Homologs of the Rml enzymes from Salmonella enterica are responsible for dTDP-beta-L-rhamnose biosynthesis in the gram-positive thermophile Aneurinibacillus thermoaerophilus DSM 10155

Shumkonia mesophila gen. nov., sp. nov., a novel representative of Shumkoniaceae fam. nov. and its potentials for extracellular polymeric substances formation and sulfur metabolism revealed by genomic analysis

A microaerophilic, mesophilic, chemoorganoheterotrophic bacterium, designated Y-P2T, was isolated from oil sludge enrichment in China. Cells of the strain were Gram-stain-negative, non-motile, non-spore-forming, rod-shaped or slightly curved with 0.8-3.0 µm in length and 0.4-0.6 µm in diameter. The strain Y-P2T grew optimally at 25 °C (range from 15 to 30 °C) and pH 7.0 (range from pH 6.0 to 7.5) without NaCl. The major cellular fatty acids were C16:0, summed feature 3 (C16:1 ω7c and/or C16:1 ω6c), summed feature 8 (C18:1 ω7c and/or C18:1 ω6c). The main polar liquids of strain Y-P2T comprised phosphatidylethanolamine (PE) and phosphatidylglycerol (PG). The respiratory quinone was Q-10. Acetate and H2 were the fermentation products of glucose. The DNA G + C content was 66.0%. Strain Y-P2T shared the highest 16S rRNA gene sequence similarity (90.3-90.6%) with species within Oceanibaculum of family Thalassobaculaceae in Rhodospirillales. Phylogenetic analyses based on 16S rRNA gene sequences and genomes showed that strain Y-P2T formed a distinct evolutionary lineage within the order Rhodospirillales. On the basis of phenotypic, phylogenetic and phylogenomic data, we propose that strain Y-P2T represents a novel species in a novel genus, for which Shumkonia mesophila gen. nov., sp. nov., within a new family Shumkoniaceae fam. nov. The type strain is Y-P2T (= CCAM 826 T = JCM 34766 T).

Novel dTDP-l-Rhamnose Synthetic Enzymes (RmlABCD) From Saccharothrix syringae CGMCC 4.1716 for One-Pot Four-Enzyme Synthesis of dTDP-l-Rhamnose

Deoxythymidine diphospho-l-rhamnose (dTDP-l-rhamnose) is used by prokaryotic rhamnosyltransferases as the glycosyl donor for the synthesis of rhamnose-containing polysaccharides and compounds that have potential in pharmaceutical development, so its efficient synthesis has attracted much attention. In this study, we successfully cloned four putative dTDP-l-rhamnose synthesis genes Ss-rmlABCD from Saccharothrix syringae CGMCC 4.1716 and expressed them in Escherichia coli. The recombinant enzymes, Ss-RmlA (glucose-1-phosphate thymidylyltransferase), Ss-RmlB (dTDP-d-glucose 4,6-dehydratase), Ss-RmlC (dTDP-4-keto-6-deoxy-glucose 3,5-epimerase), and Ss-RmlD (dTDP-4-keto-rhamnose reductase), were confirmed to catalyze the sequential formation of dTDP-l-rhamnose from deoxythymidine triphosphate (dTTP) and glucose-1-phosphate (Glc-1-P). Ss-RmlA showed maximal enzyme activity at 37°C and pH 9.0 with 2.5mMMg2+, and the K m and k cat values for dTTP and Glc-1-P were 49.56μM and 5.39s-1, and 117.30μM and 3.46s-1, respectively. Ss-RmlA was promiscuous in the substrate choice and it could use three nucleoside triphosphates (dTTP, dUTP, and UTP) and three sugar-1-Ps (Glc-1-P, GlcNH2-1-P, and GlcN3-1-P) to form nine sugar nucleotides (dTDP-GlcNH2, dTDP-GlcN3, UDP-Glc, UDP-GlcNH2, UDP-GlcN3, dUDP-Glc, dUDP-GlcNH2, and dUDP-GlcN3). Ss-RmlB showed maximal enzyme activity at 50°C and pH 7.5 with 0.02mM NAD+, and the K m and k cat values for dTDP-glucose were 98.60μM and 11.2s-1, respectively. A one-pot four-enzyme reaction system was developed by simultaneously mixing all of the substrates, reagents, and four enzymes Ss-RmlABCD in one pot for the synthesis of dTDP-l-rhamnose and dUDP-l-rhamnose with the maximal yield of 65% and 46%, respectively, under the optimal conditions. dUDP-l-rhamnose was a novel nucleotide-activated rhamnose reported for the first time.

Acute cadmium toxicity and post-stress recovery: Insights into coordinated and integrated response/recovery strategies of Anabaena sp. PCC 7120

Cyanobacteria, the first photoautotrophs have remarkable adaptive capabilities against most abiotic stresses, including Cd. A model cyanobacterium, Anabaena sp. PCC 7120 has been commonly used to understand cyanobacterial plasticity under different environmental stresses. However, very few studies have focused on the acute Cd toxicity. In this context, Anabaena was subjected to 100 μM Cd for 48 h (acute Cd stress, ACdS) and then transferred into the fresh medium for post-stress recovery (PSR). We further investigated the dynamics of morpho-ultrastructure, physiology, cytosolic proteome, thylakoidal complexes, chelators, and transporters after ACdS, as well as during early (ER), mid (MR), and late (LR) phases of PSR. The findings revealed that ACdS induced intracellular Cd accumulation and ROS production, altered morpho-ultrastructure, reduced photosynthetic pigments, and affected the structural organization of PSII, which subsequently hindered photosynthetic efficiency. Anabaena responded to ACdS and recovered during PSR by reprogramming the expression pattern of proteins/genes involved in cellular defense and repair; CO₂ access, Calvin-Benson cycle, glycolysis, and pentose phosphate pathway; protein biosynthesis, folding, and degradation; regulatory functions; PSI-based cyclic electron flow; Cd chelation; and efflux. These modulations occurred in an integrated and coordinated manner that facilitated Anabaena to detoxify Cd and repair ACdS-induced cellular damage.

