helical and 12% beta-sheet structures. A putative function of PssO in EPS synthesis and/or transport is discussed in the context of its cellular localization and the phenotypes of the deletion mutant and pssO-overexpressing strain.

Lipoprotein PssN of Rhizobium leguminosarum bv. trifolii: subcellular localization and possible involvement in exopolysaccharide export

Surface expression of exopolysaccharides (EPS) in gram-negative bacteria depends on the activity of proteins found in the cytoplasmic membrane, the periplasmic space, and the outer membrane. pssTNOP genes identified in Rhizobium leguminosarum bv. trifolii strain TA1 encode proteins that might be components of the EPS polymerization and secretion system. In this study, we have characterized PssN protein. Employing pssN-phoA and pssN-lacZ gene fusions and in vivo acylation with [3H]palmitate, we demonstrated that PssN is a 43-kDa lipoprotein directed to the periplasm by an N-terminal signal sequence. Membrane detergent fractionation followed by sucrose gradient centrifugation showed that PssN is an outer membrane-associated protein. Indirect immunofluorescence with anti-PssN and fluorescein isothiocyanate-conjugated antibodies and protease digestion of spheroplasts and intact cells of TA1 provided evidence that PssN is oriented towards the periplasmic space. Chemical cross-linking of TA1 and E. coli cells overproducing PssN-His6 protein showed that PssN might exist as a homo-oligomer of at least two monomers. Investigation of the secondary structure of purified PssN-His6 protein by Fourier transform infrared spectroscopy revealed the predominant presence of beta-structure; however, alpha-helices were also detected. Influence of an increased amount of PssN protein on the TA1 phenotype was assessed and correlated with a moderate enhancement of EPS production.

Syntenic arrangements of the surface polysaccharide biosynthesis genes in Rhizobium leguminosarum

We applied a genomic approach in the identification of genes required for the biosynthesis of different polysaccharides in Rhizobium leguminosarum bv. trifolii TA1 (RtTA1). Pulsed-field gel electrophoresis analyses of undigested genomic DNA revealed that the RtTA1 genome is partitioned into a chromosome and four large plasmids. The combination of sequencing of RtTA1 library BAC clones and PCR amplification of polysaccharide genes from the RtTA1 genome led to the identification of five large regions and clusters, as well as many separate potential polysaccharide biosynthesis genes dispersed in the genome. We observed an apparent abundance of genes possibly linked to lipopolysaccharide biosynthesis. All RtTA1 polysaccharide biosynthesis regions showed a high degree of conserved synteny between R. leguminosarum bv. viciae and/or Rhizobium etli. A majority of the genes displaying a conserved order also showed high sequence identity levels.

Membrane topology of the chromate transporter ChrA ofPseudomonas aeruginosa

The membrane topology of the plasmid-encoded Pseudomonas aeruginosa ChrA protein, which effluxes chromate ions, was determined by the analysis of translational fusions with reporter enzymes alkaline phosphatase and beta-galactosidase. A novel 13-TMS (transmembrane segments) topology, with the N-terminus located in the cytoplasm and the C-terminus in the periplasmic space, was consistent with the enzyme activities determined in both Escherichia coli and P. aeruginosa. Alignment of the two halves of ChrA showed significant sequence homology, with TMS I, II, III, IV, V and VI displaying similarity to TMS VIII, IX, X, XI, XII and XIII, respectively, although with opposite membrane orientations. This suggests that ChrA arose from the duplication of a gene encoding a 6-TMS ancestral protein, followed by the insertion of extra TMS VII. These data also suggest that the two halves of ChrA may carry out distinct functions for the transport of chromate.

Proteins exported via the PrsD-PrsE type I secretion system and the acidic exopolysaccharide are involved in biofilm formation by Rhizobium leguminosarum

