Transcriptional regulation of divergent capsule biosynthesis and transport operon promoters in serogroup B Neisseria meningitidis.

The clinically important serogroups B, C, Y, and W-135 of Neisseria meningitidis produce sialic acid capsules that are critical in pathogenesis. In each of these serogroups, the capsule transport (ctrABCD) and capsule biosynthesis (synABCD) operons are divergently transcribed from putative promoters located in a 134-bp intergenic region (J. S. Swartley, J. H. Ahn, L. J. Liu, C. M. Kahler, and D. S. Stephens, J. Bacteriol. 178:4052-4059, 1996). In this study we further assessed the role of the intergenic sequence in the transcriptional regulation of the sialic acid capsules of N. meningitidis. Insertional mutagenesis or deletions of the 134-bp sequence in the serogroup B meningococcal strain NMB resulted in a marked reduction or elimination of ctrABCD and synABCD transcription, with a concomitant loss of encapsulation. Chromosomal transcriptional lacZ-ermC reporter fusions of syn and ctr promoters were constructed through allelic exchange. Using these constructs, both operons were found to be constitutively transcribed in meningococci, the biosynthesis operon about fourfold higher than the transport operon. Both promoters showed increased activity during stationary-phase growth. In addition to the promoters, a 70-bp 5' untranslated region (UTR) upstream of synA was found to have a direct repeat and an inverted repeat that overlapped three putative integration host factor binding sites. Mutation of this 70-bp UTR and of the direct repeat upregulated both syn and ctr transcription. Regulation through the synA UTR was absent in a K1 Escherichia coli strain that produces identical capsular polysaccharide, implicating species-specific regulation. Meningococcal sialic acid capsule expression is initiated by divergent promoters in a 134-bp intergenic region, is repressed at the transcriptional level by the 5' UTR of synA, is increased during stationary-phase growth, and shows species-specific regulation. Transcriptional regulation is another important control point for sialic capsule expression in N. meningitidis.

Multiple lysophosphatidic acid acyltransferases in Neisseria meningitidis.

Lysophosphatidic acid (LPA) and phosphatidic acid (PA) are critical phospholipid intermediates in the biosynthesis of cell membranes. In Escherichia coli, LPA acyltransferase (1-acyl-sn-glycerol-3-phosphate acyltransferase; EC 2.3.1.51) catalyses the transfer of an acyl chain from either acyl-coenzyme A or acyl-acyl carrier protein onto LPA to produce PA. While E. coli possesses one essential LPA acyltransferase (PlsC), Neisseria meningitidis possesses at least two LPA acyltransferases. This study describes the identification and characterization of nlaB (neisserial LPA acyltransferase B), the second LPA acyltransferase identified in N. meningitidis. The gene was located downstream of the Tn916 insertion in N. meningitidis mutant 469 and differed in nucleotide and predicted amino acid sequence from the previously characterized neisserial LPA acyltransferase homologue nlaA. NlaB has specific LPA acyltransferase activity, as demonstrated by complementation of an E. coli plsC(Ts) mutant in trans, by decreased levels of LPA acyltransferase activity in nlaB mutants and by lack of complementation of E. coli plsB26,X50, a mutant defective in the first acyltransferase step in phospholipid biosynthesis. Meningococcal nlaA mutants accumulated LPA and demonstrated alterations in membrane phospholipid composition, yet retained LPA acyltransferase activity. In contrast, meningococcal nlaB mutants exhibited decreased LPA acyltransferase activity, but did not accumulate LPA or display any other observable membrane changes. We propose that N. meningitidis possesses at least two LPA acyltransferases to provide for the production of a greater diversity of membrane phospholipids.

Characterization of the gene cassette required for biosynthesis of the (alpha1-->6)-linked N-acetyl-D-mannosamine-1-phosphate capsule of serogroup A Neisseria meningitidis.

