Role of lipid intermediate(s) in the synthesis of serogroup B Neisseria meningitidis capsular polysaccharide

AglH, a thermophilic UDP-N-acetylglucosamine-1-phosphate:dolichyl phosphate GlcNAc-1-phosphotransferase initiating protein N-glycosylation pathway in Sulfolobus acidocaldarius, is capable of complementing the eukaryal Alg7

AglH, a predicted UDP-GlcNAc-1-phosphate:dolichyl phosphate GlcNAc-1-phosphotransferase, is initiating the protein N-glycosylation pathway in the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. AglH successfully replaced the endogenous GlcNAc-1-phosphotransferase activity of Alg7 in a conditional lethal Saccharomyces cerevisiae strain, in which the first step of the eukaryal protein N-glycosylation process was repressed. This study is one of the few examples of cross-domain complementation demonstrating a conserved polyprenyl phosphate transferase reaction within the eukaryal and archaeal domain like it was demonstrated for Methanococcus voltae (Shams-Eldin et al. 2008). The topology prediction and the alignment of the AglH membrane protein with GlcNAc-1-phosphotransferases from the three domains of life show significant conservation of amino acids within the different proposed cytoplasmic loops. Alanine mutations of selected conserved amino acids in the putative cytoplasmic loops II (D₁₀₀), IV (F₂₂₀) and V (F₂₆₄) demonstrated the importance of these amino acids for cross-domain AlgH activity in in vitro complementation assays in S. cerevisiae. Furthermore, antibiotic treatment interfering directly with the activity of dolichyl phosphate GlcNAc-1-phosphotransferases confirmed the essentiality of N-glycosylation for cell survival.

Regulation of capsule in Neisseria meningitidis

Neisseria meningitidis, a devastating pathogen exclusive to humans, expresses capsular polysaccharides that are the major meningococcal virulence determinants and the basis for successful meningococcal vaccines. With rare exceptions, the expression of capsule (serogroups A, B, C, W, X, Y) is required for systemic invasive meningococcal disease. Changes in capsule expression or structure (e.g. hypo- or hyper-encapsulation, capsule "switching", acetylation) can influence immunologic diagnostic assays or lead to immune escape. The loss or down-regulation of capsule is also critical in meningococcal biology facilitating meningococcal attachment, microcolony formation and the carriage state at human mucosal surfaces. Encapsulated meningococci contain a cps locus with promoters located in an intergenic region between the biosynthesis and the conserved capsule transport operons. The cps intergenic region is transcriptionally regulated (and thus the amount of capsule expressed) by IS element insertion, by a two-component system, MisR/MisS and through sequence changes that result in post-transcriptional RNA thermoregulation. Reversible on-off phase variation of capsule expression is controlled by slipped strand mispairing of homo-polymeric tracts and by precise insertion and excision of IS elements (e.g. IS1301) in the biosynthesis operon. Capsule structure can be altered by phase-variable expression of capsular polymer modification enzymes or "switched" through transformation and homologous recombination of different polymerases. Understanding the complex regulation of meningococcal capsule has important implications for meningococcal biology, pathogenesis, diagnostics, current and future vaccine development and vaccine strategies.

At the membrane frontier: a prospectus on the remarkable evolutionary conservation of polyprenols and polyprenyl-phosphates

Long-chain polyprenols and polyprenyl-phosphates are ubiquitous and essential components of cellular membranes throughout all domains of life. Polyprenyl-phosphates, which include undecaprenyl-phosphate in bacteria and the dolichyl-phosphates in archaea and eukaryotes, serve as specific membrane-bound carriers in glycan biosynthetic pathways responsible for the production of cellular structures such as N-linked protein glycans and bacterial peptidoglycan. Polyprenyl-phosphates are the only form of polyprenols with a biochemically-defined role; however, unmodified or esterified polyprenols often comprise significant percentages of the cellular polyprenol pool. The strong evolutionary conservation of unmodified polyprenols as membrane constituents and polyprenyl-phosphates as preferred glycan carriers in biosynthetic pathways is poorly understood. This review surveys the available research to explore why unmodified polyprenols have been conserved in evolution and why polyprenyl-phosphates are universally and specifically utilized for membrane-bound glycan assembly.

