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

Comparative genomic analysis of the flagellin glycosylation island of the Gram-positive thermophile Geobacillus

BACKGROUND

Protein glycosylation involves the post-translational attachment of sugar chains to target proteins and has been observed in all three domains of life. Post-translational glycosylation of flagellin, the main structural protein of the flagellum, is a common characteristic among many Gram-negative bacteria and Archaea. Several distinct functions have been ascribed to flagellin glycosylation, including stabilisation and maintenance of the flagellar filament, motility, surface recognition, adhesion, and virulence. However, little is known about this trait among Gram-positive bacteria.

RESULTS

Using comparative genomic approaches the flagellin glycosylation loci of multiple strains of the Gram-positive thermophilic genus Geobacillus were identified and characterized. Eighteen of thirty-six compared strains of the genus carry these loci, which show evidence of horizontal acquisition. The Geobacillus flagellin glycosylation islands (FGIs) can be clustered into five distinct types, which are predicted to encode highly variable glycans decorated with distinct and heavily modified sugars.

CONCLUSIONS

Our comparative genomic analyses showed that, while not universal, flagellin glycosylation islands are relatively common among members of the genus Geobacillus and that the encoded flagellin glycans are highly variable. This suggests that flagellin glycosylation plays an important role in the lifestyles of members of this thermophilic genus.

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.

Elucidation of sevadicin, a novel non-ribosomal peptide secondary metabolite produced by the honey bee pathogenic bacterium Paenibacillus larvae

American foulbrood (AFB) caused by the bee pathogenic bacterium Paenibacillus larvae is the most devastating bacterial disease of honey bees worldwide. From AFB-dead larvae, pure cultures of P. larvae can normally be cultivated indicating that P. larvae is able to defend its niche against all other bacteria present. Recently, comparative genome analysis within the species P. larvae suggested the presence of gene clusters coding for multi-enzyme complexes, such as non-ribosomal peptide synthetases (NRPSs). The products of these enzyme complexes are known to have a wide range of biological activities including antibacterial activities. We here present our results on antibacterial activity exhibited by vegetative P. larvae and the identification and analysis of a novel antibacterially active P. larvae tripeptide (called sevadicin; Sev) produced by a NRPS encoded by a gene cluster found in the genome of P. larvae. Identification of Sev was ultimately achieved by comparing the secretome of wild-type P. larvae with knockout mutants of P. larvae lacking production of Sev. Subsequent mass spectrometric studies, enantiomer analytics and chemical synthesis revealed the sequence and configuration of the tripeptide, D-Phe-D-ALa-Trp, which was shown to have antibacterial activity. The relevance of our findings is discussed in respect to host-pathogen interactions.

Identification and functional analysis of the S-layer protein SplA of Paenibacillus larvae, the causative agent of American Foulbrood of honey bees

The Gram-positive, spore-forming bacterium Paenibacillus larvae is the etiological agent of American Foulbrood (AFB), a globally occurring, deathly epizootic of honey bee brood. AFB outbreaks are predominantly caused by two genotypes of P. larvae, ERIC I and ERIC II, with P. larvae ERIC II being the more virulent genotype on larval level. Recently, comparative proteome analyses have revealed that P. larvae ERIC II but not ERIC I might harbour a functional S-layer protein, named SplA. We here determine the genomic sequence of splA in both genotypes and demonstrate by in vitro self-assembly studies of recombinant and purified SplA protein in combination with electron-microscopy that SplA is a true S-layer protein self-assembling into a square 2D lattice. The existence of a functional S-layer protein is novel for this bacterial species. For elucidating the biological function of P. larvae SplA, a genetic system for disruption of gene expression in this important honey bee pathogen was developed. Subsequent analyses of in vivo biological functions of SplA were based on comparing a wild-type strain of P. larvae ERIC II with the newly constructed splA-knockout mutant of this strain. Differences in cell and colony morphology suggest that SplA is a shape-determining factor. Marked differences between P. larvae ERIC II wild-type and mutant cells with regard to (i) adhesion to primary pupal midgut cells and (ii) larval mortality as measured in exposure bioassays corroborate the assumption that the S-layer of P. larvae ERIC II is an important virulence factor. Since SplA is the first functionally proven virulence factor for this species, our data extend the knowledge of the molecular differences between these two genotypes of P. larvae and contribute to explaining the observed differences in virulence. These results present an immense advancement in our understanding of P. larvae pathogenesis.