The (alpha1-->6)-linked N-acetyl-D-mannosamine-1-phosphate meningococcal capsule of serogroup A Neisseria meningitidis is biochemically distinct from the sialic acid-containing capsules produced by other disease-associated meningococcal serogroups (e.g., B, C, Y, and W-135). We defined the genetic cassette responsible for expression of the serogroup A capsule. The cassette comprised a 4,701-bp nucleotide sequence located between the outer membrane capsule transporter gene, ctrA, and galE, encoding the UDP-glucose-4-epimerase. Four open reading frames (ORFs) not found in the genomes of the other meningococcal serogroups were identified. The first serogroup A ORF was separated from ctrA by a 218-bp intergenic region. Reverse transcriptase (RT) PCR and primer extension studies of serogroup A mRNA showed that all four ORFs were cotranscribed in the opposite orientation to ctrA and that transcription of the ORFs was initiated from the intergenic region by a sigma-70-type promoter that overlapped the ctrA promoter. The first ORF exhibited 58% amino acid identity with the UDP-N-acetyl-D-glucosamine (UDP-GlcNAc) 2-epimerase of Escherichia coli, which is responsible for the conversion of UDP-GlcNAc into UDP-N-acetyl-D-mannosamine. Polar or nonpolar mutagenesis of each of the ORFs resulted in an abrogation of serogroup A capsule production as determined by colony immunoblots and enzyme-linked immunosorbent assay. Replacement of the serogroup A biosynthetic gene cassette with a serogroup B cassette by transformation resulted in capsule switching from a serogroup A capsule to a serogroup B capsule. These data indicate that assembly of the serogroup A capsule likely begins with monomeric UDP-GlcNAc and requires proteins encoded by three other genes found in the serogroup A N. meningitidis-specific operon located between ctrA and galE.

Capsule switching of Neisseria meningitidis.

The different sialic acid (serogroups B, C, Y, and W-135) and nonsialic acid (serogroup A) capsular polysaccharides expressed by Neisseria meningitidis are major virulence factors and are used as epidemiologic markers and vaccine targets. However, the identification of meningococcal isolates with similar genetic markers but expressing different capsular polysaccharides suggests that meningococcal clones can switch the type of capsule they express. We identified, except for capsule, isogenic serogroups B [(alpha2-->8)-linked polysialic acid] and C [(alpha2-->9)-linked polysialic acid] meningococcal isolates from an outbreak of meningococcal disease in the U. S. Pacific Northwest. We used these isolates and prototype serogroup A, B, C, Y, and W-135 strains to define the capsular biosynthetic and transport operons of the major meningococcal serogroups and to show that switching from the B to C capsule in the outbreak strain was the result of allelic exchange of the polysialyltransferase. Capsule switching was probably the result of transformation and horizontal DNA exchange in vivo of a serogroup C capsule biosynthetic operon. These findings indicate that closely related virulent meningococcal clones may not be recognized by traditional serogroup-based surveillance and can escape vaccine-induced or natural protective immunity by capsule switching. Capsule switching may be an important virulence mechanism of meningococci and other encapsulated bacterial pathogens. As vaccine development progresses and broader immunization with capsular polysaccharide conjugate vaccines becomes a reality, the ability to switch capsular types may have important implications for the impact of these vaccines.

Expression of sialic acid and polysialic acid in serogroup B Neisseria meningitidis: divergent transcription of biosynthesis and transport operons through a common promoter region.

We studied capsule-defective (Cap-) serogroup B meningococcal mutants created through Tn916 or omega-fragment mutagenesis. The Cap- phenotypes were the results of insertions in three of four linked genes (synX, synC, and synD) involved in CMP-N-acetylneuraminic acid and polysialic acid capsule biosynthesis, and in ctrA the first of four linked genes involved in capsule membrane transport. Mutations in the CMP-N-acetylneuraminic acid biosynthesis genes synX and synC caused defects in lipooligosaccharide sialylation but not mutations in the putative (alpha2 -> 8)-linked polysialyltransferase (synD) or in ctrA. Reverse transcriptase PCR studies indicated that the four biosynthesis genes (synX to -D) and the capsule transport genes (ctr to -D) were separately transcribed as operons. The operons were separated by a 134-bp intergenic region. Primer extension of synX and ctrA demonstrated that transcription of the operons was divergently initiated from adjacent start sites present in the intergenic region. Both transcriptional start sites were preceded by a perfect -10 Pribnow promoter binding region. The synX to -D, but not the ctrA to -D, transcriptional start site was preceded by a sequence bearing strong homology to the consensus sigma 70 -35 promoter binding sequence. Both promoters showed transcriptional activity when cloned behind a lacZ reporter gene in Escherichia coli. Our results confirm the intrinsic relationship between polysialic acid capsule biosynthesis and lipooligosaccharide sialylation pathways in group B Neisseria meningitidis. Our study also suggests that the intergenic region separating the synX to -D and ctrA to -D operons is an important control point for the regulation of group B capsule expression through coordinated transcriptional regulation of the synX to -D and drA to -D promoters.

