Common organization of gene clusters for production of different capsular polysaccharides (K antigens) in Escherichia coli

Identification of an ATP-binding cassette transport system required for translocation of lipopolysaccharide O-antigen side-chains across the cytoplasmic membrane of Klebsiella pneumoniae serotype O1

The rfbKpO1 gene cluster of Klebsiella pneumoniae O1 directs synthesis of the D-galactan I component of the lipopolysaccharide O-antigen. The first two genes in the rfbKpO1 cluster encode RfbAKpO1 and RfbBKpO1, with predicted sizes of 29.5 or 30.0 kDa and 27.4 kDa, respectively. RfbBKpO1 contains a consensus ATP-binding domain and shares homology with several proteins which function as ATP-binding components of cell surface polysaccharide transporters. RfbAKpO1 is predicted to be an integral membrane protein with five putative membrane-spanning domains and its transmembrane topology was confirmed by TnphoA mutagenesis. The hydropathy plot of RfbAKpO1 resembles KpsM, the transcytoplasmic membrane component of the capsular polysaccharide transporter from Escherichia coli K-1 and K-5. These relationships suggest that RfbAKpO1 and RfbBKpO1 belong to a family of two-component ABC (ATP-binding cassette) transporters. E. coli K-12 containing a plasmid carrying an rfbKpO1 gene cluster deleted in rfbAKpO1 and rfbBKpO1 expresses rough lipopolysaccharide molecules on its surface and accumulates cytoplasmic O-antigen. When RfbAKpO1 and RfbBKpO1 are supplied in trans by a compatible plasmid, O-polysaccharide transport is restored and smooth D-galactan I-substituted lipopolysaccharide is produced. RfbAKpO1 and RfbBKpO1 are, therefore, proposed to constitute a system required for transport of D-galactan I across the cytoplasmic membrane, where RfbAKpO1 represents the membrane-spanning translocator and RfbBKpO1 couples the energy of ATP hydrolysis ot the transport process.

Molecular characterization of cap3A, a gene from the operon required for the synthesis of the capsule of Streptococcus pneumoniae type 3: sequencing of mutations responsible for the unencapsulated phenotype and localization of the capsular cluster on the pneumococcal chromosome

The complete nucleotide sequence of the cap3A gene of Streptococcus pneumoniae, which is directly responsible for the transformation of some unencapsulated, serotype 3 mutants to the encapsulated phenotype, has been determined. This gene encodes a protein of 394 amino acids with a predicted M(r) of 44,646. Twelve independent cap3A mutations have been mapped by genetic transformation, and three of them have been sequenced. Sequence comparisons revealed that cap3A was very similar (74.4%) to the hasB gene of Streptococcus pyogenes, which encodes a UDP-glucose dehydrogenase (UDP-GlcDH) that catalyzes the conversion of UDP-glucose to UDP-glucuronic acid, the donor substances in the pneumococcal type 3 capsular polysaccharide. Furthermore, a PCR-generated cap3A+ gene restored encapsulation in our cap3A mutants as well as in a mutant previously characterized as deficient in UDP-GlcDH (R. Austrian, H. P. Bernheimer, E.E.B. Smith, and G.T. Mills, J. Exp. Med. 110:585-602, 1959). These results support the conclusion that cap3A codes for UDP-GlcDH. We have also identified a region upstream of cap3A that should contain common genes necessary for the production of capsule of any type. Pulsed-field gel electrophoresis and Southern blotting showed that the capsular genes specific for serotype 3 are located near the genes encoding PBP 2X and PBP 1A in the S. pneumoniae chromosome, whereas copies of the common genes (or part of them) appear to be present in different locations in the genome.

Molecular cloning and analysis of genes for sialic acid synthesis in Neisseria meningitidis group B and purification of the meningococcal CMP-NeuNAc synthetase enzyme

The gene encoding for the CMP-NeuNAc synthetase enzyme of Neisseria meningitidis group B was cloned by complementation of a mutant of Escherichia coli defective for this enzyme. The gene (neuA) was isolated on a 4.1-kb fragment of meningococcal chromosomal DNA. Determination of the nucleotide sequence of this fragment revealed the presence of three genes, termed neuA, neuB, and neuC, organized in a single operon. The presence of a truncated ctrA gene at one end of the cloned DNA and a truncated gene encoding for the meningococcal sialyltransferase at the other confirmed that the cloned DNA corresponded to region A and part of region C of the meningococcal capsule gene cluster. The predicted amino acid sequence of the meningococcal NeuA protein was 57% homologous to that of NeuA, the CMP-NeuNAc synthetase encoded by E. coli K1. The predicted molecular mass of meningococcal NeuA protein was 24.8 kDa, which was 6 kDa larger than that formerly predicted (U. Edwards and M. Frosch, FEMS Microbiol. Lett. 96:161-166, 1992). Purification of the recombinant meningococcal NeuA protein together with determination of the N-terminal amino acid sequence confirmed that this 24.8-kDa protein was indeed the meningococcal CMP-NeuNAc synthetase. The predicted amino acid sequences of the two other encoded proteins were homologous to those of the NeuC and NeuB proteins of E. coli K1, two proteins involved in the synthesis of NeuNAc. These results indicate that common steps exist in the biosynthesis of NeuNAc in these two microorganisms.