Paenibacillus larvae is the most devastating bacterial pathogen of honey bees. However, the molecular interactions between infected larvae and P. larvae are poorly understood and little more than speculation exist concerning virulence factors. Recently, a putative S-layer protein has been identified in P. larvae. We here demonstrate that only representatives of P. larvae genotype ERIC II harbor a functional splA-gene and that SplA is a true S-layer protein with self-assembly capability. The presence of a functional S-layer protein is novel for P. larvae. When elucidating the biological function of SplA we broke new ground by establishing primary cell culture for pupal gut cells and by developing a genetic system for disruption of gene expression in this important honey bee pathogen. By using these novel methods we were able to prove that SplA serves as a shape-determining factor, mediates adhesion to host cells, and is a key virulence factor of P. larvae ERIC II. These results present an immense advancement in our understanding of P. larvae pathogenesis. Furthermore, we propose P. larvae as a model system for the analysis of the in vivo functions of S-layer proteins because P. larvae SlpA knockout-mutants retain viability and are thus suitable for functional studies.

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.

A temperature-sensitive expression system based on the Geobacillus stearothermophilus NRS 2004/3a sgsE surface-layer gene promoter

The sgsE gene coding for the S-layer (surface layer) protein in the thermophilic Gram-positive bacterium Geobacillus stearothermophilus NRS 2004/3a is strongly induced when the culture is shifted from optimal (55 degrees C) to maximally tolerable growth temperature (67 degrees C). Here, we investigated the regulation of the sgsE promoter in G. stearothermophilus and tested the function of this promoter in Bacillus subtilis. We used EGFP (enhanced green fluorescent protein) reporter constructs and found that the sgsE promoter has very low basal activity at 28 degrees C, but is approx. 20-fold induced by elevated growth temperatures (37 and 45 degrees C). The promoter confers high expression levels, as EGFP mRNA levels at 45 degrees C were approx. 120-fold more abundant than mRNA levels of the cat (chloramphenicol resistance) gene, which was transcribed from a constitutive promoter on the same plasmid. In fluorescence-microscopic and Western-blot analysis, the EGFP protein was barely detectable at 28 degrees C, whereas intermediate and high levels were detected at 37 and 45 degrees C respectively. The potential to tune expression levels of genes driven by the sgsE promoter in B. subtilis by simple temperature adjustments presents a considerable potential for its future use as high-yield protein expression system for B. subtilis.

Construction of a gene knockout system for application in Paenibacillus alvei CCM 2051T, exemplified by the S-layer glycan biosynthesis initiation enzyme WsfP

The gram-positive bacterium Paenibacillus alvei CCM 2051T is covered by an oblique surface layer (S-layer) composed of glycoprotein subunits. The S-layer O-glycan is a polymer of [-->3)-beta-D-Galp-(1[alpha-D-Glcp-(1-->6)]-->4)-beta-D-ManpNAc-(1-->] repeating units that is linked by an adaptor of -[GroA-2-->OPO2-->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. For elucidation of the mechanism governing S-layer glycan biosynthesis, a gene knockout system using bacterial mobile group II intron-mediated gene disruption was developed. The system is further based on the sgsE S-layer gene promoter of Geobacillus stearothermophilus NRS 2004/3a and on the Geobacillus-Bacillus-Escherichia coli shuttle vector pNW33N. As a target gene, wsfP, encoding a putative UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase, representing the predicted initiation enzyme of S-layer glycan biosynthesis, was disrupted. S-layer protein glycosylation was completely abolished in the insertional P. alvei CCM 2051T wsfP mutant, according to sodium dodecyl sulfate-polyacrylamide gel electrophoresis evidence and carbohydrate analysis. Glycosylation was fully restored by plasmid-based expression of wsfP in the glycan-deficient P. alvei mutant, confirming that WsfP initiates S-layer protein glycosylation. This is the first report on the successful genetic manipulation of bacterial S-layer protein glycosylation in vivo, including transformation of and heterologous gene expression and gene disruption in the model organism P. alvei CCM 2051T.

Purification, crystallization and preliminary crystallographic analysis of WsaF, an essential rhamnosyltransferase from Geobacillus stearothermophilus

The beta1,2-rhamnosyltransferase WsaF is involved in the biosynthesis of a polyrhamnan chain which is attached to the surface-layer protein from Geobacillus stearothermophilus NRS 2004/3a. The enzyme belongs to the large retaining GT4 family. To date, no structure of a rhamnosyltransferase has been published. Recombinant purified native WsaF has been crystallized, resulting in crystals that belonged to space group P2(1)2(1)2(1) with unit-cell parameters a = 50.5, b = 56.1, c = 276.8 A and diffracted to 3.0 A resolution. Selenomethionine-variant WsaF crystallized in space group P2(1) with unit-cell parameters a = 75.9, b = 75.5, c = 78.1 A and diffracted to 2.3 A resolution.

