MOLECULAR DIVERGENCE OF THE SIA LOCUS IN DIFFERENT SEROGROUPS OF NEISSERIA-MENINGITIDIS EXPRESSING POLYSIALIC ACID CAPSULES

The serogroups B, C, W135 and Y of Neisseria meningitidis express chem ically and immunologically distinct capsular polysaccharides containin g sialic acid. In the case of serogroup B meningococci sialic acid is synthesized by the gene products of a locus termed sin and forms the h omopolymers of the capsule. The organization of the genes required for sialic acid synthesis in serogroups B, C, W135 and Y was elucidated b y PCR technology. Cloning, sequencing and the functional expression of the polysialyltransferase (PST) genes of serogroups B and C demonstra ted that the difference in capsule composition derives from the presen ce of related, but distinct siaD genes coding for PSTs. Analysis of me ningococci of serogroups W135 and Y expressing sialic acid heteropolym ers revealed that the DNA sequences of the corresponding genetic loci in these serogroups were highly homologous, but differed completely fr om the siaD genes of serogroups B and C. This finding suggests that en zymes unrelated to those of serogroups B and C are required for the fo rmation of sialic acid heteropolymers characteristic of the capsules o f serogroups W135 and Y.