Genetic analysis of type 5 capsular polysaccharide expression by Staphylococcus aureus

Capsules are produced by over 90% of Staphylococcus aureus strains, and approximately 25% of clinical isolates express type 5 capsular polysaccharide (CP5). We mutagenized the type 5 strain Reynolds with Tn918 to target genes involved in CP5 expression. From a capsule-deficient mutant, we cloned into a cosmid vector an approximately 26-kb EcoRI fragment containing the transposon insertion. In the absence of tetracycline selection, Tn918 was spontaneously excised, thereby resulting in a plasmid containing 9.4 kb of S. aureus DNA flanking the Tn918 insertion site. The 9.4-kb DNA fragment was used to screen a cosmid library prepared from the wild-type strain. Positive colonies were identified by colony hybridization, and a restriction map of one clone (pJCL19 with an approximately 34-kb insert) carrying the putative capsule gene region was constructed. Fragments of pJCL19 were used to probe genomic DNA digests from S. aureus strains of different capsular serotypes. Fragments on the ends of the cloned DNA hybridized to fragments of similar sizes in most of the strains examined. Blots hybridized to two fragments flanking the central region of the cloned DNA showed restriction fragment length polymorphism. A centrally located DNA fragment hybridized only to DNA from capsular types 2, 4, and 5. DNA from pJCL19 was subcloned to a shuttle vector for complementation studies. A 6.2-kb EcoRI-ClaI fragment complemented CP5 expression in a capsule-negative mutant derived by mutagenesis with ethyl methanesulfonate. These experiments provide the necessary groundwork for identifying genes involved in CP5 expression by S. aureus.

Nucleotide sequence and mutational analysis of the gene encoding KpsD, a periplasmic protein involved in transport of polysialic acid in Escherichia coli K1

The 17-kb kps gene cluster encodes proteins necessary for the synthesis, assembly, and translocation of the polysialic acid capsule of Escherichia coli K1. We previously reported that one of these genes, kpsD, encodes a 60-kDa periplasmic protein that is involved in the translocation of the polymer to the cell surface. The nucleotide sequence of the 2.4-kb BamHI-PstI fragment accommodating the kpsD gene was determined. Sequence analysis showed an open reading frame for a 558-amino-acid protein with a typical N-terminal prokaryotic signal sequence corresponding to the first 20 amino acids. KpsD was overexpressed, partially purified, and used to prepare polyclonal antiserum. A chromosomal insertion mutation was generated in the kpsD gene and results in loss of surface expression of the polysialic acid capsule. Immunodiffusion analysis and electron microscopy indicated that polysaccharide accumulates in the periplasmic space of mutant cells. A wild-type copy of kpsD supplied in trans complemented the chromosomal mutation, restoring extracellular expression of the K1 capsule. However, a kpsD deletion derivative (kpsD delta C11), which results in production of a truncated KpsD protein lacking its 11 C-terminal amino acids, was nonfunctional. Western blot (immunoblot) data from cell fractions expressing KpsD delta C11 suggest that the truncated protein was inefficiently exported into the periplasm and localized primarily to the cytoplasmic membrane.

Genetic and molecular characterization of capsular polysaccharide biosynthesis in Streptococcus pneumoniae type 3

To achieve a better understanding of the genetics of capsular polysaccharide synthesis in Streptococcus pneumoniae, we have identified and characterized mutants deficient in type 3 capsule production. We identified a clone that restored encapsulation in one of our mutants and in a mutant deficient in UDP-glucose dehydrogenase. By hybridization, we developed a chromosomal map of the type 3-specific region and identified a flanking region containing DNA common to all capsule types examined. Insertion mutations were used to identify chromosomal loci required for capsule synthesis, and to map transcription within the region. Using non-destructive insertions linked to type-specific genes of type 2, 3, or 5, we were able to select for the transformation of all necessary genes specific for capsule type. Our data provide molecular evidence to show that all the type-specific genes are linked in a cassette and can be transferred as a unit during transformation.

Contribution of genes from the capsule gene complex (cps) to lipooligosaccharide biosynthesis and serum resistance in Neisseria meningitidis

Within the capsule gene complex (cps) of Neisseria meningitidis B a 5.5 kb DNA fragment encodes proteins with strong homologies to enzymes of the lipopolysaccharide biosynthetic pathway of Salmonella typhimurium and Escherichia coli, GalE, RfbB, RfbC and RfbD. A meningococcal galE mutant expressed a truncated lipooligosaccharide (LOS), which terminated at the glucose residue between inner and outer core, and a second galE gene present outside the cps cluster was found to be transcriptionally and functionally inactive and, thus, unable to complement this defect. Because of the defect in the outer core, the LOS of the galE-defective meningococcal mutant was not sialylated. In contrast, carbohydrate analysis of the LOS of an rfb-defective meningococcal mutant revealed no difference from the LOS of the wild-type strain, suggesting that the rfb genes are inactive. This was supported by Northern blot analysis, which showed that expression of the rfb gene products was transcriptionally regulated. The inability of the meningococcal galE mutant, which cannot sialylate the LOS, allowed us to investigate the significance of LOS sialylation in relation to the presence of the polysialic acid capsule. Sialylated LOS, but not the polysialic acid capsule, is necessary to confer complete serum resistance on the meningococcus by inhibition of the alternative complement pathway.