Mutations in Sugar-Nucleotide Synthesis Genes Restore Holdfast Polysaccharide Anchoring to Caulobacter crescentus Holdfast Anchor Mutants

Attachment is essential for microorganisms to establish interactions with both biotic and abiotic surfaces. Stable attachment of Caulobacter crescentus to surfaces requires an adhesive polysaccharide holdfast, but the exact composition of the holdfast is unknown. The holdfast is anchored to the cell envelope by outer membrane proteins HfaA, HfaB, and HfaD. Holdfast anchor gene mutations result in holdfast shedding and reduced cell adherence. Translocation of HfaA and HfaD to the cell surface requires HfaB. The Wzx homolog HfsF is predicted to be a bacterial polysaccharide flippase. An hfsF deletion significantly reduced the amount of holdfast produced per cell and slightly reduced adherence. A ΔhfsF ΔhfaD double mutant was completely deficient in adherence. A suppressor screen that restored adhesion in the ΔhfsF ΔhfaD mutant identified mutations in three genes: wbqV, rfbB, and rmlA Both WbqV and RfbB belong to a family of nucleoside-diphosphate epimerases, and RmlA has similarity to nucleotidyltransferases. The loss of wbqV or rfbB in the ΔhfsF ΔhfaD mutant reduced holdfast shedding but did not restore holdfast synthesis to parental levels. Loss of wbqV or rfbB did not restore adherence to a ΔhfsF mutant but did restore adherence and holdfast anchoring to a ΔhfaD mutant, confirming that suppression occurs through restoration of holdfast anchoring. The adherence and holdfast anchoring of a ΔhfaA ΔhfaD mutant could be restored by wbqV or rfbB mutation, but such mutations could not suppress these phenotypes in the ΔhfaB mutant. We hypothesize that HfaB plays an additional role in holdfast anchoring or helps to translocate an unknown factor that is important for holdfast anchoring.IMPORTANCE Biofilm formation results in increased resistance to both environmental stresses and antibiotics. Caulobacter crescentus requires an adhesive holdfast for permanent attachment and biofilm formation, but the exact mechanism of polysaccharide anchoring to the cell and the holdfast composition are unknown. Here we identify novel polysaccharide genes that affect holdfast anchoring to the cell. We identify a new role for the holdfast anchor protein HfaB. This work increases our specific knowledge of the polysaccharide adhesin involved in Caulobacter attachment and the general knowledge regarding production and anchoring of polysaccharide adhesins by bacteria. This work also explores the interactions between different polysaccharide biosynthesis and secretion systems in bacteria.

Characterization of the dTDP-Fuc3N and dTDP-Qui3N biosynthetic pathways in Campylobacter jejuni 81116

The Gram-negative bacterium Campylobacter jejuni 81116 (Penner serotype HS:6) has a class E lipooligosaccharide (LOS) biosynthesis locus containing 19 genes, which encode for 11 putative glycosyltransferases, 1 lipid A acyltransferase and 7 enzymes thought to be involved in the biosynthesis of dideoxyhexosamine (ddHexN) moieties. Although the LOS outer core structure of C. jejuni 81116 is still unknown, recent mass spectrometry analyses suggest that it contains acetylated forms of two ddHexN residues. For this investigation, five of the genes encoding enzymes reportedly involved in the biosyntheses of these sugar residues were examined, rmlA, rmlB, wlaRA, wlaRB and wlaRG. Specifically, these genes were cloned and expressed in Escherichia coli, and the corresponding enzymes were purified and tested for biochemical activity. Here we present data demonstrating that RmlA functions as a glucose-1-phosphate thymidylyltransferase and that RmlB is a thymidine diphosphate (dTDP)-glucose 4,6-dehydratase. We also show, through nuclear magnetic resonance spectroscopy and mass spectrometry analyses, that WlaRG, when utilized in coupled assays with either WlaRA or WlaRB and dTDP-4-keto-6-deoxyglucose, results in the production of either dTDP-3-amino-3,6-dideoxy-d-galactose (dTDP-Fuc3N) or dTDP-3-amino-3,6-dideoxy-d-glucose (dTDP-Qui3N), respectively. In addition, the X-ray crystallographic structures of the 3,4-ketoisomerases, WlaRA and WlaRB, were determined to 2.14 and 2.0 Å resolutions, respectively. Taken together, the data reported herein demonstrate that C. jejuni 81116 utilizes five enzymes to synthesize dTDP-Fuc3N or dTDP-Qui3N and that WlaRG, an aminotransferase, can function on sugars with differing stereochemistry about their C-4' carbons. Importantly, the data reveal that C. jejuni 81116 has the ability to synthesize two isomeric ddHexN forms.

Structure of the Bacillus anthracis dTDP-L-rhamnose-biosynthetic enzyme dTDP-4-dehydrorhamnose reductase (RfbD)

Bacillus anthracis is the causative agent of the deadly disease Anthrax. Its use in bioterrorism and its ability to re-emerge have brought renewed interest in this organism. B. anthracis is a Gram-positive bacterium that adds L-rhamnose to its cell-wall polysaccharides using the activated donor dTDP-β-L-rhamnose. The enzymes involved in the biosynthesis of the activated donor are absent in humans, which make them ideal targets for therapeutic development to combat pathogens. Here, the 2.65 Å resolution crystal structure of the fourth enzyme in the dTDP-β-L-rhamnose-biosynthetic pathway from B. anthracis, dTDP-4-dehydro-β-L-rhamnose reductase (RfbD), is presented in complex with NADP+. This enzyme catalyzes the reduction of dTDP-4-dehydro-β-L-rhamnose to dTDP-β-L-rhamnose. Although the protein was co-crystallized in the presence of Mg2+, the protein lacks the conserved residues that coordinate Mg2+.

A 96-well microtiter plate assay for high-throughput screening of Mycobacterium tuberculosis dTDP-d-glucose 4,6-dehydratase inhibitors

Mycobacterium tuberculosis dTDP-d-glucose 4,6-dehydratase (RmlB) is the second enzyme for the biosynthesis of dTDP-l-rhamnose, which is a sugar donor to the synthesis of the cell wall linker, d-N-acetylglucosamine-l-rhamnose. RmlB is essential to mycobacterial growth and is not found in humans; therefore, it is a potential target for developing new anti-tuberculosis drugs. So far, there has been no suitable method for high-throughput screening of RmlB inhibitors. Here, the recombinant M. tuberculosis RmlB was purified and an absorbance-based microtiter plate assay was developed for RmlB activity. It could be used for high-throughput screening of RmlB inhibitors. The kinetic properties of M. tuberculosis RmlB, including optimal pH, optimal temperature, the effect of metal ions, and the kinetic parameters, were determined with this assay. The inhibitory effects of dTTP and dTDP on M. tuberculosis RmlB were also studied with the assay.

One-pot four-enzyme synthesis of thymidinediphosphate-l-rhamnose

A concise and effective one-pot four-enzyme synthesis of dTDP-Rha, the substrate of rhamnosyltransferases, is described.

Bacterial cell-envelope glycoconjugates

Prokaryotic glycosylation fulfills an important role in maintaining and protecting the structural integrity and function of the bacterial cell wall, as well as serving as a flexible adaption mechanism to evade environmental and host-induced pressure. The scope of bacterial and archaeal protein glycosylation has considerably expanded over the past decade(s), with numerous examples covering the glycosylation of flagella, pili, glycosylated enzymes, as well as surface-layer proteins. This article addresses structure, analysis, function, genetic basis, biosynthesis, and biomedical and biotechnological applications of cell-envelope glycoconjugates, S-layer glycoprotein glycans, and "nonclassical" secondary-cell wall polysaccharides. The latter group of polymers mediates the important attachment and regular orientation of the S-layer to the cell wall. The structures of these glycopolymers reveal an enormous diversity, resembling the structural variability of bacterial lipopolysaccharides and capsular polysaccharides. While most examples are presented for Gram-positive bacteria, the S-layer glycan of the Gram-negative pathogen Tannerella forsythia is also discussed. In addition, archaeal S-layer glycoproteins are briefly summarized.