Biochemical characterization of the O-linked glycosylation pathway in Neisseria gonorrhoeae responsible for biosynthesis of protein glycans containing N,N'-diacetylbacillosamine

The O-linked protein glycosylation pathway in Neisseria gonorrhoeae is responsible for the synthesis of a complex oligosaccharide on undecaprenyl diphosphate and subsequent en bloc transfer of the glycan to serine residues of select periplasmic proteins. Protein glycosylation (pgl) genes have been annotated on the basis of bioinformatics and top-down mass spectrometry analysis of protein modifications in pgl-null strains [Aas, F. E., et al. (2007) Mol. Microbiol. 65, 607-624; Vik, A., et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 4447-4452], but relatively little biochemical analysis has been performed to date. In this report, we present the expression, purification, and functional characterization of seven Pgl enzymes. Specifically, the enzymes studied are responsible for synthesis of an uncommon uridine diphosphate (UDP)-sugar (PglD, PglC, and PglB-acetyltransferase domain), glycan assembly (PglB-phospho-glycosyltransferase domain, PglA, PglE, and PglH), and final oligosaccharide transfer (PglO). UDP-2,4-diacetamido-2,4,6-trideoxy-α-d-hexose (DATDH), which is the first sugar in glycan biosynthesis, was produced enzymatically, and the stereochemistry was assigned as uridine diphosphate N'-diacetylbacillosamine (UDP-diNAcBac) by nuclear magnetic resonance characterization. In addition, the substrate specificities of the phospho-glycosyltransferase, glycosyltransferases, and oligosaccharyltransferase (OTase) were analyzed in vitro, and in most cases, these enzymes exhibited strong preferences for the native substrates relative to closely related glycans. In particular, PglO, the O-linked OTase, and PglB(Cj), the N-linked OTase from Campylobacter jejuni, preferred the native N. gonorrhoeae and C. jejuni substrates, respectively. This study represents the first comprehensive biochemical characterization of this important O-linked glycosylation pathway and provides the basis for further investigations of these enzymes as antibacterial targets.

Translocation and surface expression of lipidated serogroup B capsular Polysaccharide in Neisseria meningitidis

The capsule of N. meningitidis serogroup B, (alpha2-->8)-linked polysialic acid and the capsules of other meningococcal serogroups and of other gram-negative bacterial pathogens are anchored in the outer membrane through a 1,2-diacylglycerol moiety. Previous work on the meningococcal cps complex in Escherichia coli K-12 indicated that deletion of genes designated lipA and lipB caused intracellular accumulation of hyperelongated capsule polymers lacking the phospholipid substitution. To better understand the role of lip and lipB in capsule expression in a meningococcal background, the location, sequence, and relationship to related bacterial capsule genes were defined and specific mutations in lipA and lipB were generated in the serogroup B meningococcal strain NMB. The lipA and lipB genes are located on the 3' end of the ctr operon and are most likely transcribed independently. Inactivation of lipA, lipB, and both resulted in the same total levels of capsular polymer production as in the parental controls; however, these mutants were as sensitive as an unencapsulated mutant to killing by normal human serum. Immunogold electron microscopy and flow cytometric analyses revealed intracellular inclusions of capsular polymers in lipA, lipB, and lipA lipB mutants. Capsular polymers purified from lipA, lipB, and lipA lipB mutants were lipidated. The phospholipid anchor was shown by gas chromatography-mass spectroscopy analysis to be a phosphodiester-linked 1,2-dipalmitoyl (C16:0) glycerol moiety and was identical in structure to that found on the wild-type meningococcal capsule polymers. Thus, lipA and lipB do not encode proteins responsible for diacylglycerophosphatidic acid substitution of the meningococcal capsule polymer; rather, they are required for proper translocation and surface expression of the lipidated polymer.