Structure, function, and evolution of bacterial ATP-binding cassette systems

SUMMARY

ATP-binding cassette (ABC) systems are universally distributed among living organisms and function in many different aspects of bacterial physiology. ABC transporters are best known for their role in the import of essential nutrients and the export of toxic molecules, but they can also mediate the transport of many other physiological substrates. In a classical transport reaction, two highly conserved ATP-binding domains or subunits couple the binding/hydrolysis of ATP to the translocation of particular substrates across the membrane, through interactions with membrane-spanning domains of the transporter. Variations on this basic theme involve soluble ABC ATP-binding proteins that couple ATP hydrolysis to nontransport processes, such as DNA repair and gene expression regulation. Insights into the structure, function, and mechanism of action of bacterial ABC proteins are reported, based on phylogenetic comparisons as well as classic biochemical and genetic approaches. The availability of an increasing number of high-resolution structures has provided a valuable framework for interpretation of recent studies, and realistic models have been proposed to explain how these fascinating molecular machines use complex dynamic processes to fulfill their numerous biological functions. These advances are also important for elucidating the mechanism of action of eukaryotic ABC proteins, because functional defects in many of them are responsible for severe human inherited diseases.

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.

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.

The role of horizontal transfer in the evolution of a highly variable lipopolysaccharide biosynthesis locus in xanthomonads that infect rice, citrus and crucifers

Background

Lipopolysaccharide (LPS) is a pathogen associated molecular pattern (PAMP) of animal and plant pathogenic bacteria. Variation at the interstrain level is common in LPS biosynthetic gene clusters of animal pathogenic bacteria. This variation has been proposed to play a role in evading the host immune system. Even though LPS is a modulator of plant defense responses, reports of interstrain variation in LPS gene clusters of plant pathogenic bacteria are rare.

Results

In this study we report the complete sequence of a variant 19.9 kb LPS locus present in the BXO8 strain of Xanthomonas oryzae pv. oryzae (Xoo), the bacterial blight pathogen of rice. This region is completely different in size, number and organization of genes from the LPS locus present in most other strains of Xoo from India and Asia. Surprisingly, except for one ORF, all the other ORFs at the BXO8 LPS locus are orthologous to the genes present at this locus in a sequenced strain of X. axonopodis pv. citri (Xac; a pathogen of citrus plants). One end of the BXO8 LPS gene cluster, comprised of ten genes, is also present in the related rice pathogen, X. oryzae pv. oryzicola (Xoc). In Xoc, the remainder of the LPS gene cluster, consisting of seven genes, is novel and unrelated to LPS gene clusters of any of the sequenced xanthomonads. We also report substantial interstrain variation suggestive of very recent horizontal gene transfer (HGT) at the LPS biosynthetic locus of Xanthomonas campestris pv. campestris (Xcc), the black rot pathogen of crucifers.

Conclusion

Our analyses indicate that HGT has altered the LPS locus during the evolution of Xanthomonas oryzae pathovars and suggest that the ancestor of all Xanthomonas oryzae pathovars had an Xac type of LPS gene cluster. Our finding of interstrain variation in two major xanthomonad pathogens infecting different hosts suggests that the LPS locus in plant pathogenic bacteria, as in animal pathogens, is under intense diversifying selection.

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.

The dTDP-4-dehydro-6-deoxyglucose reductase encoding fcd gene is part of the surface layer glycoprotein glycosylation gene cluster of Geobacillus tepidamans GS5-97T

The glycan chain of the S-layer protein of Geobacillus tepidamans GS5-97(T) consists of disaccharide repeating units composed of L-rhamnose and D-fucose, the latter being a rare constituent of prokaryotic glycoconjugates. Although biosynthesis of nucleotide-activated L-rhamnose is well established, D-fucose biosynthesis is less investigated. The conversion of alpha-D-glucose-1-phosphate into thymidine diphosphate (dTDP)-4-dehydro-6-deoxyglucose by the sequential action of RmlA (glucose-1-phosphate thymidylyltransferase) and RmlB (dTDP-glucose-4,6-dehydratase) is shared between the dTDP-D-fucose and the dTDP-L-rhamnose biosynthesis pathway. This key intermediate is processed by the dTDP-4-dehydro-6-deoxyglucose reductase Fcd to form dTDP-alpha-D-fucose. We identified the fcd gene in G. tepidamans GS5-97(T) by chromosome walking and performed functional characterization of the recombinant 308-amino acid enzyme. The in vitro activity of the enzymatic cascade (RmlB and Fcd) was monitored by high-performance liquid chromatography and the reaction product was confirmed by (1)H and (13)C nuclear magnetic resonance spectroscopy. This is the first characterization of the dTDP-alpha-D-fucopyranose biosynthesis pathway in a Gram-positive organism. fcd was identified as 1 of 20 open reading frames contained in a 17471-bp S-layer glycosylation (slg) gene cluster on the chromosome of G. tepidamans GS5-97(T). The sgtA structural gene is located immediately upstream of the slg gene cluster with an intergenic region of 247 nucleotides. By comparison of the SgtA amino acid sequence with the known glycosylation pattern of the S-layer protein SgsE of Geobacillus stearothermophilus NRS 2004/3a, two out of the proposed three glycosylation sites on SgtA could be identified by electrospray ionization quadrupole-time-of-flight mass spectrometry to be at positions Ser-792 and Thr-583.