Genetic and biochemical characterizations of enzymes involved in Streptococcus pneumoniae serotype 2 capsule synthesis demonstrate that Cps2T (WchF) catalyzes the committed step by addition of β1-4 rhamnose, the second sugar residue in the repeat unit

Five genes (cps2E, cps2T, cps2F, cps2G, and cps2I) are predicted to encode the glycosyltransferases responsible for synthesis of the Streptococcus pneumoniae serotype 2 capsule repeat unit, which is polymerized to yield a branched surface structure containing glucose-glucuronic acid linked to a glucose-rhamnose-rhamnose-rhamnose backbone. Cps2E is the initiating glycosyltransferase, but experimental evidence supporting the functions of the remaining glycosyltransferases is lacking. To biochemically characterize the glycosyltransferases, the donor substrate dTDP-rhamnose was first synthesized using recombinant S. pneumoniae enzymes Cps2L, Cps2M, Cps2N, and Cps2O. In in vitro assays with each of the glycosyltransferases, only reaction mixtures containing recombinant Cps2T, dTDP-rhamnose, and the Cps2E product (undecaprenyl pyrophosphate glucose) generated a new product, which was consistent with lipid-linked glucose-rhamnose. cps2T, cps2F, and cps2I deletion mutants produced no detectable capsule, but trace amounts of capsule were detectable in Δcps2G mutants, suggesting that Cps2G adds a nonbackbone sugar. All Δcps2F, Δcps2G, and Δcps2I mutants contained different secondary suppressor mutations in cps2E, indicating that the initial mutations were lethal in the absence of reduced repeat unit synthesis. Δcps2T mutants did not contain secondary mutations affecting capsule synthesis. The requirement for secondary mutations in mutants lacking Cps2F, Cps2G, and Cps2I indicates that these activities occur downstream of the committed step in capsule synthesis and reveal that Cps2T catalyzes this step. Therefore, Cps2T is the β1-4 rhamnosyltransferase that adds the second sugar to the repeat unit and, as the committed step in type 2 repeat unit synthesis, is predicted to be an important point of capsule regulation.

Biosynthesis of UDP-glucuronic acid and UDP-galacturonic acid in Bacillus cereus subsp. cytotoxis NVH 391-98

The food borne pathogen Bacillus cereus produces uronic acid-containing glycans that are secreted in a shielding biofilm environment, and certain alkaliphilic Bacillus deposit uronate-glycan polymers in the cell wall when adapting to alkaline environments. The source of these acidic sugars is unknown and, in the present study, we describe the functional identification of an operon in Bacillus cerues subsp. cytotoxis NVH 391-98 that comprises genes involved in the synthesis of UDP-uronic acids in Bacillus spp. Within the operon, a UDP-glucose 6-dehydrogenase converts UDP-glucose in the presence of NAD(+) to UDP-glucuronic acid and NADH, and a UDP-GlcA 4-epimerase (UGlcAE) converts UDP-glucuronic acid to UDP-galacturonic acid. Interestingly, in vitro, both enzymes can utilize the TDP-sugar forms as well, albeit at lower catalytic efficiency. Unlike most of the very few bacterial 4-epimerases that have been characterized, which are promiscuous, the B. cereus UGlcAE enzyme is very specific and cannot use UDP-glucose, UDP-N-acetylglucosamine, UDP-N-acetylglucosaminuronic acid or UDP-xylose as substrates. Size exclusion chromatography suggests that UGlcAE is active as a monomer, unlike the dimeric form of plant enzymes; the Bacillus UDP-glucose 6-dehydrogenase is also found as a monomer. Phylogenic analysis further suggests that the Bacillus UGlcAE may have evolved separately from other bacterial and plant epimerases. Our results provide insight into the formation and function of uronic acid-containing glycans in the lifecycle of B. cereus and related species containing homologous operons, as well as a basis for determining the importance of these acidic glycans. We also discuss the ability to target UGlcAE as a drug candidate.

Development of a colorimetric assay and kinetic analysis for Mycobacterium tuberculosis D-glucose-1-phosphate thymidylyltransferase

dTDP-L-rhamnose as a sugar donor provides L-rhamnosyl residue in the synthesis of disaccharide linker (D-N-acetylglucosamine-L-rhamnose), the key structure of the Mycobacterium tuberculosis cell wall. Four enzymes are involved in the formation of dTDP-L-rhamnose and D-glucose-1-phosphate thymidylyltransferase (RmlA) catalyzes the first step of D-glucose-1-phosphate and dTTP to dTDP-D-glucose and PPi. The previous studies on RmlA essentiality proved RmlA as a potential target for antituberculosis drugs. However, there has not been a suitable assay for RmlA to screen inhibitors currently. In this study, the authors reported a microtiter plate-based colorimetric assay for RmlA enzyme activity. Using this assay, the kinetic properties of M. tuberculosis RmlA including initial velocity, optimal temperature, optimal pH, the effect of Mg(2+), and kinetic parameters were determined. The establishment of the accurate and rapid colorimetric assay and kinetic analysis of M. tuberculosis RmlA will facilitate high-throughput screening of RmlA inhibitors.

The lipopolysaccharide of the mastitis isolate Escherichia coli strain 1303 comprises a novel O-antigen and the rare K-12 core type

Mastitis represents one of the most significant health problems of dairy herds. The two major causative agents of this disease are Escherichia coli and Staphylococcus aureus. Of the first, its lipopolysaccharide (LPS) is thought to play a prominent role during infection. Here, we report the O-antigen (OPS, O-specific polysaccharide) structure of the LPS from bovine mastitis isolate E. coli 1303. The structure was determined utilizing chemical analyses, mass spectrometry, and 1D and 2D NMR spectroscopy methods. The O-repeating unit was characterized as -[→4)-β-d-Quip3NAc-(1→3)-α-l-Fucp2OAc-(1→4)-β-d-Galp-(1→3)-α-d-GalpNAc-(1→]- in which the O-acetyl substitution was non-stoichiometric. The nucleotide sequence of the O-antigen gene cluster of E. coli 1303 was also determined. This cluster, located between the gnd and galF genes, contains 13 putative open reading frames, most of which represent unknown nucleotide sequences that have not been described before. The O-antigen of E. coli 1303 was shown to substitute O-7 of the terminal ld-heptose of the K-12 core oligosaccharide. Interestingly, the non-OPS-substituted core oligosaccharide represented a truncated version of the K-12 outer core – namely terminal ld-heptose and glucose were missing; however, it possessed a third Kdo residue in the inner core. On the basis of structural and genetic data we show that the mastitis isolate E. coli 1303 represents a new serotype and possesses the K-12 core type, which is rather uncommon among human and bovine isolates.