Naturally acquired passive protective activity against Neisseria meningitidis Group C in the absence of serum bactericidal activity

The hallmark of immunity to meningococcal disease is a bactericidal titer in serum of > or =1:4 measured with human complement, but this threshold titer may underestimate the extent of protection. We used the infant rat model of meningococcal bacteremia to measure group C passive protective activity in serum samples from 91 unimmunized adults living in California. A total of 35 sera (38.5%) had passive protective activity. Sera with complement-mediated bactericidal titers of > or =1:4 were 3.4-fold more likely to confer protection (89%) than nonbactericidal sera (26%; P < 0.0001). Thus, bactericidal titers of > or =1:4 are a marker of protection, but this threshold lacks sensitivity for predicting protective activity. We investigated the 73 sera with bactericidal titers of <1:4 to determine the basis of protective activity. The 19 sera with protective activity had a higher geometric mean group C anticapsular antibody concentration (0.72 microg/ml) than the 54 sera that lacked protective activity (0.16 microg/ml; P < 0.001). Thus, protective activity in the absence of bactericidal activity was associated with higher concentrations of anticapsular antibodies, but not all sera with anticapsular antibodies conferred protection. Of 18 nonbactericidal sera with anticapsular antibody concentrations between 0.31 and 0.99 microg/ml, the 11 sera that conferred protection had a higher mean antibody avidity constant (21.9 nM(-1)) than the 7 nonprotective sera (14.6 nM(-1); P < 0.03). Thus, in sera with titers of <1:4, protective activity is associated with higher-avidity group C anticapsular antibodies, which are present in concentrations insufficient to elicit complement-mediated bacteriolysis in vitro but sufficient to confer protection in an in vivo bacteremia model.

The Neisseria lipooligosaccharide-specific alpha-2,3-sialyltransferase is a surface-exposed outer membrane protein

Neisseria gonorrhoeae and Neisseria meningitidis express an approximately 43-kDa alpha-2,3-sialyltransferase (Lst) that sialylates the surface lipooligosaccharide (LOS) by using exogenous (in all N. gonorrhoeae strains and some N. meningitidis serogroups) or endogenous (in other N. meningitidis serogroups) sources of 5'-cytidinemonophospho-N-acetylneuraminic acid (CMP-NANA). Sialylation of LOS can protect N. gonorrhoeae and N. meningitidis from complement-mediated serum killing and from phagocytic killing by neutrophils. The precise subcellular location of Lst has not been determined. We confirm and extend previous studies by demonstrating that Lst is located in the outer membrane and is surface exposed in both N. gonorrhoeae and N. meningitidis. Western immunoblot analysis of subcellular fractions of N. gonorrhoeae strain F62 and N. meningitidis strain MC58 not subset 3 (an acapsulate serogroup B strain) performed with rabbit antiserum raised against recombinant Lst revealed an approximately 43-kDa protein exclusively in outer membrane preparations of both pathogens. Inner membrane, periplasmic, cytoplasmic, and culture supernatant fractions were devoid of Lst, as determined by Western blot analysis. Consistent with this finding, outer membrane fractions of N. gonorrhoeae were significantly enriched for sialyltransferase enzymatic activity. A trace of enzymatic activity was detected in inner membrane fractions, which may have represented Lst in transit to the outer membrane or may have represented inner membrane contamination of outer membrane preparations. Subcellular preparations of an isogenic lst insertion knockout mutant of N. gonorrhoeae F62 (strain ST01) expressed neither a 43-kDa immunoreactive protein nor sialyltransferase activity. Anti-Lst rabbit antiserum bound to whole cells of N. meningitidis MC58 not subset 3 and wild-type N. gonorrhoeae F62 but not to the Lst mutant ST01, indicating the surface exposure of the enzyme. Although the anti-Lst antiserum avidly bound enzymatically active, recombinant Lst, it inhibited Lst (sialyltransferase) activity by only about 50% at the highest concentration of antibody used. On the contrary, anti-Lst antiserum did not inhibit sialylation of whole N. gonorrhoeae cells in the presence of exogenous CMP-NANA, suggesting that the antibody did not bind to or could not access the enzyme active site on the surface of viable Neisseria cells. Taken together, these results indicate that Lst is an outer membrane, surface-exposed glycosyltransferase. To our knowledge, this is the first demonstration of the localization of a bacterial glycosyltransferase to the outer membrane of gram-negative bacteria.

