Biosynthesis of a Mycobacterial Lipopolysaccharide

Whole cells of Mycobacterium phlei, incubated with methyl-14C-l-methionine, synthesize a 6-O-methyl-d-glucose-containing lipopolysaccharide in which the methyl groups of the 6-O-methylglucose become extensively labeled. A soluble enzyme was obtained from sonically disrupted M. phlei cells which catalyzed the transfer of methyl groups from methyl-14C-S-adenosylmethionine to endogenous acceptor, yielding labeled lipopolysaccharide. Nucleotides containing 6-O-methylglucose could not be detected in such incubations or in incubations with whole cells. d-Gluco-oligosaccharides with α-(1 → 4)-linkages, prepared by partial acetolysis of amylose, were good acceptors in the methyltransferase system. The most active oligosaccharide fraction contained 7 to 10 glucose units, and the product of the methyltransferase reaction with such an acceptor contained methylated sequences of at least 6 glucose units. Oligosaccharides prepared from cellulose were not active as acceptors. The methyltransferase, with amylo-oligosaccharides as the acceptor, had a pH optimum of 7.0, was activated by ammonium and magnesium ions, and had an apparent Km of 10-4 m for S-adenosyl-l-methionine. Since the lipopolysaccharide of Mycobacterium species contains a sequence of about 10 α-(1 → 4)-linked 6-O-methyl-d-glucose units, and since the methyltransferase described here catalyzed the methylation of α-(1 → 4)-linked d-gluco-oligosaccharides, we conclude that the polysaccharide component of the lipopolysaccharide is probably methylated at the polymer level during its biosynthesis.