The genetic basis of the phase variation repertoire of lipopolysaccharide immunotypes in Neisseria meningitidis

Neisseria meningitidis strains express a diverse range of lipopolysaccharid e (LPS) structures that have been classified into 12 immunotypes. A feature of meningococcal LPS is the reversible, high-frequency switching of expres sion (phase variation) of terminal LPS structures. A number of studies are strongly suggestive of a key role for these terminal structures, and their phase-variable expression, in pathogenesis. In a previous study, a locus of three LPS biosynthetic genes, IgtABE, involved in the biosynthesis of one of these terminal structures, lacto-N-neotetraose, was described, The molec ular mechanism of phase-variable expression of this structure is by high-fr equency mutation in a homopolymeric tract of G residues in the IgtA gene. T o investigate the genetic basis of the structural differences between the i mmunotypes, and the potential for strains to express alternative immunotype s, this locus was examined in all of the immunotype strains. Initially, the Igt locus of strain 126E, an L1 immunotype strain, was cloned and sequence d, revealing two active genes, IgtC and IgtE. The remnants of the IgtA and IgtB genes and an inactive IgtD gene were also present, indicating that the locus may have once contained five active genes, similar to a locus previo usly reported in Neisseria gonorrhoeae strain F62. Probes based on each of the Igt genes (ABCDE), and the recently reported IgtC gene, were used to de termine the presence or absence of Igt genes within individual strains, all owing the prediction of the phase variation repertoire of these strains. Se quencing to determine the nature of homopolymeric tract regions within the Igt genes was carried out to establish the potential for LPS switching. In general, the set of strains examined could be sorted into two distinct grou ps: one group which phase-vary the alpha-chain extension via IgtA or IgtC b ut cannot make beta-chain; the second group phase-vary the beta-chain exten sion via IgtG but do not vary alpha-chain (lacto-N-neotetraose).