The s-layer glycome-adding to the sugar coat of bacteria

The amazing repertoire of glycoconjugates present on bacterial cell surfaces includes lipopolysaccharides, capsular polysaccharides, lipooligosaccharides, exopolysaccharides, and glycoproteins. While the former are constituents of Gram-negative cells, we review here the cell surface S-layer glycoproteins of Gram-positive bacteria. S-layer glycoproteins have the unique feature of self-assembling into 2D lattices providing a display matrix for glycans with periodicity at the nanometer scale. Typically, bacterial S-layer glycans are O-glycosidically linked to serine, threonine, or tyrosine residues, and they rely on a much wider variety of constituents, glycosidic linkage types, and structures than their eukaryotic counterparts. As the S-layer glycome of several bacteria is unravelling, a picture of how S-layer glycoproteins are biosynthesized is evolving. X-ray crystallography experiments allowed first insights into the catalysis mechanism of selected enzymes. In the future, it will be exciting to fully exploit the S-layer glycome for glycoengineering purposes and to link it to the bacterial interactome.

Protein tyrosine O-glycosylation--a rather unexplored prokaryotic glycosylation system

Glycosylation is a frequent and heterogeneous posttranslational protein modification occurring in all domains of life. While protein N-glycosylation at asparagine and O-glycosylation at serine, threonine or hydroxyproline residues have been studied in great detail, only few data are available on O-glycosidic attachment of glycans to the amino acid tyrosine. In this study, we describe the identification and characterization of a bacterial protein tyrosine O-glycosylation system. In the Gram-positive, mesophilic bacterium Paenibacillus alvei CCM 2051(T), a polysaccharide consisting of [-->3)-beta-d-Galp-(1[alpha-d-Glcp-(1-->6)] -->4)-beta-d-ManpNAc-(1-->] repeating units is O-glycosidically linked via an adaptor with the structure -[GroA-2-->OPO(2)-->4-beta-d-ManpNAc-(1-->4)] -->3)-alpha-l-Rhap-(1-->3)-alpha-l-Rhap-(1-->3)-alpha-l-Rhap-(1-->3)-beta-d-Galp-(1--> to specific tyrosine residues of the S-layer protein SpaA. A +AH4-24.3-kb S-layer glycosylation (slg) gene cluster encodes the information necessary for the biosynthesis of this glycan chain within 18 open reading frames (ORF). The corresponding translation products are involved in the biosynthesis of nucleotide-activated monosaccharides, assembly and export as well as in the transfer of the completed polysaccharide chain to the S-layer target protein. All ORFs of the cluster, except those encoding the nucleotide sugar biosynthesis enzymes and the ATP binding cassette (ABC) transporter integral transmembrane proteins, were disrupted by the insertion of the mobile group II intron Ll.LtrB, and S-layer glycoproteins produced in mutant backgrounds were analyzed by mass spectrometry. There is evidence that the glycan chain is synthesized in a process comparable to the ABC-transporter-dependent pathway of the lipopolysaccharide O-polysaccharide biosynthesis. Furthermore, with the protein WsfB, we have identified an O-oligosaccharyl:protein transferase required for the formation of the covalent beta-d-Gal-->Tyr linkage between the glycan chain and the S-layer protein.

Genetic diversity of capsular polysaccharide biosynthesis in Klebsiella pneumoniae clinical isolates

Klebsiella pneumoniae is an enteric pathogen causing community-acquired and hospital-acquired infections in humans. Epidemiological studies have revealed significant diversity in capsular polysaccharide (CPS) type and clinical manifestation of K. pneumoniae infection in different geographical areas of the world. We have sequenced the capsular polysaccharide synthesis (cps) region of seven clinical isolates and compared the sequences with the publicly available cps sequence data of five strains: NTUH-K2044 (K1 serotype), Chedid (K2 serotype), MGH78578 (K52 serotype), A1142 (K57 serotype) and A1517. Among all strains, six genes at the 5' end of the cps clusters that encode proteins for CPS transportation and processing at the bacterial surface are highly similar to each other. The central region of the cps gene clusters, which encodes proteins for polymerization and assembly of the CPS subunits, is highly divergent. Based on the collected sequence, we found that either the wbaP gene or the wcaJ gene exists in a given K. pneumoniae strain, suggesting that there is a major difference in the CPS biosynthesis pathway and that the K. pneumoniae strains can be classified into at least two distinct groups. All isolates contain gnd, encoding gluconate-6-phosphate dehydrogenase, at the 3' end of the cps gene clusters. The rmlBADC genes were found in CPS K9-positive, K14-positive and K52-positive strains, while manC and manB were found in K1, K2, K5, K14, K62 and two undefined strains. Our data indicate that, while overall genomic organization is similar between different pathogenic K. pneumoniae strains, the genetic variation of the sugar moiety and polysaccharide linkage generate the diversity in CPS molecules that could help evade host immune attack.

Functional characterization and substrate specificity of spinosyn rhamnosyltransferase by in vitro reconstitution of spinosyn biosynthetic enzymes

Spinosyn, a potent insecticide, is a novel tetracyclic polyketide decorated with d-forosamine and tri-O-methyl-L-rhamnose. Spinosyn rhamnosyltransferase (SpnG) is a key biocatalyst with unique sequence identity and controls the biosynthetic maturation of spinosyn. The rhamnose is critical for the spinosyn insecticidal activity and cell wall biosynthesis of the spinosyn producer, Saccharopolyspora spinosa. In this study, we have functionally expressed and characterized SpnG and the three enzymes, Gdh, Epi, and Kre, responsible for dTDP-L-rhamnose biosynthesis in S. spinosa by purified enzymes from Escherichia coli. Most notably, the substrate specificity of SpnG was thoroughly characterized by kinetic and inhibition experiments using various NDP sugar analogs made by an in situ combination of NDP-sugar-modifying enzymes. SpnG was found to exhibit striking substrate promiscuity, yielding corresponding glycosylated variants. Moreover, the critical residues presumably involved in catalytic mechanism of Gdh and SpnG were functionally evaluated by site-directed mutagenesis. The information gained from this study has provided important insight into molecular recognition and mechanism of the enzymes, especially SpnG. The results have made possible the structure-activity characterization of SpnG, as well as the use of SpnG or its engineered form to serve as a combinatorial tool to make spinosyn analogs with altered biological activities and potency.

Stereoselective synthesis of sugar nucleotides using neighboring group participation

A straightforward, efficient method for the chemical synthesis of sugar nucleotides derived from D-mannose and L-fucose precursors is described. This synthetic strategy involves the coupling of acylated glycosyl bromides with nucleoside 5'-diphosphates, which enables the exploitation of neighboring group participation to exclusively prepare diastereomerically pure sugar nucleotides of desired 1,2-trans anomeric configuration. This is the first stereoselective direct coupling approach to sugar nucleotide synthesis. Following deprotection using triethylamine and purification via C18 reversed-phase ion-pair chromatography, UDP- and GDP-alpha-D-mannose as well as UDP- and GDP-beta-L-fucose were obtained in good yield in only four synthetic steps from D-mannose and L-fucose.

