The recent cloning of the lipooligosaccharide (LOS) A-2,3-sialyltransferase from Neisseria meningitidis immunotype L3 permitted us to examine other immunotypes for this structural gene. We identified the gene and measured the enzyme activity in the L1 immunotype strain which had previously been reported to lack sialic acid in its LOS because it contains a terminal A-linked galactose which was thought not to be an acceptor for the sialyltransferase. This finding prompted us to re-examine the structure of the LOS from the L1 immunotype, which revealed the presence of sialic acid on the terminal A-linked galactose. Oligosaccharides derived from the LOS were shown to be sialylated by composition and methylation analysis, mass spectrometry and nuclear magnetic resonance. The detailed structural analysis showed the sialic acid to occur only at O6 of the teminal A-D-galactopyranose residue of the A-D-Gal-1,4-ÎČ-D-Gal-1,4-ÎČ-D-glc trisaccharide (P k epitope) chain of the LOS, in the A-D configuration. These data are the first report of a A-2,6-linked sialic acid in a bacterial LOS or lipopolysaccharide, and also the first report of a sialylated P k epitope.Keywords : sialyltransferase ; Neisseria meningitidis; glycosyltransferase; lipooligosaccharide.The A-2,3-sialyltransferase involved in the biosynthesis of The mucosal pathogens in the genera Neisseria, Haemophilus, Moraxella, Campylobacter and Bordetella all possess a ma-sialyllacto-N-neotetraose has been reported to occur in the major cell surface lipooligosaccharide (LOS) which has features jority of the N. meningitidis immunotypes as well as N. gonorthat distinguish it from the enterobacterial lipopolysaccharide rhoeae isolates [8]. The distibution of enzyme activity does not (LPS) [1]. Perhaps the most important distinction is that struc-always correlate with the presence or absence of lacto-N-neotures displayed in LOS from these bacteria have some structural tetraose in the LOS. We previously reported that the N. meninsimilarity to human glycolipids. The LOS molecules elaborated gitidis A-2,3-sialyltransferase has a relaxed acceptor specificity by strains of N. meningitidis and N. gonorrhoeae that have been in that it would use synthetic acceptors which presented terminal shown to be important in the disease mechanism include those N-acetyllactosamine, lactose or galactose [9]. We have extended presenting the following oligosaccharide structures: Lacto-N-our examination of this enzyme and have reported recently that neotetraose, Sialyllacto-N-neotetraose, and the P k blood group the A-2,3-sialyltransferase from N. meningitidis immunotypes L3 antigen [1] (Fig. 1). These oligosaccharides provide these patho-and L1 can also use the terminal A-D-galactose, to make an Agens with a means of evading the host immune response through 2,3-sialyl-P k -trisaccharide in vitro with a synthetic acceptor molmolecular mimicry and are therefore potent virulence factors [2]. ecule [10]. Since we had demonstrated that an A-D-galactose resThe genetics of LPS biosynthesis have been ext...