Biosynthesis of a bacterial lipopolysaccharide. VI. Mechanism of incorporation of abequose into the O-antigen of Salmonella typhimurium.

Enzymatic incorporation of abequose into the repeating unit of the O-antigen is catalyzed by the particulate cell envelope fraction of Salmonella typhimurium. The initial step is the formation of a lipid-linked tetrasaccharide intermediate according to the reaction cytidine diphosphate abequose + mannosyl-rhamnosyl-galactose1 PP-lipid -+ abequosyl-mannosyl-rhamnosyl-galactose -1 -PP-lipid. The tetrasaccharide-lipid intermediate has been isolated by extraction into chloroform-methanol, and the oligosaccharide moiety identified. Subsequent polymerization reactions lead to formation of O-antigen polysaccharide chains containing tetrasaccharide repeating units. Isolation of a lipid-linked oligosaccharide corresponding to a dhner of the repeating unit provides direct evidence that the growing O-antigen chain remains attached to lipid during polymerization. Evidence that the structure of the enzymatically synthesized polysaccharide corresponds to that of authentic O-antigen has been obtained by isolation and characterization of abequose-containing oligosaccharides following degradation of the polymers with periodate.