Molecular basis of S-layer glycoprotein glycan biosynthesis in Geobacillus stearothermophilus

The Gram-positive bacterium Geobacillus stearothermophilus NRS 2004/3a possesses a cell wall containing an oblique surface layer (S-layer) composed of glycoprotein subunits. O-Glycans with the structure [-->2)-alpha-L-Rhap-(1-->3)-beta-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->](n) (= 13-18), a2-O-methyl group capping the terminal repeating unit at the nonreducing end and a -->2)-alpha-L-Rhap-[(1-->3)-alpha-L-Rhap](n) (= 1-2)(1-->3)- adaptor are linked via a beta-D-Galp residue to distinct sites of the S-layer protein SgsE. S-layer glycan biosynthesis is encoded by a polycistronic slg (surface layer glycosylation) gene cluster. Four assigned glycosyltransferases named WsaC-WsaF, were investigated by a combined biochemical and NMR approach, starting from synthetic octyl-linked saccharide precursors. We demonstrate that three of the enzymes are rhamnosyltransferases that are responsible for the transfer of L-rhamnose from a dTDP-beta-L-Rha precursor to the nascent S-layer glycan, catalyzing the formation of the alpha1,3- (WsaC and WsaD) and beta1,2-linkages (WsaF) present in the adaptor saccharide and in the repeating units of the mature S-layer glycan, respectively. These enzymes work in concert with a multifunctional methylrhamnosyltransferase (WsaE). The N-terminal portion of WsaE is responsible for the S-adenosylmethionine-dependent methylation reaction of the terminal alpha1,3-linked L-rhamnose residue, and the central and C-terminal portions are involved in the transfer of L-rhamnose from dTDP-beta-L-rhamnose to the adaptor saccharide to form the alpha1,2- and alpha1,3-linkages during S-layer glycan chain elongation, with the methylation and the glycosylation reactions occurring independently. Characterization of these enzymes thus reveals the complete molecular basis for S-layer glycan biosynthesis.

Biosynthesis of dTDP-3-acetamido-3,6-dideoxy-alpha-D-glucose

Derivatives of 3-amino-3,6-dideoxyhexoses are widespread in Nature. They are part of the repeating units of lipopolysaccharide O-antigens, of the glycan moiety of S-layer (bacterial cell surface layer) glycoproteins and also of many antibiotics. In the present study, we focused on the elucidation of the biosynthesis pathway of dTDP-alpha-D-Quip3NAc (dTDP-3-acetamido-3,6-dideoxy-alpha-D-glucose) from the Gram-positive, anaerobic, thermophilic organism Thermoanaerobacterium thermosaccharolyticum E207-71, which carries Quip3NAc in its S-layer glycan. The biosynthesis of dTDP-alpha-D-Quip3NAc involves five enzymes, namely a transferase, a dehydratase, an isomerase, a transaminase and a transacetylase, and follows a pathway similar to that of dTDP-alpha-D-Fucp3NAc (dTDP-3-acetamido-3,6-dideoxy-alpha-D-galactose) biosynthesis in Aneurinibacillus thermoaerophilus L420-91(T). The ORFs (open reading frames) of interest were cloned, overexpressed in Escherichia coli and purified. To elucidate the enzymatic cascade, the different products were purified by HPLC and characterized by NMR spectroscopy. The initiating reactions catalysed by the glucose-1-phosphate thymidylyltransferase RmlA and the dTDP-D-glucose-4,6-dehydratase RmlB are well established. The subsequent isomerase was shown to be capable of forming a dTDP-3-oxo-6-deoxy-D-glucose intermediate from the RmlB product dTDP-4-oxo-6-deoxy-D-glucose, whereas the isomerase involved in the dTDP-alpha-D-Fucp3NAc pathway synthesizes dTDP-3-oxo-6-deoxy-D-galactose. The subsequent reaction steps of either pathway involve a transaminase and a transacetylase, leading to the specific production of nucleotide-activated 3-acetamido-3,6-dideoxy-alpha-D-glucose and 3-acetamido-3,6-dideoxy-alpha-D-galactose respectively. Sequence comparison of the ORFs responsible for the biosynthesis of dTDP-alpha-D-Quip3NAc revealed homologues in Gram-negative as well as in antibiotic-producing Gram-positive bacteria. There is strong evidence that the elucidated biosynthesis pathway may also be valid for LPS (lipopolysaccharide) O-antigen structures and antibiotic precursors.

S-layer nanoglycobiology of bacteria

Cell surface layers (S-layers) are common structures of the bacterial cell envelope with a lattice-like appearance that are formed by a self-assembly process. Frequently, the constituting S-layer proteins are modified with covalently linked glycan chains facing the extracellular environment. S-layer glycoproteins from organisms of the Bacillaceae family possess long, O-glycosidically linked glycans that are composed of a great variety of sugar constituents. The observed variations already exceed the display found in eukaryotic glycoproteins. Recent investigations of the S-layer protein glycosylation process at the molecular level, which has lagged behind the structural studies due to the lack of suitable molecular tools, indicated that the S-layer glycoprotein glycan biosynthesis pathway utilizes different modules of the well-known biosynthesis routes of lipopolysaccharide O-antigens. The genetic information for S-layer glycan biosynthesis is usually present in S-layer glycosylation (slg) gene clusters acting in concert with housekeeping genes. To account for the nanometer-scale cell surface display feature of bacterial S-layer glycosylation, we have coined the neologism 'nanoglycobiology'. It includes structural and biochemical aspects of S-layer glycans as well as molecular data on the machinery underlying the glycosylation event. A key aspect for the full potency of S-layer nanoglycobiology is the unique self-assembly feature of the S-layer protein matrix. Being aware that in many cases the glycan structures associated with a protein are the key to protein function, S-layer protein glycosylation will add a new and valuable component to an 'S-layer based molecular construction kit'. In our long-term research strategy, S-layer nanoglycobiology shall converge with other functional glycosylation systems to produce 'functional' S-layer neoglycoproteins for diverse applications in the fields of nanobiotechnology and vaccine technology. Recent advances in the field of S-layer nanoglycobiology have made our overall strategy a tangible aim of the near future.

Enzyme-catalyzed synthesis of furanosyl nucleotides

A bacterial alpha-d-glucopyranosyl-1-phosphate thymidylyltransferase was found to couple four hexofuranosyl-1-phosphates, as well as a pentofuranosyl-1-phosphate, with deoxythymidine 5'-triphosphate, providing access to furanosyl nucleotides. The enzymatic reaction mixtures were analyzed by electrospray ionization mass spectrometry and NMR spectroscopy to determine the anomeric stereochemistry of furanosyl nucleotide products. This is the first demonstration of a nucleotidylyltransferase discriminating between diastereomeric mixtures of sugar-1-phosphates to produce stereopure, biologically relevant furanosyl nucleotides.

