Membrane-associated nucleotide sugar reactions: influence of mutations affecting lipopolysaccharide on the first enzyme of O-antigen synthesis

Both the synthesis of lipopolysaccharide O-antigen and the synthesis of peptidoglycan in Salmonella typhimurium proceed via membrane-bound glycosylated lipid intermediates. The first enzyme of each pathway transfers a sugar phosphate from a nucleotide sugar to the glycosyl carrier lipid (P-GCL). Each enzyme catalyzes an exchange reaction between the reaction product urine monophosphate, and the nucleotide sugar substrate. Several strains of S. typhimurium defective in lipopolysaccharide synthesis accumulate glycosylated lipid intermediates under appropriate conditions. In addition, strains lysogenic for phage P22 synthesize a glucose derivative of the carrier lipid. These strains were used to demonstrate the P/GCL requirement of the exchange reaction catalyzed by galactose-diphosphoglycosyl carrier lipid (GCL-PP-Gal) synthetase, the first enzyme of O-antigen synthesis. Enzyme activity is greatly reduced when glycosylated P-GCL accumulates on the cytoplasmic membrane. The exchange reaction catalyzed by the first enzyme of peptidoglycan synthesis is unaffected by the accumulation of O-antigen fragments on the carrier lipid and may interact with a different pool of P-GCL within the membrane. GCL-PP-Gal synthetase activity cannot be detected in the membranes of two rfa mutants that synthesize incomplete lipopolysaccharide core. Either the synthesis of GCL-PP-Gal synthetase or the stable integration of the enzyme into the membrane structure may be disrupted in the rfa mutants. Peptidoglycan synthesis is unaffected by the mutations affecting the core glycosyltransferases.