An extraintestinal, pathogenic isolate of Escherichia coli (O4/K54/H5) can produce a group 1 capsule which is divergently regulated from its constitutively produced group 2, K54 capsular polysaccharide

We are studying an O4/K54/H5 Escherichia coli bacteremic isolate (CP9) as a model pathogen for extraintestinal infection. Its group 2, K54 capsular polysaccharide is an important virulence determinant and confers serum resistance. In this study the effect of the group 1 capsule regulators, RcsA, RcsB, and Lon protease, on the regulation of CP9's capsular polysaccharides was assessed. It was established that in the presence of multicopy rcsA or with disruption of lon, CP9 can be induced to produce a group 1 capsule. RcsA, RcsB, and Lon are present in this K54 background and regulate group 1 capsule expression in a fashion similar to that described for K-12 strains. Two independent group 2 capsule gene protein fusions (cl1.29::TnphoA and cl1.137::TnphoA) were used to evaluate the effects of these regulators on group 2 K54 capsule production. Disruption of lon resulted in 1.9-fold (TR293 [cl1.29::TnphoA lon-146]) and 3.4-fold (TR1373 [cl1.137::TnphoA lon-146]) decreases in fusion activity at 28 degrees C, relative to the baseline level. However, decreases in fusion activity at 42 degrees C were only 1.2- and 1.4-fold, respectively. Inactivation of both lon and rcsA or lon and rcsB restored fusion activity to baseline levels at 28 degrees C, but only a partial restoration of activity was seen at higher temperatures. To assess whether these differences in fusion activity reflected a functional change in capsule production, the effects of 80% normal human serum (NHS) were tested against CP9 and TR93 (lon-146). Since the group 2 K54 capsule protects against the bactericidal activity of 80% NHS, a decrease in its production results in an increase in serum sensitivity. Viable counts of CP9 increased 10-fold in 80% NHS over 3 h at 28 degrees C, as expected. In contrast to CP9, TR93 (lon-146) incurred a 10-fold loss in viability under the same conditions. The levels of RcsA are increased in TR93 (lon 146) as consequence of lon disruption; therefore, these results in conjunction with the cl1::TnphoA protein fusion data establish RcsA as a negative regulator of the group 2 K54 capsular polysaccharide. Furthermore, these results also suggest existence of another Lon-sensitive negative regulator of group 2 K54 capsule production, which is active higher temperatures.

Synthesis of the K5 (group II) capsular polysaccharide in transport-deficient recombinant Escherichia coli

The genes directing the expression of group II capsules in Escherichia coli are organized into three regions. The central region 2 is type specific and thought to determine the synthesis of the respective polysaccharide, whilst the flanking regions 1 and 3 are common to all group II gene clusters and direct the surface expression of the capsular polysaccharide. In this communication we analyze the involvement of region 1 and 3 genes in the synthesis of the capsular KS polysaccharide. Recombinant E. coli strains harboring all KS specific region 2 genes and having various combinations of region 1 and 3 genes were studied using immunoelectron microscopy. Membranes from these bacteria were incubated with UDP[14C]GlcA and UDPGlcNAc in the absence or presence of KS polysaccharide as an exogenous acceptor. It was found that recombinant strains with only gene region 2 did not produce the K5 polysaccharide. Membranes of such strains did not synthesize the polymer and did not elongate K5 polysaccharide added as an exogenous acceptor. An involvement of genes from region 1 (notably kpsC and kpsS) and from region 3 (notably kpsT) in the K5 polysaccharide synthesis was apparent and is discussed.

Expression of the capsular K5 polysaccharide of Escherichia coli: biochemical and electron microscopic analyses of mutants with defects in region 1 of the K5 gene cluster

The gene cluster of the capsular K5 polysaccharide, a representative of group II capsular antigens of Escherichia coli, has been cloned previously, and three regions responsible for polymerization and surface expression have been defined (I.S. Roberts, R. Mountford, R. Hodge, K. B. Jann, and G. J. Boulnois, J. Bacteriol. 170:1305-1330, 1988). Region 1 has now been sequenced, and five open reading frames (kpsEDUCS) have been defined (C. Pazzani, C. Rosenow, G. J. Boulnois, D. Bronner, K. Jann, and I. S. Roberts, J. Bacteriol. 175:5978-5983, 1993). In this study, we characterized region 1 mutants by immunoelectron microscopy, membrane-associated polymerization activity, cytoplasmic CMP-2-keto-3-deoxyoctonate (KDO) synthetase activity, and chemical analysis of their K5 polysaccharides. Certain mutations within region 1 not only effected polysaccharide transport (lack of region 1 gene products) but also impaired the polymerization capacity of the respective membranes, reflected in reduced amounts of polysaccharide but not in its chain length. KDO and phosphatidic acid (phosphatidyl-KDO) substitution was found with extracellular and periplasmic polysaccharide and not with cytoplasmic polysaccharide. This and the fact that the K5 polysaccharide is formed in a kpsU mutant (defective in capsule-specific K-CMP-KDO synthetase) showed that CMP-KDO is engaged not in initiation of polymerization but in translocation of the polysaccharide.