Lipophilic sugar nucleotide synthesis by structure-based design of nucleotidylyltransferase substrates

Structure-based design of alkyl sugar-1-phosphates provides an efficient nucleotidylyltransferase-catalyzed synthesis of a series of new lipophilic sugar nucleotides possessing long or branched alkyl chains, thereby demonstrating the utility of nucleotidylyltransferases to catalyze the synthesis of sugar nucleotides with potential applications in lipopolysaccharide and lipoglycopeptide biosynthesis.

Biochemical characterization of dTDP-D-Qui4N and dTDP-D-Qui4NAc biosynthetic pathways in Shigella dysenteriae type 7 and Escherichia coli O7

O-antigen variation due to the presence of different types of sugars and sugar linkages is important for the survival of bacteria threatened by host immune systems. The O antigens of Shigella dysenteriae type 7 and Escherichia coli O7 contain 4-(N-acetylglycyl)amino-4,6-dideoxy-d-glucose (d-Qui4NGlyAc) and 4-acetamido-4,6-dideoxy-d-glucose (d-Qui4NAc), respectively, which are sugars not often found in studied polysaccharides. In this study, we characterized the biosynthetic pathways for dTDP-d-Qui4N and dTDP-d-Qui4NAc (the nucleotide-activated precursors of d-Qui4NGlyAc and d-Qui4NAc in O antigens). Predicted genes involved in the synthesis of the two sugars were cloned, and the gene products were overexpressed and purified as His-tagged fusion proteins. In vitro enzymatic reactions were carried out using the purified proteins, and the reaction products were analyzed by capillary electrophoresis, electrospray ionization-mass spectrometry, and nuclear magnetic resonance spectroscopy. It is shown that in S. dysenteriae type 7 and E. coli O7, dTDP-d-Qui4N is synthesized from alpha-d-glucose-1-phosphate in three reaction steps catalyzed by glucose-1-phosphate thymidyltransferase (RmlA), dTDP-d-glucose 4,6-dehydratase (RmlB), and dTDP-4-keto-6-deoxy-d-glucose aminotransferase (VioA). An additional acetyltransferase (VioB) catalyzes the conversion of dTDP-d-Qui4N into dTDP-d-Qui4NAc in E. coli O7. Kinetic parameters and some other properties of VioA and VioB are described and differences between VioA proteins from S. dysenteriae type 7 (VioA(D7)) and E. coli O7 (VioA(O7)) discussed. To our knowledge, this is the first time that functions of VioA and VioB have been biochemically characterized. This study provides valuable enzyme sources for the production of dTDP-d-Qui4N and dTDP-d-Qui4NAc, which are potentially useful in the pharmaceutical industry for drug development.

Novel biocatalysts based on S-layer self-assembly of Geobacillus stearothermophilus NRS 2004/3a: a nanobiotechnological approach

The crystalline cell-surface (S) layer sgsE of Geobacillus stearothermophilus NRS 2004/3a represents a natural protein self-assembly system with nanometer-scale periodicity that is evaluated as a combined carrier/patterning element for the conception of novel types of biocatalyst aiming at the controllable display of biocatalytic epitopes, storage stability, and reuse. The glucose-1-phosphate thymidylyltransferase RmlA is used as a model enzyme and chimeric proteins are constructed by translational fusion of rmlA to the C-terminus of truncated forms of sgsE (rSgsE (131-903), rSgsE(331-903)) and used for the construction of three principal types of biocatalysts: soluble (monomeric), self-assembled in aqueous solution, and recrystallized on negatively charged liposomes. Enzyme activity of the biocatalysts reaches up to 100 % compared to sole RmlA cloned from the same bacterium. The S-layer portion of the biocatalysts confers significantly improved shelf life to the fused enzyme without loss of activity over more than three months, and also enables biocatalyst recycling. These nanopatterned composites may open up new functional concepts for biocatalytic applications in nanobiotechnology.

Substrate spectrum of L-rhamnulose kinase related to models derived from two ternary complex structures

The enzyme L-rhamnulose kinase from Escherichia coli participates in the degradation pathway of L-rhamnose, a common natural deoxy-hexose. The structure of the enzyme in a ternary complex with its substrates ADP and L-rhamnulose has been determined at 1.55A resolution and refined to R(cryst)/R(free) values of 0.179/0.209. The result was compared with the lower resolution structure of a corresponding complex containing L-fructose instead of L-rhamnulose. In light of the two established sugar positions and conformations, a number of rare sugars have been modeled into the active center of L-rhamnulose kinase and the model structures have been compared with the known enzymatic phosphorylation rates. Rare sugars are of rising interest for the synthesis of bioactive compounds.

Functional characterization of the initiation enzyme of S-layer glycoprotein glycan biosynthesis in Geobacillus stearothermophilus NRS 2004/3a

The glycan chain of the S-layer glycoprotein of Geobacillus stearothermophilus NRS 2004/3a is composed of repeating units [-->2)-alpha-l-Rhap-(1-->3)-beta-l-Rhap-(1-->2)-alpha-l-Rhap-(1-->], with a 2-O-methyl modification of the terminal trisaccharide at the nonreducing end of the glycan chain, a core saccharide composed of two or three alpha-l-rhamnose residues, and a beta-d-galactose residue as a linker to the S-layer protein. In this study, we report the biochemical characterization of WsaP of the S-layer glycosylation gene cluster as a UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase that primes the S-layer glycoprotein glycan biosynthesis of Geobacillus stearothermophilus NRS 2004/3a. Our results demonstrate that the enzyme transfers in vitro a galactose-1-phosphate from UDP-galactose to endogenous phosphoryl-polyprenol and that the C-terminal half of WsaP carries the galactosyltransferase function, as already observed for the UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase WbaP from Salmonella enterica. To confirm the function of the enzyme, we show that WsaP is capable of reconstituting polysaccharide biosynthesis in WbaP-deficient strains of Escherichia coli and Salmonella enterica serovar Typhimurium.

Gene cloning, functional expression and secretion of the S-layer protein SgsE from Geobacillus stearothermophilus NRS 2004/3a in Lactococcus lactis

The ~93-kDa surface layer protein SgsE of Geobacillus stearothermophilus NRS 2004/3a forms a regular crystalline array providing a nanopatterned matrix for the future display of biologically relevant molecules. Lactococcus lactis NZ9000 was established as a safe expression host for the controlled targeted production of SgsE based on the broad host-range plasmid pNZ124Sph, into which the nisA promoter was introduced. SgsE devoid of its signal peptide-encoding sequence was cloned into the new vector and purified from the cytoplasm at a yield of 220 mg l- of expression culture. Secretion constructs were based on the signal peptide of the Lactobacillus brevis SlpA protein or the L. lactis Usp45 protein, allowing isolation of 95 mg of secreted rSgsE l-1. N-terminal sequencing confirmed correct processing of SgsE in L. lactis NZ9000. The ability of rSgsE to self-assemble in suspension and to recrystallize on solid supports was demonstrated by electron and atomic force microscopy.