The biochemistry and genetics of capsular polysaccharide production in bacteria

Bacterial polysaccharides are usually associated with the outer surface of the bacterium. They can form an amorphous layer of extracellular polysaccharide (EPS) surrounding the cell that may be further organized into a distinct structure termed a capsule. Additional polysaccharide molecules such as lipopolysaccharide (LPS) or lipooligosaccharide (LOS) may also decorate the cell surface. Polysaccharide capsules may mediate a number of biological processes, including invasive infections of human beings. Discussed here are the genetics and biochemistry of selected bacterial capsular polysaccharides and the basis of capsule diversity but not the genetics and biochemistry of LPS biosynthesis (for reviews see 100, 140).

Thermoregulation of kpsF, the first region 1 gene in the kps locus for polysialic acid biosynthesis in Escherichia coli K1

The kps locus for biosynthesis of the capsular polysialic acid virulence factor in Escherichia coli K1 contains at least two convergently transcribed operons, designated region 1 and regions 2 plus 3. On the basis of DNA sequence analysis, kpsF appeared to be a good candidate for the first gene of region 1 (M. J. Cieslewicz, S. M. Steenbergen, and E. R. Vimr, J. Bacteriol. 175:8018-8023, 1993). A preliminary indication that kpsF is required for capsule production is the capsule-negative phenotype of an aph T insertion in the chromosomal copy of kpsF. The present communication describes the isolation and phenotypic characterization of this mutant. Although transcription through kpsF was required for capsule production, complementation analysis failed to indicate a clear requirement for the KpsF polypeptide. However, since E. coli contains at least two other open reading frames that could code for homologs of KpsF, the apparent dispensability of KpsF remains provisional. DNA sequence analysis of 1,100 bp upstream from the kpsF translational start site did not reveal any open reading frames longer than 174 nucleotides, consistent with kpsF being the first gene of region 1. Since kpsF appeared to be the first gene of a region whose gene products are required for polysialic acid transport and because capsule production is known to be thermoregulated, primer extension analyses were carried out with total RNA isolated from cells grown at permissive (37 degrees C) and nonpermissive (20 degrees C) temperatures. The results revealed a potentially complex kpsF promoter-like region that was transcriptionally silent at the nonpermissive temperature, suggesting that thermoregulation of region 1 may be exerted through variations in kpsF expression. Additional evidence supporting this conclusion was obtained by demonstrating the effects of temperature on expression of the gene kpsE, immediately downstream of kpsF. Chloramphenicol acetyltransferase assays were carried out with constructs containing the kpsF 5' untranslated region fused to a promoterless cat cassette, providing further evidence that kpsF is thermoregulated. Although the function of KpsF is unclear, primary structure analysis indicated two motifs commonly observed in regulatory proteins and homology with glucosamine synthase from Rhizobium meliloti.

Biosynthesis of the polysialic acid capsule in Escherichia coli K1

The extracellular polysaccharides elaborated by most or all bacterial species function in cell-to-cell and cell-substratum adhesion, cell signaling, and avoidance or inhibition of noxious agents in animal hosts or free-living environments. Recent advances in our understanding of exopolysaccharide synthesis have been facilitated by comparative approaches in both plant and animal pathogens, as well as in microorganisms of industrial importance. One of the best understood of these systems is the kps locus for polysialic acid synthesis in Escherichia coli K1. The genes for sialic acid synthesis, activation, polymerization and translocation have been identified and assigned at least tentative functions in the synthetic and export pathways. Initial studies of kps thermoregulation suggest that genetic control mechanisms will be involved which are distinct from those already described for several other exopolysaccharides. Information about the common as well as unique features of polysialic acid biosynthesis will increase our knowledge of microbial cell surfaces which in turn may suggest novel targets for therapeutic or industrial interventions.