Molecular analysis of region 1 of the Escherichia coli K5 antigen gene cluster: a region encoding proteins involved in cell surface expression of capsular polysaccharide

The nucleotide sequence of region 1 of the K5 antigen gene cluster of Escherichia coli was determined. This region is postulated to encode functions which, at least in part, participate in translocation of polysaccharide across the periplasmic space and onto the cell surface. Analysis of the nucleotide sequence revealed five genes that encode proteins with predicted molecular masses of 75.7, 60.5, 44, 43, and 27 kDa. The 27-kDa protein was 70.7% homologous to the CMP-2-keto-3-deoxyoctulosonic acid synthetase enzyme encoded by the E. coli kdsB gene, indicating the presence of a structural gene for a similar enzyme within the region 1 operon. The 43-kDa protein was homologous to both the Ctrb and BexC proteins encoded by the Neisseria meningitidis and Haemophilus influenzae capsule gene clusters, respectively, indicating common stages in the expression of capsules in these gram-negative bacteria. However, no homology was detected between the 75.7, 60.5-, and 44-kDa proteins and any of the proteins so far described for the H. influenzae and N. meningitidis capsule gene clusters.

Generation of isogenic K54 capsule-deficient Escherichia coli strains through TnphoA-mediated gene disruption

Transposon mutagenesis, using IS50L::phoA(Tn-phoA), was performed in a K54/O4/H5 blood isolate of Escherichia coli (CP9), to generate a library of random mutants. Five hundred and twenty-six independent CP9 TnphoA mutants were isolated with active gene fusions to alkaline phosphatase. From this mutant library, eight capsule-deficient strains were detected and were found to have a single copy of TnphoA. Sixteen additional capsule deficient mutants with TnphoA inserts were subsequently obtained that did not possess active PhoA fusions. In conjunction with the initial eight capsule-deficient isolates we have defined genes on three different XbaI fragments as being involved in capsule production. Generalized transduction with the bacteriophage T4 established that these insertions were responsible for the loss of capsule and that they are linked. These capsule-deficient strains can be used to assess the pathogenic role of the K54 capsular polysaccharide.

The presence of a novel type of surface polysaccharide in Rhizobium meliloti requires a new fatty acid synthase-like gene cluster involved in symbiotic nodule development

Bacterial exopolysaccharide (EPS) and lipopolysaccharide (LPS) molecules have been shown to play important roles in plant-bacterium interactions. Here we have demonstrated that the fix-23 loci, which compensate for exo mutations during symbiotic nodule development, are involved in the production of a novel polysaccharide that is rich in 3-deoxy-D-manno-2-octulosonic acid (Kdo) but is not the classical LPS. This molecule is likely to be a surface antigen since antiserum to whole Rhizobium meliloti cells reacts strongly with it, and since mutations in fix-23 result in an inability to produce this polysaccharide and to bind bacteriophage 16-3. It is likely that this Kdo-rich polysaccharide is analogous to certain Escherichia coli K-antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one gene cluster of this region revealed that the predicted protein products of six genes exhibit a high degree of homology and similar organization to those of the rat fatty acid synthase multifunctional enzyme domains.

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.

A gene cluster for amylovoran synthesis in Erwinia amylovora: characterization and relationship to cps genes in Erwinia stewartii

A large ams gene cluster required for production of the acidic extracellular polysaccharide (EPS) amylovoran by the fire blight pathogen Erwinia amylovora was cloned. Tn5 mutagenesis and gene replacement were used to construct chromosomal ams mutants. Five complementation groups, essential for amylovoran synthesis and virulence in E. amylovora, were identified and designated ams A-E. The ams gene cluster is about 7 kb in size and functionally equivalent to the cps gene cluster involved in EPS synthesis by the related pathogen Erwinia stewartii. Mucoidy and virulence were restored to E. stewartii mutants in four cps complementation groups by the cloned E. amylovora ams genes. Conversely, the E. stewartii cps gene cluster was able to complement mutations in E. amylovora ams genes. Correspondence was found between the amsA-E complementation groups and the cpsB-D region, but the arrangement of the genes appears to be different. EPS production and virulence were also restored to E. amylovora amsE and E. stewartii cpsD mutants by clones containing the Rhizobium meliloti exo A gene.

Cloning and sequencing of a gene involved in the synthesis of the capsular polysaccharide of Streptococcus pneumoniae type 3

A 4.5 kb ScaI chromosomal DNA fragment of a clinical isolate of Streptococcus pneumoniae serotype 3 was cloned in Escherichia coli. Combined genetic and molecular analyses have allowed the localization, in a 781 bp EcoRV subfragment, of a gene (cap3-1) directly responsible for the transformation of an unencapsulated, serotype 3 mutant to the capsulated phenotype. Comparison of the deduced amino acid sequence of CAP3-1 with the protein sequences compiled in the data banks revealed that the CAP3-1 polypeptide was highly similar to the amino-terminus of the GDP-mannose dehydrogenase of Pseudomonas aeruginosa, an enzyme that participates in the synthesis of the mucoid polysaccharide of this species. In addition, the 32 N-terminal amino acids of CAP3-1 perfectly matched structures common to NAD(+)-binding domains of many dehydrogenases. Our results indicate that the 4.5 kb ScaI fragment might also contain genes common to 13 different pneumococcal serogroups or serotypes tested. To the best of our knowledge, this is the first time that a gene of the capsular complex of S. pneumoniae has been cloned and sequenced. The findings reported here provide new insights for the study of the molecular biology of the main virulence factor responsible for the pathogenesis of pneumococcal infections and might represent a basic step in the identification of cross-reactive antigens that should allow the preparation of new and improved vaccines.