Nodule development induced by Mesorhizobium loti mutant strains affected in polysaccharide synthesis

The role of Mesorhizobium loti surface polysaccharides on the nodulation process is not yet fully understood. In this article, we describe the nodulation phenotype of mutants affected in the synthesis of lipopolysaccharide (LPS) and beta(1,2) cyclic glucan. M. loti lpsbeta2 mutant produces LPS with reduced amount of O-antigen, whereas M. loti lpsbeta1 mutant produces LPS totally devoid of O-antigen. Both genes are clustered in the chromosome. Based on amino acid sequence homology, LPS sugar composition, and enzymatic activity, we concluded that lpsbeta2 codes for an enzyme involved in the transformation of dTDP-glucose into dTDP-rhamnose, the sugar donor of rhamnose for the synthesis of O-antigen. On the other hand, lpsbeta1 codes for a glucosyltransferase involved in the biosynthesis of the O-antigen. Although LPS mutants elicited normal nodules, both show reduced competitiveness compared with the wild type. M. loti beta(1-2) cyclic glucan synthase (cgs) mutant induces white, empty, ineffective pseudonodules in Lotus tenuis. Cgs mutant induces normal root hair curling but is unable to induce the formation of infection threads. M. loti cgs mutant was more sensitive to deoxycholate and displayed motility impairment compared with the wild-type strain. This pleiotropic effect depends on calcium concentration and temperature.

Cold Adaptation of the Antarctic Archaeon, Methanococcoides burtonii Assessed by Proteomics Using ICAT

Using isotope coded affinity tag (ICAT) chromatography and liquid chromatography-mass spectrometry, 163 proteins were identified from the cold-adapted archaeon, Methanococcoides burtonii. 14 proteins were differentially expressed during growth at 4 degrees C and 23 degrees C. Knowledge of protein abundance, protein identity and gene arrangement was used to determine mechanisms of cold adaptation. Growth temperature was found to affect proteins involved in energy generation and biosynthesis linked to methanogenesis, membrane transport, transcription and protein folding, as well as affecting the expression of two hypothetical proteins. Pooling the data from this ICAT study with data from a previous two-dimensional gel electrophoresis study highlighted consistencies and differences between the two methods, and led us to conclude that the two approaches were generally complementary. This is the first report of ICAT applied to Archaea, or for the study of cold adaptation in any organism.

Genetic organization of chromosomal S-layer glycan biosynthesis loci of Bacillaceae

S-layer glycoproteins are cell surface glycoconjugates that have been identified in archaea and in bacteria. Usually, S-layer glycoproteins assemble into regular, crystalline arrays covering the entire bacterium. Our research focuses on thermophilic Bacillaceae, which are considered a suitable model system for studying bacterial glycosylation. During the past decade, investigations of S-layer glycoproteins dealt with the elucidation of the highly variable glycan structures by a combination of chemical degradation methods and nuclear magnetic resonance spectroscopy. It was only recently that the molecular characterization of the genes governing the formation of the S-layer glycoprotein glycan chains has been initiated. The S-layer glycosylation (slg) gene clusters of four of the 11 known S-layer glycan structures from members of the Bacillaceae have now been studied. The clusters are approximately 16 to approximately 25 kb in size and transcribed as polycistronic units. They include nucleotide sugar pathway genes that are arranged as operons, sugar transferase genes, glycan processing genes, and transporter genes. So far, the biochemical functions only of the genes required for nucleotide sugar biosynthesis have been demonstrated experimentally. The presence of insertion sequences and the decrease of the G + C content at the slg locus suggest that the investigated organisms have acquired their specific S-layer glycosylation potential by lateral gene transfer. In addition, S-layer protein glycosylation requires the participation of housekeeping genes that map outside the cluster. The gene encoding the respective S-layer target protein is transcribed monocistronically and independently of the slg cluster genes. Its chromosomal location is not necessarily in close vicinity to the slg gene cluster.

Analysis of Campylobacter jejuni capsular loci reveals multiple mechanisms for the generation of structural diversity and the ability to form complex heptoses

We recently demonstrated that Campylobacter jejuni produces a capsular polysaccharide (CPS) that is the major antigenic component of the classical Penner serotyping system distinguishing Campylobacter into >60 groups. Although the wide variety of C. jejuni serotypes are suggestive of structural differences in CPS, the genetic mechanisms of such differences are unknown. In this study we sequenced biosynthetic cps regions, ranging in size from 15 to 34 kb, from selected C. jejuni strains of HS:1, HS:19, HS:23, HS:36, HS:23/36 and HS:41 serotypes. Comparison of the determined cps sequences of the HS:1, HS:19 and HS:41 strains with the sequenced strain, NCTC11168 (HS:2), provides evidence for multiple mechanisms of structural variation including exchange of capsular genes and entire clusters by horizontal transfer, gene duplication, deletion, fusion and contingency gene variation. In contrast, the HS:23, HS:36 and HS:23/36 cps sequences were highly conserved. We report the first detailed structural analysis of 81-176 (HS:23/36) and G1 (HS:1) and refine the previous structural interpretations of the HS:19, HS:23, HS:36 and HS:41 serostrains. For the first time, we demonstrate the commonality and function of a second heptose biosynthetic pathway for Campylobacter CPS independent of the pathway for lipooligosaccharide (LOS) biosynthesis and identify a novel heptosyltransferase utilized by this alternate pathway. Furthermore, we show the retention of two functional heptose isomerases in Campylobacter and the sharing of a phosphatase for both LOS and CPS heptose biosynthesis.

Synthesis of the heteropolysaccharide O antigen of Escherichia coli O52 requires an ABC transporter: structural and genetic evidence

The structural and genetic organization of the Escherichia coli O52 O antigen was studied. As identified by sugar and methylation analysis and nuclear magnetic resonance spectroscopy, the O antigen of E. coli O52 has a partially O-acetylated disaccharide repeating unit (O unit) containing D-fucofuranose and 6-deoxy-D-manno-heptopyranose, as well as a minor 6-deoxy-3-O-methylhexose (most likely, 3-O-methylfucose). The O-antigen gene cluster of E. coli O52, which is located between the galF and gnd genes, was found to contain putative genes for the synthesis of the O-antigen constituents, sugar transferase genes, and ABC-2 transporter genes. Further analysis confirmed that O52 employs an ATP-binding cassette (ABC) transporter-dependent pathway for translocation and polymerization of the O unit. This is the first report of an ABC transporter being involved in translocation of a heteropolysaccharide O antigen in E. coli. Genes specific for E. coli O52 were also identified.