Co-transcription of a homologue of the formamidopyrimidine-DNA glycosylase (fpg) and lysophosphatidic acid acyltransferase (nlaA) in Neisseria meningitidis.

We report the identification of an open reading frame in a serogroup B isolate of Neisseria meningitidis that exhibits high nucleotide and predicted amino acid identity with the fpg gene of Escherichia coli, and its product, formamidopyrimidine-DNA glycosylase (Fapy-DNA glycosylase), a DNA repair enzyme. We further show that the meningococcal fpg is co-transcribed with nlaA, encoding a lysophosphatidic acid acyltransferase, and suggest that the DNA repair enzyme may be involved in the regulation of nlaA or its gene product.

Membrane glycerophospholipid biosynthesis in Neisseria meningitidis and Neisseria gonorrhoeae: identification, characterization, and mutagenesis of a lysophosphatidic acid acyltransferase.

Lysophosphatidic acid (LPA) acyltransferases of Neisseria meningitidis and Neisseria gonorrhoeae were identified which share homology with other prokaryotic and eukaryotic LPA acyltransferases. In Escherichia coli, the conversion of LPA to phosphatidic acid, performed by the 1-acyl-sn-glycerol-3-phosphate acyltransferase PlsC, is a critical intermediate step in the biosynthesis of membrane glycerophospholipids. A Tn916-generated mutant of a serogroup B meningococcal strain was identified that exhibited increased amounts of capsular polysaccharide, as shown by colony immunoblots, and a threefold increase in the number of assembled pili. The single, truncated 3.8 kb Tn916 insertion in the meningococcal mutant was localized within a 771 bp open reading frame, The gonococcal equivalent of this gene was identified by transformation with the cloned meningococcal mutant gene. In N. gonorrhoeae, the mutation increased piliation fivefold. The insertions were found to be within a gene that was subsequently designated nlaA (neisserial LPA acyltransferase). The predicted neisserial LPA acyltransferases were homologous (>20% identity, >40% amino acid similarity) to the family of PlsC protein homologues. A cloned copy of the meningococcal nlaA gene complemented in trans a temperature-sensitive E. coli PlsCts- mutant. Tn916 and omega-cassette insertional inactivations of the neisserial nlaA genes altered the membrane glycerophospholipid compositions of both N. meningitidis and N. gonorrhoeae but were not lethal. Therefore, the pathogenic Neisseria spp. appear to be able to utilize alternative enzyme(s) to produce phosphatidic acid. This hypothesis is supported by the observation that, although the amounts of mature glycerophospholipids were altered in the meningococcal and the gonococcal nlaA mutants, glycerophospholipid synthesis was detectable at significant levels. In addition, acyltransferase enzymatic activity, while reduced in the gonococcal nlaA mutant, was increased in the meningococcal nlaA mutant. We postulate that the pathogenic Neisseria spp. are able to utilize alternate acyltransferases to produce glycerophospholipids in the absence of nlaA enzymatic activity. Implementation of these secondary enzymes results in alterations of glycerophospholipid composition that lead to pleiotropic effects on the cell surface components, including effects on capsule and piliation.

Identification of a genetic locus involved in the biosynthesis of N-acetyl-D-mannosamine, a precursor of the (alpha 2-->8)-linked polysialic acid capsule of serogroup B Neisseria meningitidis.

We characterized the genetic defect of a capsule-deficient serogroup B meningococcal strain created by Tn916 mutagenesis. The transposon insertion interrupts a capsule biosynthesis gene, synX, which is involved in the production of N-acetyl-D-mannosamine, a precursor of the (alpha 2-->8)-linked polysialic acid capsule of serogroup B meningococci.

Deletions of Tn916-like transposons are implicated in tetM-mediated resistance in pathogenic Neisseria.