The NodL and NodJ proteins from Rhizobium and Bradyrhizobium strains are similar to capsular polysaccharide secretion proteins from gram-negative bacteria

The NodL and NodJ nodulation proteins have been described in different Rhizobium and Bradyrhizobium species. The nodLJ genes belong to the nod regulon. Other genes from this regulon are involved in the biosynthesis and modification of lipo-oligosaccharide molecule(s) which are morphogenic signals when acting on legume roots. It has been proposed that the NodL and NodJ proteins belong to a bacterial inner membrane transport system of small molecules. Nucleotide sequencing of Mudll PR13 insertions in the nodulation region of the symbiotic plasmid from a Rhizobium leguminosarum bv. phaseoli strain CE3 has revealed the presence of nodL and nodJ-related sequences downstream of nodC. Computer nucleotide sequence analysis of the entire NodL and NodJ sequences from R. leguminosarum bv. viciae and Bradyrhizobium japonicum strains show that both proteins are similar to the KpsT and KpsM proteins from Escherichia coli K1 and K5 strains, to the BexB and BexA proteins from Haemophilis influenzae and to the CtrD and CtrC proteins from Neisseria meningitidis, respectively. Except for the NodL and NodJ proteins, all of them have been involved in the mechanism of secretion of polysaccharides in each of their harbouring species. On the basis of the similarity found, we propose that the NodL and the NodJ proteins could be involved in the export of a lipo-oligosaccharide.

Genome diversity at the serA-linked capsule locus in Escherichia coli

Individual isolates of Escherichia coli synthesize one of more than 70 chemically distinct polysaccharides which form the capsule. In this article we review the genetics of capsule production in E. coli and highlight what this is beginning to reveal in terms of the genetic basis of the structural diversity of polysaccharides. The serA-linked capsule locus can take three different allelic forms. Two of these are associated with capsule genes and are themselves internally variant, whilst the third form has not so far been implicated in capsule biogenesis. Thus the serA-linked region of the E. coli genome is strikingly polymorphic.

Cloning of genes affecting capsule expression in Staphylococcus aureus strain M

Staphylococcus aureus strain M produces large amounts of capsular polysaccharide. It produces a non-encapsulated variant at a frequency of 0.01% at 37 degrees C. At high temperature (43 degrees C), the frequency of capsule loss was shown to be 1-38%. A 19 kb plasmid and a prophage were found to be carried by the M strain, but curing of these elements did not affect capsular production. To clone the capsular (cap) genes, a plasmid library of S. aureus M was constructed directly in S. aureus RN4200. The library was then infected with phage 80 alpha. After transduction of the phage lysates to a Cap- mutant derived from M strain, a recombinant plasmid was obtained which complemented the mutant to a Cap+ phenotype. Chromosomal walking experiments were used to clone additional nearby cap genes. Complementation tests using a collection of Cap- mutants showed that most of the mutants were complemented by a 19.4 kb DNA fragment, suggesting that the majority of the cap genes affecting capsule production are clustered together.

Horizontal transfer of multiple penicillin-binding protein genes, and capsular biosynthetic genes, in natural populations of Streptococcus pneumoniae

Multiply antibiotic-resistant serotype 23F isolates of Streptococcus pneumoniae are prevalent in Spain and have also been recovered recently in the United Kingdom and the United States. Analysis of populations of these isolates by multilocus enzyme electrophoresis, and restriction endonuclease cleavage electrophoretic profiling of penicillin-binding protein (PBP) genes, has demonstrated that these isolates are a single clone (Muñoz et al., 1991). Here we report studies of non-serotype 23F penicillin-resistant pneumococci isolated in Spain and the United Kingdom. One of the isolates expressed serotype 19 capsule but was otherwise indistinguishable from the serotype 23F clone on the basis of multilocus enzyme electrophoresis, antibiotic resistance profiling, and restriction endonuclease patterns of genes encoding PBP1A, PBP2B and PBP2X, a result which suggests that horizontal transfer of capsular biosynthesis genes had occurred. These same techniques revealed that six other resistant isolates, all expressing serotype 9 polysaccharide capsule, represent a clone. Interestingly, the chromosomal lineage of this clone is not closely related to the 23F clone; however, the serotype 9 and 23F clones harbour apparently identical PBP1A, -2B and -2X genes. To explain these data, we favour the interpretation that horizontal gene transfer in natural populations has distributed genes encoding altered forms of PBP1A, -2B and -2X to distinct evolutionary lineages of S. pneumoniae.

Identification of two genes, kpsM and kpsT, in region 3 of the polysialic acid gene cluster of Escherichia coli K1

The polysialic acid capsule of Escherichia coli K1, a causative agent of neonatal septicemia and meningitis, is an essential virulence determinant. The 17-kb kps gene cluster, which is divided into three functionally distinct regions, encodes proteins necessary for polymer synthesis and expression at the cell surface. The central region, 2, encodes products required for synthesis, activation, and polymerization of sialic acid, while flanking regions, 1 and 3, are thought to be involved in polymer assembly and transport. In this study, we identified two genes in region 3, kpsM and kpsT, which encode proteins with predicted sizes of 29.6 and 24.9 kDa, respectively. The hydrophobicity profile of KpsM suggests that it is an integral membrane protein, while KpsT contains a consensus ATP-binding domain. KpsM and KpsT belong to a family of prokaryotic and eukaryotic proteins involved with a variety of biological processes, including membrane transport. A previously described kpsT chromosomal mutant that accumulates intracellular polysialic acid was characterized and could be complemented in trans. Results of site-directed mutagenesis of the putative ATP-binding domain of KpsT are consistent with the view that KpsT is a nucleotide-binding protein. KpsM and KpsT have significant similarity to BexB and BexA, two proteins that are essential for polysaccharide capsule expression in Haemophilus influenzae type b. We propose that KpsM and KpsT constitute a system for transport of polysialic acid across the cytoplasmic membrane.