S-layer glycan-specific loci on the chromosome of Geobacillus stearothermophilus NRS 2004/3a and dTDP-L-rhamnose biosynthesis potential of G. stearothermophilus strains

The approximately 16.5 kb surface layer (S-layer) glycan biosynthesis (slg) gene cluster of the Gram-positive thermophile Geobacillus stearothermophilus NRS 2004/3a has been sequenced. The cluster is located immediately downstream of the S-layer structural gene sgsE and consists of 13 ORFs that have been identified by database sequence comparisons. The cluster encodes dTDP-L-rhamnose biosynthesis (rml operon), required for building up the polyrhamnan S-layer glycan, as well as for assembly and export of the elongated glycan chain, and its transfer to the S-layer protein. This is the first report of a gene cluster likely to be involved in the glycosylation of an S-layer protein. There is evidence that this cluster is transcribed as a polycistronic unit, whereas sgsE is transcribed monocistronically. To get insights into the regulatory mechanisms underlying glycosylation of the S-layer protein, the influence of growth temperature on the S-layer was investigated in seven closely related G. stearothermophilus strains, of which only strain NRS 2004/3a possessed a glycosylated S-layer. Chromosomal DNA preparations of these strains were screened for the presence of the rml operon, because L-rhamnose is a frequent constituent of S-layer glycans. From rml-positive strains, flanking regions of the operon were sequenced. Comparison with the slg gene cluster of G. stearothermophilus NRS 2004/3a revealed sequence homologies between adjacent genes. The temperature inducibility of S-layer protein glycosylation was investigated in those strains by raising the growth temperature from 55 degrees C to 67 degrees C; no change of either the protein banding pattern or the glycan staining behaviour was observed on SDS-PAGE gels, although the sgsE transcript was several-fold more abundant at 67 degrees C. Cell-free extracts of the strains were capable of converting dTDP-D-glucose to dtdp-L-rhamnose. Taken together, the results indicate that the rml locus is highly conserved among G. stearothermophilus strains, and that in the investigated rml-containing strains, dTDP-L-rhamnose is actively synthesized in vitro. However, in contrast to previous reports for G. stearothermophilus wild-type strains, an increase in growth temperature did not switch an S-layer protein phenotype to an S-layer glycoprotein phenotype, via the de novo generation of a new S-layer gene sequence.

Biosynthesis of dTDP-3-acetamido-3,6-dideoxy-alpha-D-galactose in Aneurinibacillus thermoaerophilus L420-91T

The glycan chain of the S-layer protein of Aneurinibacillus thermoaerophilus L420-91T (DSM 10154) consists of d-rhamnose and 3-acetamido-3,6-dideoxy-d-galactose (d-Fucp3NAc). Thymidine diphosphate-activated d-Fucp3NAc serves as precursor for the assembly of structural polysaccharides in Gram-positive and Gram-negative organisms. The biosynthesis of dTDP-3-acetamido-3,6-dideoxy-alpha-d-galactose (dTDP-d-Fucp3NAc) involves five enzymes. The first two steps of the reaction are catalyzed by enzymes that are part of the well studied dTDP-l-rhamnose biosynthetic pathway, namely d-glucose-1-phosphate thymidyltransferase (RmlA) and dTDP-d-glucose-4,6-dehydratase (RmlB). The enzymes catalyzing the last three synthesis reactions have not been characterized biochemically so far. These steps include an isomerase, a transaminase, and a transacetylase. We identified all five genes involved by chromosome walking in the Gram-positive organism A. thermoaerophilus L420-91T and overexpressed the three new enzymes heterologously in Escherichia coli. The activities of these enzymes were monitored by reverse phase high performance liquid chromatography, and the intermediate products formed were characterized by 1H and 13C nuclear magnetic resonance spectroscopy analysis. Alignment of the newly identified proteins with known sequences revealed that the elucidated pathway in this Gram-positive organism may also be valid in the biosynthesis of the O-antigen of lipopolysaccharides of Gram-negative organisms. The key enzyme in the biosynthesis of dTDP-d-Fucp3NAc has been identified as an isomerase, which converts the 4-keto educt into the 3-keto product, with concomitant epimerization at C-4 to produce a 6-deoxy-d-xylo configuration. This is the first report of the functional characterization of the biosynthesis of dTDP-d-Fucp3NAc and description of a novel type of isomerase capable of synthesizing dTDP-6-deoxy-d-xylohex-3-ulose from dTDP-6-deoxy-d-xylohex-4-ulose.

Biosynthesis of 2-acetamido-2,6-dideoxy-L-hexoses in bacteria follows a pattern distinct from those of the pathways of 6-deoxy-L-hexoses

6-Deoxy-L-hexoses have been shown to be synthesized from dTDP-D-glucose or GDP-D-mannose so that the gluco/galacto-configuration is converted into the manno/talo-configuration, and manno/talo is switched to gluco/galacto. Our laboratory has been investigating the biosynthesis of 2-acetamido-2,6-dideoxy-L-hexoses in both Gram-positive and Gram-negative bacteria, and in a recent paper we described the biosynthesis of the talo (pneumosamine) and galacto (fucosamine) derivatives from UDP-D-N-acetylglucosamine a 2-acetamido sugar [Kneidinger, O'Riordan, Li, Brisson, Lee and Lam (2003) J. Biol. Chem. 278, 3615-3627]. In the present study, we undertake the task to test the hypothesis that UDP-D-N-acetylglucosamine is the common precursor for the production of 2-acetamido-2,6-dideoxy-L-hexoses in the gluco-, galacto-, manno- and talo-configurations. We present data to reveal the steps for the biosynthesis of the gluco (quinovosamine)- and manno (rhamnosamine)-configured compounds. The corresponding enzymes WbvB, WbvR and WbvD from Vibrio cholerae serotype O37 have been overexpressed and purified to near homogeneity. The enzymic reactions have been analysed by capillary electrophoresis and NMR spectroscopy. Our data have revealed a general feature of reaction cascades due to the three enzymes. First, UDP-D-N-acetylglucosamine is catalysed by the multi-functional enzyme WbvB, whereby dehydration occurs at C-4, C-6 and epimerization at C-5, C-3 to produce UDP-2-acetamido-2,6-dideoxy-L-lyxo-4-hexulose. Secondly, this intermediate is converted by the C-4 reductase, WbvR, in a stereospecific reaction to yield UDP-2-acetamido-L-rhamnose. Thirdly, UDP-2-acetamido-L-rhamnose is epimerized at C-2 to UDP-2-acetamido-L-quinovose by WbvD. Interestingly, WbvD is also an orthologue of WbjD, but not vice versa. Incubation of purified WbvD with UDP-2-acetamido-2,6-dideoxy-L-talose and analysing the reaction products by capillary electrophoresis revealed the same product peak as when WbjD was used. This sugar nucleotide is a specific substrate for WbjD and is a C-4 epimer of UDP-2-acetamido-L-rhamnose.