Interaction of undecaprenyl phosphate with phospholipid bilayers

The effect of undecaprenyl phosphate (C55-P) on dioleoylphosphatidylcholine (DOPC) bilayer lipid membranes has been studied. The current-voltage characteristics, steady-state diffusion potentials, membrane conductance-temperature relationships, membrane electric capacitance and membrane breakdown voltage have been measured for different mixtures of undecaprenyl phosphate and DOPC. The ratio of permeability coefficients for sodium and chloride ions, the activation energy for ion migration across the membrane and membrane thickness have been determined. The electrical measurements showed that undecaprenyl phosphate decreases membrane-normalized conductance, membrane ionic permeability, membrane hydrophobic thickness and membrane selectivity for chloride ions, and increases the activation energy for ion transport, membrane nonlinearity potential, membrane specific capacitance, membrane electromechanical stability and membrane selectivity for sodium ions. From the results, we suggest that the interaction of the gradient of electric transmembrane potential with the negative charge of the phosphate group of C55-P determines the dynamics, conformation and aggregation behaviour of undecaprenyl phosphate in phospholipid membranes. Some implications of these findings for a possible regulation of the C55-P-dependent expression of polysialic acid capsule in Escherichia coli K1 bacterial cells are indicated.

Region II of the Haemophilus influenzae type be capsulation locus is involved in serotype-specific polysaccharide synthesis

The central (serotype-specific) Region II of the Haemophilus influenzae Type b capsulation locus cap is 8.3 kb long and contains a cluster of four genes. We show that these genes, designated orf1 to orf4, are involved in the biosynthetic steps required for the formation of the Type b capsular polysaccharide and that orf1 probably encodes a CDP-ribitolpyrophosphorylase. We present evidence that growth of polysaccharide chains takes place through the alternating addition of single sugar nucleotides.

Current status of meningococcal group B vaccine candidates: capsular or noncapsular?

Meningococcal meningitis is a severe, life-threatening infection for which no adequate vaccine exists. Current vaccines, based on the group-specific capsular polysaccharides, provide short-term protection in adults against serogroups A and C but are ineffective in infants and do not induce protection against group B strains, the predominant cause of infection in western countries, because the purified serogroup B polysaccharide fails to elicit human bactericidal antibodies. Because of the poor immunogenicity of group B capsular polysaccharide, different noncapsular antigens have been considered for inclusion in a vaccine against this serogroup: outer membrane proteins, lipooligosaccharides, iron-regulated proteins, Lip, pili, CtrA, and the immunoglobulin A proteases. Alternatively, attempts to increase the immunogenicity of the capsular polysaccharide have been made by using noncovalent complexes with outer membrane proteins, chemical modifications, and structural analogs. Here, we review the strategies employed for the development of a vaccine for Neisseria meningitidis serogroup B; the difficulties associated with the different approaches are discussed.

Microbial sialidases: does bigger always mean better?

Sialidases are a superfamily of N-acylneuraminate-releasing (sialic-acid-releasing) exoglycosidases found mainly in higher eukaryotes and in some, mostly pathogenic, viruses, bacteria and protozoans. The functions of sialidases are poorly understood and, until recently, their biochemical and evolutionary relationships were unclear. A comparative approach has demonstrated the remarkable similarities and differences between nonviral sialidases, and is providing clues about their functions.

Polysialic acids

1. Polysialic acids are linear homopolymers of N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc) and deaminated neuraminic acid (KDN) residues joined by alpha 2,8, alpha 2-9 or alpha 2,8/alpha 2,9 ketosidic linkages. 2. They occur in glycoproteins of embryonic neural membranes (playing a role of neural cell adhesion molecules), in non-neural tissues (postnatal kidney), tumours, (neuroectodermal tumours), fish eggs and in the capsule of certain bacteria such as Neisseria meningitidis group B. 3. These polymers are synthesized through reactions which involve (a) the synthesis of sialic acid; (b) its activation to a cytidine monophosphate sugar nucleotide and (c) the polymerization of the different residues by a polysialyl-transferase complex. 4. Polysialic acids are involved in organogenesis and in cell growth. In several tissues they act as oncodevelopmental antigens, and in bacteria are also virulent determinants.