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.

Evidence for a common molecular origin of the capsule gene loci in gram-negative bacteria expressing group II capsular polysaccharides

Capsular polysaccharides of Gram-negative bacteria contribute to a large extent to the pathogenicity of these organisms. We show here that the molecular organization of the capsule gene loci in different serogroups of Neisseria meningitidis is similar to that of Haemophilus influenzae and Escherichia coli. A common molecular origin of the mechanisms of encapsulation is indicated by strong homology of the genes involved in transport of the capsular polysaccharides to the cell surface in all these organisms. The proteins involved in capsular polysaccharide transport fit the characteristics of ABC (ATP-binding cassette) transporters. Furthermore, by sequence comparison of the sialytransferases of N. meningitidis B and E. coli K1, the capsule of which is composed of alpha 2,8-linked polyneuraminic acid, a significant degree of homology was observed, indicating that the capsular polysaccharide type itself has the same evolutionary origin in these two pathogens.

Map position and genomic organization of the kps cluster for polysialic acid synthesis in Escherichia coli K1

The multigenic kps cluster in Escherichia coli K1 encodes functions for synthesis of a polysialic acid capsule. DNA probes flanking each side of the cluster were hybridized to lambda clones bearing overlapping E. coli W3110 genomic fragments. These fragments covered the region between 60 and 70 map units on the chromosome. The results located kps to an accretion domain near 64 map units and established the orientation of kps cluster genes. Acquisition of kps by the E. coli genome was apparently the result of an ancestral transpositionlike addition event.

Regulation of Rhizobium meliloti exo genes in free-living cells and in planta examined by using TnphoA fusions

The exo loci of Rhizobium meliloti are necessary for the production of an acidic exopolysaccharide, EPS I, that is needed for alfalfa nodule invasion by strain Rm1021. We have isolated and characterized alkaline phosphatase fusions made with TnphoA in several exo loci of R. meliloti and used these fusions to examine the subcellular localization of exo gene products and the regulation of exo genes in free-living cells and in planta. In the course of this work, we isolated a new exo locus, exoT. We have obtained evidence that several of the exo loci may encode membrane proteins. The activity of TnphoA fusions in several exo loci is increased two- to fivefold in the presence of the regulatory mutations exoR95 and exoS96. While examining the regulation of the exo gens by exoR95 and exoS96, we found that certain classes of exo mutations are lethal in an exoR95 or exoS96 background unless a plasmid complementing the exo mutation is present. This result has possible implications for the role of these exo loci in EPS I biosynthesis. We have developed a method for staining nodules specifically for the alkaline phosphatase activity present in the inducing bacteria and used this method to show that an exoF::TnphoA fusion is expressed mainly in the invasion zone of the nodule.

Virulence factors in Escherichia coli urinary tract infection

Uropathogenic strains of Escherichia coli are characterized by the expression of distinctive bacterial properties, products, or structures referred to as virulence factors because they help the organism overcome host defenses and colonize or invade the urinary tract. Virulence factors of recognized importance in the pathogenesis of urinary tract infection (UTI) include adhesins (P fimbriae, certain other mannose-resistant adhesins, and type 1 fimbriae), the aerobactin system, hemolysin, K capsule, and resistance to serum killing. This review summarizes the virtual explosion of information regarding the epidemiology, biochemistry, mechanisms of action, and genetic basis of these urovirulence factors that has occurred in the past decade and identifies areas in need of further study. Virulence factor expression is more common among certain genetically related groups of E. coli which constitute virulent clones within the larger E. coli population. In general, the more virulence factors a strain expresses, the more severe an infection it is able to cause. Certain virulence factors specifically favor the development of pyelonephritis, others favor cystitis, and others favor asymptomatic bacteriuria. The currently defined virulence factors clearly contribute to the virulence of wild-type strains but are usually insufficient in themselves to transform an avirulent organism into a pathogen, demonstrating that other as-yet-undefined virulence properties await discovery. Virulence factor testing is a useful epidemiological and research tool but as yet has no defined clinical role. Immunological and biochemical anti-virulence factor interventions are effective in animal models of UTI and hold promise for the prevention of UTI in humans.

Molecular analysis of the Escherichia coli K5 kps locus: identification and characterization of an inner-membrane capsular polysaccharide transport system

The complete nucleotide sequence has been determined of a region of the Escherichia coli K5 antigen gene cluster postulated to encode functions for the translocation of capsular polysaccharide across the inner membrane. This revealed two genes, designated kpsM and kpsT, organized in a single transcriptional unit. Analysis of the predicted amino acid sequence of the KpsM and KpsT proteins indicates that they may function as dual components in a polysaccharide export system analogous to the periplasmic binding protein-dependent transport systems of Gram-negative bacteria. We propose that the KpsT protein acts as an energizer, coupling ATP hydrolysis to the transport process mediated by the KpsM protein. Extensive sequence homology between the KpsM and KpsT proteins and the products of the bexB and bexA genes present in the capsulation (cap) locus of Haemophilus influenzae, indicates that a common mechanism for the export of polysaccharide across the inner membrane may exist in these two micro-organisms.