New insights into the glycosylation of the surface layer protein SgsE from Geobacillus stearothermophilus NRS 2004/3a

The surface of Geobacillus stearothermophilus NRS 2004/3a cells is covered by an oblique surface layer (S-layer) composed of glycoprotein subunits. To this S-layer glycoprotein, elongated glycan chains are attached that are composed of [-->2)-alpha-l-Rhap-(1-->3)-beta-l-Rhap-(1-->2)-alpha-L-Rhap-(1-->] repeating units, with a 2-O-methyl modification of the terminal trisaccharide at the nonreducing end of the glycan chain and a core saccharide as linker to the S-layer protein. On sodium dodecyl sulfate-polyacrylamide gels, four bands appear, of which three represent glycosylated S-layer proteins. In the present study, nanoelectrospray ionization time-of-flight mass spectrometry (MS) and infrared matrix-assisted laser desorption/ionization orthogonal time-of-flight mass spectrometry were adapted for analysis of this high-molecular-mass and water-insoluble S-layer glycoprotein to refine insights into its glycosylation pattern. This is a prerequisite for artificial fine-tuning of S-layer glycans for nanobiotechnological applications. Optimized MS techniques allowed (i) determination of the average masses of three glycoprotein species to be 101.66 kDa, 108.68 kDa, and 115.73 kDa, (ii) assignment of nanoheterogeneity to the S-layer glycans, with the most prevalent variation between 12 and 18 trisaccharide repeating units, and the possibility of extension of the already-known -->3)-alpha-l-Rhap-(1-->3)-alpha-l-Rhap-(1--> core by one additional rhamnose residue, and (iii) identification of a third glycosylation site on the S-layer protein, at position threonine-590, in addition to the known sites threonine-620 and serine-794. The current interpretation of the S-layer glycoprotein banding pattern is that in the 101.66-kDa glycoprotein species only one glycosylation site is occupied, in the 108.68-kDa glycoprotein species two glycosylation sites are occupied, and in the 115.73-kDa glycoprotein species three glycosylation sites are occupied, while the 94.46-kDa band represents nonglycosylated S-layer protein.

Gene cluster responsible for validamycin biosynthesis in Streptomyces hygroscopicus subsp. jinggangensis 5008

A gene cluster responsible for the biosynthesis of validamycin, an aminocyclitol antibiotic widely used as a control agent for sheath blight disease of rice plants, was identified from Streptomyces hygroscopicus subsp. jinggangensis 5008 using heterologous probe acbC, a gene involved in the cyclization of D-sedoheptulose 7-phosphate to 2-epi-5-epi-valiolone of the acarbose biosynthetic gene cluster originated from Actinoplanes sp. strain SE50/110. Deletion of a 30-kb DNA fragment from this cluster in the chromosome resulted in loss of validamycin production, confirming a direct involvement of the gene cluster in the biosynthesis of this important plant protectant. A sequenced 6-kb fragment contained valA (an acbC homologue encoding a putative cyclase) as well as two additional complete open reading frames (valB and valC, encoding a putative adenyltransferase and a kinase, respectively), which are organized as an operon. The function of ValA was genetically demonstrated to be essential for validamycin production and biochemically shown to be responsible specifically for the cyclization of D-sedoheptulose 7-phosphate to 2-epi-5-epi-valiolone in vitro using the ValA protein heterologously overexpressed in E. coli. The information obtained should pave the way for further detailed analysis of the complete biosynthetic pathway, which would lead to a complete understanding of validamycin biosynthesis.