Phospholipid substitution of capsular polysaccharides and mechanisms of capsule formation in Neisseria meningitidis

Within the capsule gene complex (cps) of Neisseria meningitidis two functional regions B and C are involved in surface translocation of the cytoplasmically synthesized capsular polysaccharide, which is a homopolymer of alpha-2,8 polyneuraminic acid. The region-C gene products share characteristics with transporter proteins of the ABC (ATP-binding cassette) superfamily of active transporters. For analysis of the role of region B in surface translocation of the capsular polysaccharide we purified the polysaccharides of region B- and region C-defective Escherichia coli clones by affinity chromatography. The molecular weights of the polysaccharides were determined by gel filtration and the polysaccharides were analysed for phospholipid substitution by polyacrylamide gel electrophoresis and immunoblotting. The results indicate that the full-size capsular polysaccharide with a phospholipid anchor is synthesized intracellularly and that lipid modification is a strong requirement for translocation of the polysaccharide to the cell surface. Proteins encoded by region B are involved in phospholipid substitution of the capsular polysaccharide. Nucleotide sequence analysis of region B revealed two open reading frames, which encode proteins with molecular masses of 45.1 and 48.7 kDa.

Functional analysis of the sialyltransferase complexes in Escherichia coli K1 and K92

The polysialyltransferase (polyST) structural gene, neuS, for poly alpha 2,8sialic acid (PSA) capsule synthesis in Escherichia coli K1 was previously mapped near the kps region 1 and 2 junction (S. M. Steenbergen and E. R. Vimr, Mol. Microbiol. 4:603-611, 1990). Present Southern and colony blot hybridization results confirmed that neuS was a region 2 locus and indicated apparent homology with neuS from E. coli K92, bacteria that synthesize a sialyl alpha 2,8-2,9-linked polymer. A K1- mutant with an insertion mutation in neuS was complemented in trans by K92 neuS, providing direct evidence that neuS encoded the PSA polymerase. A 2.9-kb E. coli K1 kps subclone was sequenced to better characterize polyST. In addition to neuS, the results identified a new open reading frame, designated neuE, the linker sequence between regions 1 and 2, and the last gene of region 1, kpsS. The kpsS translational reading frame was confirmed by sequencing across the junction of a kpsS'-lacZ+ fusion. PolyST was identified by maxicell analysis of nested deletions and coupled in vitro transcription-translation assays. PolyST's derived primary structure predicted a 47,500-Da basic polypeptide without extensive similarity to other known proteins. PolyST activity was increased 31-fold and was membrane localized when neuS was cloned into an inducible expression vector, suggesting, together with the polyST primary structure, that polyST is a peripheral inner membrane glycosyltransferase. However, polyST could not initiate de novo PSA synthesis, indicating a functional requirement for other kps gene products. The existence of a sialyltransferase distinct from polyST was suggested by identification of a potential polyprenyl-binding motif in a C-terminal membrane-spanning domain of the predicted neuE gene product. Direct evidence for a quantitatively minor sialyltransferase activity, which could function to initiate PSA synthesis, was obtained by phenotypic analysis of mutants with multiple defects in sialic acid synthesis, degradation, and polymerization. The results provide an initial molecular description of K1 and K92 sialyltransferase complexes and suggest a possible common function for accessory kps gene products.

Biosynthesis of the Escherichia coli K5 polysaccharide, a representative of group II capsular polysaccharides: polymerization in vitro and characterization of the product

Biosynthesis of the capsular K5 polysaccharide of Escherichia coli, which has the structure 4)-beta GlcA-1,4-alpha GlcNAc-(1, was studied with membrane preparations from an E. coli K5 wild-type strain and from a recombinant K-12 strain expressing the K5 capsule. Polymerization occurs at the inner face of the cytoplasmic membrane without the participation of lipid-linked oligosaccharides. The serological K5 specificity of the in vitro product was determined with a K5-specific monoclonal antibody in an antigen-binding assay. The K5 polysaccharide, as obtained from the membranes after an in vitro incubation, has 2-keto-3-deoxyoctulosonic acid as the reducing sugar, which indicates that the polysaccharide grows by chain elongation at the nonreducing end.