Using the tetM-containing conjugative transposon Tn916 as a mutagenesis tool, we identified two distinct classes of transposon insertions in the meningococcal chromosome. Class I insertions have an intact copy of Tn916 that appears to have transposed by a novel recombinational mechanism, similar to the transposition of conjugative transposons in Gram-positive bacteria. Class II insertions were characterized by deletions of Tn916 but preservation of the tetM determinant. In addition, we identified Class II Tn916-like insertions in the naturally occurring 25.2 MDa tetM-containing plasmids of both Neisseria meningitidis and Neisseria gonorrhoeae. The turncated Tn916-like insertions appeared to be present in the same site in these two plasmids; however, the deletions of the transposon were different. Plasmid sequence adjacent to the truncated transposon in the 25.2 MDa plasmids was found in a tetracycline-sensitive N. gonorrhoeae 24.5 MDa conjugative plasmid. These data suggest that the 25.2 MDa plasmids are the result of one or a series of Class II Tn916-like insertions into 24.5 MDa conjugative plasmids. Class II insertions of Tn916-like transposons are implicated in the dissemination of tetM resistance in pathogenic Neisseria.

Effect of the (alpha 2-->8)-linked polysialic acid capsule on adherence of Neisseria meningitidis to human mucosal cells.

The effect of (alpha 2-->8)-linked polysialic acid on the adherence of Neisseria meningitidis to human mucosal cells was examined using a serogroup B-encapsulated strain and a capsule-defective (Cap-) mutant of this strain. The Cap- mutant contains a single truncated insert of Tn916 in a 3.8-kb HaeIII chromosomal fragment. The Tn916 insert was shown to be responsible for the phenotype by linkage studies and by demonstration that loss of the insert restored encapsulation. The Cap- mutant consistently adhered to human buccal epithelial cell in greater numbers than the encapsulated parent, but the increase in adherence was less than twofold. Adherence of the Cap- mutant during infection of human nasopharyngeal organ cultures was 1.3- to 6.5-fold greater than that of the encapsulated parent. However, specificity of adherence of meningococci for nonciliated nasopharyngeal epithelial cells and the ability to be internalized by these cells was not due to the (alpha 2-->8)-linked polysialic acid capsule.

Insertion of Tn916 in Neisseria meningitidis resulting in loss of group B capsular polysaccharide.

We recently found that the 16.4-kb conjugative transposon Tn916 could be introduced into Neisseria meningitidis by transformation and that it appeared to transpose to many different sites in the chromosome of recipient meningococci. In order to identify transposon-induced alterations of specific meningococcal virulence determinants, a library of meningococcal Tetr transformants containing Tn916 was made and screened for those altered in the production of group B capsular polysaccharide. A capsule-defective mutant, M7, was identified by using monoclonal and polyclonal antisera to group B polysaccharide in immunoblot and agar antiserum procedures. Growth of M7 was similar to that of the parent strain. M7 produced no group B capsular polysaccharide by rocket immunoelectrophoresis, and the mutation was stable during laboratory passage. The capsule-defective phenotype was linked to Tetr, as demonstrated by immunoblot and Southern blot analysis of progeny Tetr transformants (transformants of the parent strain obtained with DNA from M7). A capsule-deficient mutant, O8, was identified by using a similar approach. Analysis of the Tn916 insertions in M7 and O8 indicated that a significant portion of the transposon on either side of the tetM determinant had been lost. The ability of Tn916 to generate defined, stable mutations in meningococcal virulence determinants is demonstrated by our study.

Gene encoding the major subunit of CS1 pili of human enterotoxigenic Escherichia coli.

Some enterotoxigenic strains of Escherichia coli (ETEC) utilize the CS1 pilus for colonization of human intestinal epithelium. We have cloned the gene which encodes the major CS1 subunit and called it cooA (for coli surface antigen one). Hybridization showed that the ETEC strain from which it was cloned carried cooA on a plasmid different from the one encoding its positive regulator, rns. Based on the cooA DNA sequence, cleavage with signal peptidase would be expected to produce a mature protein of 15.2 kDa; a 16-kDa polypeptide that reacted with CS1-specific antiserum was observed on electrophoresis. At the protein level, there was 92% similarity and 55% identity between cooA and cfaB, the major colonization factor antigen I (CFA/I) antigen. However, CS1-specific antisera did not react with CfaB. No hybridization was seen between either of two different cooA probes and total DNA from ETEC strains expressing AFA-1, CFA/I, CS2, CS3, CS4, CS5, or CS6.