MOLECULAR ANALYSIS OF A LOCUS FOR THE BIOSYNTHESIS AND PHASE-VARIABLEEXPRESSION OF THE LACTO-N-NEOTETRAOSE TERMINAL LIPOPOLYSACCHARIDE STRUCTURE IN NEISSERIA-MENINGITIDIS

Lipopolysaccharide (LPS) is a major determinant of Neisseria meningiti dis virulence. A key feature of meningococcal LPS is the phase-variabl e expression of terminal structures which are proposed to have dispara te roles in pathogenesis. In order to identify the biosynthetic genes for terminal LPS structures and the control mechanisms for their phase -variable expression, the lic2A gene, which is involved in LPS biosynt hesis in Haemophilus influenzae, was used as a hybridization probe to identify a homologous gene in N. meningitidis strain MC58. The homolog ous region of DNA was cloned and nucleotide sequence analysis revealed three open reading frames (ORFs), two of which were homologous to the H. influenzae lic2A gene. All three ORFs were mutagenized by the inse rtion of antibiotic-resistance cassettes and the LPS from these mutant strains was analysed to determine if the genes had a role in LPS bios ynthesis. Immunological and tricine-SDS-PAGE analysis of LPS from the mutant strains indicated that all three genes were probably transferas es in the biosynthesis of the terminal lacto-N-neotetraose structure o f meningococcal LPS. The first ORF of the locus contains a homopolymer ic tract of 14 guanosine residues within the B-end of the coding seque nce. As the lacto-N-neotetraose structure in meningococcal LPS is subj ect to phase-variable expression, colonies that no longer expressed th e terminal structure, as determined by monoclonal antibody binding, we re isolated. Analysis of an 'off' phase variant revealed a change in t he number of guanosine residues resulting in a frameshift mutation, in dicating that a slipped-strand mispairing mechanism, operating in the first ORF, controls the phase-variable expression of lacto-N-neotetrao se.