Functional and evolutionary relatedness of genes for exopolysaccharide synthesis in Rhizobium meliloti and Rhizobium sp. strain NGR234

Rhizobium meliloti SU47 and Rhizobium sp. strain NGR234 produce distinct exopolysaccharides that have some similarities in structure. R. meliloti has a narrow host range, whereas Rhizobium strain NGR234 has a very broad host range. In cross-species complementation and hybridization experiments, we found that several of the genes required for the production of the two polysaccharides were functionally interchangeable and similar in evolutionary origin. NGR234 exoC and exoY corresponded to R. meliloti exoB and exoF, respectively. NGR234 exoD was found to be an operon that included genes equivalent to exoM, exoA, and exoL in R. meliloti. Complementation of R. meliloti exoP, -N, and -G by NGR234 R'3222 indicated that additional equivalent genes remain to be found on the R-prime. We were not able to complement NGR234 exoB with R. meliloti DNA. In addition to functional and evolutionary equivalence of individual genes, the general organization of the exo regions was similar between the two species. It is likely that the same ancestral genes were used in the evolution of both exopolysaccharide biosynthetic pathways and probably of pathways in other species as well.

Further electron microscopic studies on the expression of Escherichia coli group II capsules

The de novo expression of Escherichia coli K1, K5, and K12 capsules was analyzed with immunoelectron microscopy in temperature upshift experiments, with upshift from 18 degrees C (capsule restrictive) to 37 degrees C (capsule permissive). Newly produced capsular polysaccharides appeared at the cell surface atop membrane adhesion sites (Bayer's junctions). After plasmolysis of the bacteria at an early expression stage, the capsular polysaccharides were labeled at discrete sites in the periplasm by the immunogold technique. After temperature upshift in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) or chloramphenicol, the polysaccharides were labeled in the cytoplasm.

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.

Expression of the Escherichia coli K5 capsular antigen: immunoelectron microscopic and biochemical studies with recombinant E. coli

The capsular K5 polysaccharide, a representative of group II capsular antigens of Escherichia coli, has been cloned previously, and three gene regions responsible for polymerization and surface expression have been defined (I. S. Roberts, R. Mountford, R. Hodge, K. B. Jann, and G. J. Boulnois, J. Bacteriol. 170:1305-1310, 1988). In this report, we describe the immunoelectron microscopic analysis of recombinant bacteria expressing the K5 antigen and of mutants defective in either region 1 or region 3 gene functions, as well as the biochemical analysis of the K5 capsular polysaccharide. Whereas the K5 clone expressed the K5 polysaccharide as a well-developed capsule in about 25% of its population, no capsule was observed in whole mount preparations and ultrathin sections of the expression mutants. Immunogold labeling of sections from the region 3 mutant revealed the capsular K5 polysaccharide in the cytoplasm. With the region 1 mutant, the capsular polysaccharide appeared associated with the cell membrane, and, unlike the region 3 mutant polysaccharide, the capsular polysaccharide could be detected in the periplasm after plasmolysis of the bacteria. Polysaccharides were isolated from the homogenized mutants with cetyltrimethylammonium bromide. The polysaccharide from the region 1 mutant had the same size as that isolated from the capsule of the original K5 clone, and both polysaccharides were substituted with phosphatidic acid. The polysaccharide from the region 3 mutant was smaller and was not substituted with phosphatidic acid. These results prompt us to postulate that gene region 3 products are involved in the translocation of the capsular polysaccharide across the cytoplasmic membrane and that region 1 directs the transport of the lipid-substituted capsular polysaccharide through the periplasm and across the outer membrane.

Visualization of the bacterial polysaccharide capsule

The highly hydrated capsule of E. coli strains is composed of a large number of polysaccharide fibers of which the thinnest measure about 2 nm in width. The fibers may span the entire distance from the outer membrane to the outer rim of the capsule and show a propensity to associate with each other to form thicker filaments. Presence of thick filaments may also indicate a partial collapse of the capsular organization due to removal of water. The in vivo capsule represents a relatively open structure with the negatively charged polysaccharide fibers permitting the binding of large quantities of water and ions, and providing intracellular space for diffusing molecules to access the envelope membranes even in conditions of high cell density. Negative charge and steric hindrance of the polysaccharide strands protect the cells against attack by a large variety of harmful macromolecules and against infection by most bacteriophages. Two types of procedure have been most successful in maintaining the size and overall structure of the capsule: (a) the interaction of cationic molecules with the in vivo capsule, and (b) the use of antibody to stabilize capsules for subsequent dehydration and plastic embedding. A further type of potentially useful procedure, cryofixation and cryosubstitution, has shown interesting results in a number of cases. These techniques are expected to play a significant role in structural studies in the near future. The sites of export of capsular antigen have been described in earlier conventional electron microscopic studies. Data obtained from the recent technique of "on-section" labeling support the model that both the capsular antigen and the O antigen are assembled at junctions of the inner and outer membrane. It is anticipated that one will be able to discern in greater ultrastructural detail the membranes at which the antigen is translocated. Novel membrane fixation and isolation techniques will have to be established and employed in a combination of sensitive microscopic techniques and immuno- and enzyme localization methods. These developments will make it possible to explore questions pertaining to the maintenance and structural organization of microbial capsules and the functional interaction of polysaccharides with natural surfaces, man-made substances and drugs.

