MUTATIONS IN FIRA, ENCODING THE 2ND ACYLTRANSFERASE IN LIPOPOLYSACCHARIDE BIOSYNTHESIS, AFFECT MULTIPLE STEPS IN LIPOPOLYSACCHARIDE BIOSYNTHESIS

The product of the firA (ssc) gene is essential for growth and for the integrity of the outer membrane of Escherichia coli and Salmonella ty phimurium. Recently, Kelly and coworkers (T. M. Kelly, S. A. Stachula, C. R. H. Raetz, and M. S. Anderson, J. Biol. Chem., 268:19866-19874, 1993) identified firA as the gene encoding UDP-3-O-(R-3-hydroxymyristo yl)-glucosamine N-acyltransferase, the third step in lipid A biosynthe sis. We studied the effects of six different mutations in firA on lipo polysaccharide synthesis. All of the firA mutants of both E. coli and S. typhimurium examined had a decreased lipopolysaccharide synthesis r ate. E. coli and S. typhimurium strains defective in firA produced a l ipid A that contains a seventh fatty acid, a hexadecanoic acid, when g rown at the nonpermissive temperature. Analysis of the enzymatic activ ity of other enzymes involved in lipid A biosynthesis revealed that th e firA mutations pleiotropically affect lipopolysaccharide biosynthesi s. In addition to that of UDP-3-O-(R-3-hydroxymyristoyl)-glucosamine N -acyltransferase, the enzymatic activity of the lipid A 4' kinase (the sixth step of lipid A biosynthesis) was decreased in strains with eac h of the firA mutations examined. However, overproduction of FirA was not accompanied by overexpression of the lipid A 4' kinase.