The C-terminal domain of the nucleotide-binding domain protein Wzt determines substrate specificity in the ATP-binding cassette transporter for the lipopolysaccharide O-antigens in Escherichia coli serotypes O8 and O9a

The polymannan O-antigenic polysaccharides (O-PSs) of Escherichia coli O8 and O9a are synthesized via an ATP-binding cassette (ABC) transporter-dependent pathway. The group 2 capsular polysaccharides of E. coli serve as prototypes for polysaccharide synthesis and export via this pathway. Here, we show that there are some fundamental differences between the ABC transporter-dependent pathway for O-PS biosynthesis and the capsular polysaccharide paradigm. In the capsule system, mutants lacking the ABC transporter are viable, and membranes isolated from these strains are no longer able to synthesize polymer using an endogenous acceptor. In contrast, E. coli strains carrying mutations in the membrane component (Wzm) and/or the nucleotide-binding component (Wzt) of the O8 and O9a polymannan transporters are nonviable under conditions permissive to O-PS biosynthesis and take on an aberrant elongated cell morphology. Whereas the ABC transporters for capsular polysaccharides with different structures are functionally interchangeable, the O8 and O9a exporters are specific for their cognate polymannan substrates. The E. coli O8 and O9a Wzt proteins contain a C-terminal domain not present in the corresponding nucleotide-binding protein (KpsT) from the capsule exporter. Whereas the Wzm components are functionally interchangeable, albeit with reduced efficiency, the Wzt components are not, indicating a specific role for Wzt in substrate specificity. Chimeric Wzt proteins were constructed in order to localize the region involved in substrate specificity to the C-terminal domain.

N-acetylmuramic acid as capping element of alpha-D-fucose-containing S-layer glycoprotein glycans from Geobacillus tepidamans GS5-97T

Geobacillus tepidamans GS5-97(T) is a novel Gram-positive, moderately thermophilic bacterial species that is covered by a glycosylated surface layer (S-layer) protein. The isolated and purified S-layer glycoprotein SgtA was ultrastructurally and chemically investigated and showed several novel properties. By SDS-PAGE, SgtA was separated into four distinct bands in an apparent molecular mass range of 106-166 kDa. The three high molecular mass bands gave a positive periodic acid-Schiff staining reaction, whereas the 106-kDa band was nonglycosylated. Glycosylation of SgtA was investigated by means of chemical analyses, 600-MHz nuclear magnetic resonance spectroscopy, and electrospray ionization quadrupole time-of-fight mass spectrometry. Glycopeptides obtained after Pronase digestion revealed the glycan structure [-->2)-alpha-L-Rhap-(1-->3)-alpha-D-Fucp-(1-->](n=approximately 20), with D-fucopyranose having never been identified before as a constituent of S-layer glycans. The rhamnose residue at the nonreducing end of the terminal repeating unit of the glycan chain was di-substituted. For the first time, (R)-N-acetylmuramic acid, the key component of prokaryotic peptidoglycan, was found in an alpha-linkage to carbon 3 of the terminal rhamnose residue, serving as capping motif of an S-layer glycan. In addition, that rhamnose was substituted at position 2 with a beta-N-acetylglucosamine residue. The S-layer glycan chains were bound via the trisaccharide core -->2)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1--> to carbon 3 of beta-D-galactose, which was attached in O-glycosidic linkage to serine and threonine residues of SgtA of G. tepidamans GS5-97(T).

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.

Nonreducing Terminal Modifications Determine the Chain Length of Polymannose O Antigens of Escherichia coli and Couple Chain Termination to Polymer Export via an ATP-binding Cassette Transporter

The chain length of bacterial lipopolysaccharide O antigens is regulated to give a modal distribution that is critical for pathogenesis. This paper describes the process of chain length determination in the ATP-binding cassette (ABC) transporter-dependent pathway, a pathway that is widespread among Gram-negative bacteria. Escherichia coli O8 and O9/O9a polymannans are synthesized in the cytoplasm, and an ABC transporter exports the nascent polymer across the inner membrane prior to completion of the LPS molecule. The polymannan O antigens have nonreducing terminal methyl groups. The 3-O-methyl group in serotype O8 is transferred from S-adenosylmethionine by the WbdD(O8) enzyme, and this modification terminates polymerization. Methyl groups are added to the O9a polymannan in a reaction dependent on preceding phosphorylation. The bifunctional WbdD(O9a) catalyzes both reactions, but only the kinase activity controls chain length. Chain termination occurs in a mutant lacking the ABC transporter, indicating that it precedes export. An E. coli wbdD(O9a) mutant accumulated O9a polymannan in the cytoplasm, indicating that WbdD activity coordinates polymannan chain termination with export across the inner membrane.