The type I protein secretion system of Rhizobium leguminosarum bv. viciae encoded by the prsD and prsE genes is responsible for secretion of the exopolysaccharide (EPS)-glycanases PlyA and PlyB. The formation of a ring of biofilm on the surface of the glass in shaken cultures by both the prsD and prsE secretion mutants was greatly affected. Confocal laser scanning microscopy analysis of green-fluorescent-protein-labeled bacteria showed that during growth in minimal medium, R. leguminosarum wild type developed microcolonies, which progress to a characteristic three-dimensional biofilm structure. However, the prsD and prsE secretion mutants were able to form only an immature biofilm structure. A mutant disrupted in the EPS-glycanase plyB gene showed altered timing of biofilm formation, and its structure was atypical. A mutation in an essential gene for EPS synthesis (pssA) or deletion of several other pss genes involved in EPS synthesis completely abolished the ability of R. leguminosarum to develop a biofilm. Extracellular complementation studies of mixed bacterial cultures confirmed the role of the EPS and the modulation of the biofilm structure by the PrsD-PrsE secreted proteins. Protein analysis identified several additional proteins secreted by the PrsD-PrsE secretion system, and N-terminal sequencing revealed peptides homologous to the N termini of proteins from the Rap family (Rhizobium adhering proteins), which could have roles in cellular adhesion in R. leguminosarum. We propose a model for R. leguminosarum in which synthesis of the EPS leads the formation of a biofilm and several PrsD-PrsE secreted proteins are involved in different aspects of biofilm maturation, such as modulation of the EPS length or mediating attachment between bacteria.

Rhizobial exopolysaccharides: genetic control and symbiotic functions

Specific complex interactions between soil bacteria belonging to Rhizobium, Sinorhizobium, Mesorhizobium, Phylorhizobium, Bradyrhizobium and Azorhizobium commonly known as rhizobia, and their host leguminous plants result in development of root nodules. Nodules are new organs that consist mainly of plant cells infected with bacteroids that provide the host plant with fixed nitrogen. Proper nodule development requires the synthesis and perception of signal molecules such as lipochitooligosaccharides, called Nod factors that are important for induction of nodule development. Bacterial surface polysaccharides are also crucial for establishment of successful symbiosis with legumes. Sugar polymers of rhizobia are composed of a number of different polysaccharides, such as lipopolysaccharides (LPS), capsular polysaccharides (CPS or K-antigens), neutral β-1, 2-glucans and acidic extracellular polysaccharides (EPS). Despite extensive research, the molecular function of the surface polysaccharides in symbiosis remains unclear.

This review focuses on exopolysaccharides that are especially important for the invasion that leads to formation of indetermined (with persistent meristem) type of nodules on legumes such as clover, vetch, peas or alfalfa. The significance of EPS synthesis in symbiotic interactions of Rhizobium leguminosarum with clover is especially noticed. Accumulating data suggest that exopolysaccharides may be involved in invasion and nodule development, bacterial release from infection threads, bacteroid development, suppression of plant defense response and protection against plant antimicrobial compounds. Rhizobial exopolysaccharides are species-specific heteropolysaccharide polymers composed of common sugars that are substituted with non-carbohydrate residues. Synthesis of repeating units of exopolysaccharide, their modification, polymerization and export to the cell surface is controlled by clusters of genes, named exo/exs, exp or pss that are localized on rhizobial megaplasmids or chromosome. The function of these genes was identified by isolation and characterization of several mutants disabled in exopolysaccharide synthesis. The effect of exopolysaccharide deficiency on nodule development has been extensively studied. Production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen or sulphur. There is a strong suggestion that production of a variety of symbiotically active polysaccharides may allow rhizobial strains to adapt to changing environmental conditions and interact efficiently with legumes.

Topological and transcriptional analysis of pssL gene product: a putative Wzx-like exopolysaccharide translocase in Rhizobium leguminosarum bv. trifolii TA1

An identified pssL gene is yet another one, besides the pssT, pssN and pssP genes, encoding for a protein engaged in polysaccharide polymerization and export in Rhizobium leguminosarum bv. trifolii strain TA1 (RtTA1). Amino acid sequence similarity and hypothetical protein secondary structure placed the PssL protein within Wzx (RfbX) translocases with putative flippase function that belong to the polysaccharide specific transport (PST) family. The predicted secondary structure of the PssL membrane protein was examined with a series of PssL-PhoA and PssL-LacZ translational fusions. The results support the hypothesis of PssL being a member of PST protein family comprising transporters with 12 membrane spanning segments and amino and carboxyl termini located in the cytoplasm. Results of semi-quantitative RT-PCR showed that the initial abundance of mRNA encoding PssL protein was relatively lower when compared to the quantity of the previously identified PssT membrane protein. PssL might be a good candidate for Wzx-like protein that together with PssT (Wzy protein) could be responsible for Wzx/Wzy-like-dependent EPS polymerization and translocation in RtTA1.