Bacterial polysaccharide capsule synthesis, export and evolution of structural diversity

Elaboration of a capsule composed of one of a range of acidic polysaccharides is a common feature of many bacteria, particularly those capable of causing serious infections in humans. Biochemical and genetical analyses of capsule biogenesis in Escherichia coli are beginning to reveal new aspects of polysaccharide biosynthesis. Genes have been identified which are thought to encode products responsible for the translocation of these high molecular-weight polysaccharides across the cytoplasmic and outer membranes, and the organization of exported polysaccharide into a capsule. Their further analysis should provide new insights into membrane biology, particularly since the genes in question are absent from the often used laboratory strains of E. coli. Genetic analysis of capsule diversity is beginning to suggest possible mechanisms for the generation of the structural diversity of polysaccharides.

Common organization of chromosomal loci for production of different capsular polysaccharides in Haemophilus influenzae

Cloned Haemophilus influenzae type b capsulation genes were used as hybridization probes to isolate DNA from the capsulation loci (cap) of other serotypes of H. influenzae. Mapping of the resulting clones and Southern hybridization analysis of chromosomal DNAs from type a, b, c, and d strains showed that in each strain cap was organized in the same way: a central DNA segment specific to each serotype flanked by DNA segments of common structure. We infer that enzymes necessary for the synthesis of specific capsular polysaccharide are encoded in the central segment of cap, while proteins involved in a more general way in the process of capsulation are encoded in the flanking segments. Studies of the function of the DNA in one of these non-serotype-specific flanking segments (J. S. Kroll, I. Hopkins, and E. R. Moxon, Cell 53:347-356, 1988) have previously identified a gene encoding a protein necessary for polysaccharide export, an event now deduced to proceed by a mechanism independent of the nature of the disaccharide subunit in the polysaccharide. The near-total duplication of cap that has been found in most type b strains was not found at the analogous locus in the other serotypes. This reinforces our previous hypothesis, based on study of type b strains alone, that while such a duplication is unnecessary for capsulation, it confers some unexplained survival advantage on the widely prevalent strains with this clinically important serotype.

Activity of CMP-2-keto-3-deoxyoctulosonic acid synthetase in Escherichia coli strains expressing the capsular K5 polysaccharide implication for K5 polysaccharide biosynthesis

The activity of the cytoplasmic CMP-2-keto-3-deoxyoctulosonic acid synthetase (CMP-KDO synthetase), which is low in Escherichia coli rough strains such as E. coli K-12 and in uncapsulated strains such as E. coli O111, was significantly elevated in encapsulated E. coli O10:K5 and O18:K5. This enzyme activity was even higher in an E. coli clone expressing the K5 capsule. This and the following findings suggest a correlation between elevated CMP-KDO synthetase activity and the biosynthesis of the capsular K5 polysaccharide. (i) Expression of the K5 polysaccharide and elevated CMP-KDO synthetase activity were observed with bacteria grown at 37 degrees C but not with cells grown at 20 degrees C or below. (ii) The recovery kinetics of capsule expression of intact bacteria, in vitro K5 polysaccharide-synthesizing activity of bacteria, and CMP-KDO synthetase activity of bacteria after temperature upshift from 18 to 37 degrees C were the same. (iii) Chemicals which inhibit capsule (polysaccharide) expression also inhibited the elevation of CMP-KDO synthetase activity. The chromosomal location of the gene responsible for the elevation of this enzyme activity was narrowed down to the distal segment of the transport region of the K5 expression genes.

Molecular characterization and expression in Escherichia coli of the gene complex encoding the polysaccharide capsule of Neisseria meningitidis group B

The gene complex encoding all determinants of the biosynthesis pathway of the capsule of group B meningococci (cps) has been cloned in Escherichia coli. A 24-kilobase large chromosomal fragment is necessary for capsule expression on the E. coli surface. By transposon and deletion mutagenesis, two separate steps in transport of the polysaccharide from the cytoplasm to the periplasm and further to the cell surface became evident. Mutants were also isolated that accumulate soluble poly(sialic acid) in the cytoplasm. The cloned cps complex conferred to E. coli strain GC6 sensitivity for E. coli K1-specific phages; phage sensitivity was enhanced in two distinct classes of cps mutants. Southern blot experiments revealed homology to some or all other Neisseria meningitidis capsular types and other Neisseria species, depending on the fragment of the cps complex used as probe.

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.

Genetic analysis of a cluster of genes required for synthesis of the calcofluor-binding exopolysaccharide of Rhizobium meliloti

Rhizobium meliloti produces an acidic, Calcofluor-binding exopolysaccharide which plays a role in nodulation of alfalfa plants by this bacterium. We constructed and mapped 102 transposon insertions in a 48-kilobase (kb) region previously shown to contain several exo genes. Mutations affecting production of the Calcofluor-binding exopolysaccharide were clustered in a 22-kb region and fell into 12 complementation groups. Strains carrying mutations in seven of the complementation groups (exoA, exoB, exoF, exoL, exoM, exoP, and exoQ) produced no Calcofluor-binding exopolysaccharide and induced non-nitrogen-fixing nodules on alfalfa. Mutants in an eighth complementation group, exoH (Leigh et al., Cell 51:579-587, 1987), produce an altered exopolysaccharide and also induce the formation of non-nitrogen-fixing nodules. Mutants in the remaining four complementation groups produced less Calcofluor-binding material than the wild type. Mutants carrying mutations in two of these complementation groups (exoK and exoN) formed apparently normal, nitrogen-fixing nodules, while mutants in the other two groups (exoG and exoJ) formed normal nodules less efficiently than the wild type.