Cellulose biosynthesis and function in bacteria

The current model of cellulose biogenesis in plants, as well as bacteria, holds that the membranous cellulose synthase complex polymerizes glucose moieties from UDP-Glc into beta-1,4-glucan chains which give rise to rigid crystalline fibrils upon extrusion at the outer surface of the cell. The distinct arrangement and degree of association of the polymerizing enzyme units presumably govern extracellular chain assembly in addition to the pattern and width of cellulose fibril deposition. Most evident for Acetobacter xylinum, polymerization and assembly appear to be tightly coupled. To date, only bacteria have been effectively studied at the biochemical and genetic levels. In A. xylinum, the cellulose synthase, composed of at least two structurally similar but functionally distinct subunits, is subject to a multicomponent regulatory system. Regulation is based on the novel nucleotide cyclic diguanylic acid, a positive allosteric effector, and the regulatory enzymes maintaining its intracellular turnover: diguanylate cyclase and Ca2(+)-sensitive bis-(3',5')-cyclic diguanylic acid (c-di-GMP) phosphodiesterase. Four genes have been isolated from A. xylinum which constitute the operon for cellulose synthesis. The second gene encodes the catalytic subunit of cellulose synthase; the functions of the other three gene products are still unknown. Exclusively an extracellular product, bacterial cellulose appears to fulfill diverse biological roles within the natural habitat, conferring mechanical, chemical, and physiological protection in A. xylinum and Sarcina ventriculi or facilitating cell adhesion during symbiotic or infectious interactions in Rhizobium and Agrobacterium species. A. xylinum is proving to be most amenable for industrial purposes, allowing the unique features of bacterial cellulose to be exploited for novel product applications.

Mechanism of polysialic acid chain elongation in Escherichia coli K1

Understanding the mechanisms of polysialic acid synthesis in Escherichia coli K1 requires a molecular description of the polymerase complex. Since the number of potential models explaining polysialic acid assembly would be constrained if only one sialyltransferase were required for this process, the phenotypes of a sialyltransferase null mutation generated by transposon mutagenesis were investigated. The chromosomal insertion mutation was mapped by Southern hybridization analysis and by complementation with plasmid subclones, demonstrating that sialyltransferase is encoded by neuS, a gene implicated previously as coding for the polymerase (Vimr et al., 1989). As expected, if only one gene encoded sialyltransferase, the null mutant had undetectable polymerase activity when assayed with endogenous or exogenous acceptors, and accumulated sugar nucleotide precursors intracellularly. Nested deletion analysis of neuS ruled out polarity effects of transposon insertion mutation and provided more precise mapping of the sialyltransferase structural gene. Maxicell analysis of the nested deletion set implicated a 34,000 molecular weight polypeptide as the neuS gene product. These results, together with biochemical characterization of sialyltransferase reaction products in the wild type, indicated that CMP-sialic acid is the probable sialosyl donor for polysialic acid elongation and that chain growth is by sequential addition of monomeric units.

Genetic analysis of chromosomal mutations in the polysialic acid gene cluster of Escherichia coli K1

The kps gene cluster of Escherichia coli K1 encodes functions for sialic acid synthesis, activation, polymerization, and possibly translocation of polymer to the cell surface. The size and complexity of this membrane polysaccharide biosynthetic cluster have hindered genetic mapping and functional descriptions of the kps genes. To begin a detailed investigation of the polysialic acid synthetic mechanism, acapsular mutants were characterized to determine their probable defects in polymer synthesis. The mutants were tested for complementation with kps fragments subcloned from two separately isolated, functionally intact kps gene clusters. Complementation was assayed by immunological and biochemical methods and by sensitivity to the K1-specific bacteriophage K1F. The kps cluster consisted of a central 5.8-kilobase region that contained at least two genes coding for sialic acid synthetic enzymes, a gene encoding the sialic acid-activating enzyme, and a gene encoding the sialic acid polymerase. This biosynthetic region is flanked on one side by an approximately 2.8-kilobase region that contains a potential regulatory locus and at least one structural gene for a polypeptide that appears to function in polysialic acid assembly. Flanking the biosynthetic region on the opposite side is a 6- to 8.4-kilobase region that codes for at least three proteins which may also function in polymer assembly and possibly in translocating polymer to the outer cell surface. Results of transduction crosses supported these conclusions and indicated that some of the kps genes flanking the central biosynthetic region may not function directly in transporting polymer to the cell surface. The results also demonstrate that the map position and probable function of most of the kps cluster genes have been identified.