PnuC and the utilization of the nicotinamide riboside analog 3-aminopyridine in Haemophilus influenzae

The utilization pathway for the uptake of NAD and nicotinamide riboside was previously characterized for Haemophilus influenzae. We now report on the cellular location, topology, and substrate specificity of PnuC. pnuC of H. influenzae is only distantly related to pnuC of Escherichia coli and Salmonella enterica serovar Typhimurium. When E. coli PnuC was expressed in an H. influenzae pnuC mutant, it was able to take up only nicotinamide riboside and not nicotinamide mononucleotide. Therefore, we postulated that PnuC transporters in general possess specificity for nicotinamide riboside. Earlier studies showed that 3-aminopyridine derivatives (e.g., 3-aminopyridine adenine dinucleotide) are inhibitory for H. influenzae growth. By testing characterized strains with mutations in the NAD utilization pathway, we show that 3-aminopyridine riboside is inhibitory to H. influenzae and is taken up by the NAD-processing and nicotinamide riboside route. 3-Aminopyridine riboside is utilized effectively in a pnuC+ background. In addition, we demonstrate that 3-aminopyridine adenine dinucleotide resynthesis is produced by NadR. 3-Aminopyridine riboside-resistant H. influenzae isolates were characterized, and mutations in nadR could be detected. We also tested other species of the family Pasteurellaceae, Pasteurella multocida and Actinobacillus actinomycetemcomitans, and found that 3-aminopyridine riboside does not act as a growth inhibitor; hence, 3-aminopyridine riboside represents an anti-infective agent with a very narrow host range.

Membrane topology analysis of cyclic glucan synthase, a virulence determinant of Brucella abortus

Brucella abortus cyclic glucan synthase (Cgs) is a 316-kDa (2,831-amino-acid) integral inner membrane protein that is responsible for the synthesis of cyclic beta-1,2-glucan by a novel mechanism in which the enzyme itself acts as a protein intermediate. B. abortus Cgs uses UDP-glucose as a sugar donor and has the three enzymatic activities necessary for synthesis of the cyclic polysaccharide (i.e., initiation, elongation, and cyclization). Cyclic glucan is required in B. abortus for effective host interaction and complete expression of virulence. To gain further insight into the structure and mechanism of action of B. abortus Cgs, we studied the membrane topology of the protein using a combination of in silico predictions, a genetic approach involving the construction of fusions between the cgs gene and the genes encoding alkaline phosphatase (phoA) and beta-galactosidase (lacZ), and site-directed chemical labeling of lysine residues. We found that B. abortus Cgs is a polytopic membrane protein with the amino and carboxyl termini located in the cytoplasm and with six transmembrane segments, transmembrane segments I (residues 419 to 441), II (residues 452 to 474), III (residues 819 to 841), IV (residues 847 to 869), V (residues 939 to 961), and VI (residues 968 to 990). The six transmembrane segments determine four large cytoplasmic domains and three very small periplasmic regions.

Complexity of phenotypes and symbiotic behaviour of Rhizobium leguminosarum biovar trifolii exopolysaccharide mutants

Rhizobium leguminosarum biovar trifolii strain TA1 polysaccharide synthesis (pss) mutants in the pssD, pssP, pssT and pssO genes and altered in exopolysaccharide (EPS) synthesis were investigated. EPS-deficient mutants were also changed in lipopolysaccharide structure. All mutants exhibited varied sensitivities to detergents, ethanol and antibiotics, thus indicating changes in bacterial membrane integrity. Using pss mutants marked with the gusA gene, EPS-deficient mutants were found to have abnormalities in nodule development and to provoke severe plant defence reactions. The pss mutants that produced altered quantities of EPS with a changed degree of polymerisation generally occupied the younger developmental zones of the nodules and elicited moderate plant defence reactions.

Environmental modulation of the pssTNOP gene expression in Rhizobium leguminosarum bv. trifolii

Exopolysaccharide production by Rhizobium leguminosarum bv. trifolii is required for successful establishment of nitrogen-fixing symbiosis with clover (Trifolium pratense L.). Using plasmid-borne transcriptional fusions of promoters of pss genes with promoterless lacZ the effect of root exudate, phosphate, and ammonia on expression of pssT, pssN, pssO, and pssP genes in wild-type strain RtTA1 background was determined. A stimulating effect of these environmental factors on pssO and pssP gene expression was observed. The putative pssO gene promoter was determined to be a strong promoter within which the divergent nod-box element was identified. The pssO promoter was slightly inducible in a flavonoid-dependent manner in wild-type R. leguminosarum bv. trifolii strains RtTA1 and ANU843 and very weakly active in a mutant of strain ANU843 that lacks the regulatory nodD gene. The expression of pssO and pssP genes in planta was investigated using plasmid-borne pssO-gusA and pssP-gusA fusions under different phosphate availability to clover. The level of pssO-gusA fusion expression was shown to be dependent on phosphate concentration in the plant growth medium.