Effect of Cold Shock on Lipid A Biosynthesis inEscherichia coli

Abstract

Palmitoleate is not present in lipid A isolated from Escherichia coli grown at 30 °C or higher, but it comprises ~11% of the fatty acyl chains of lipid A in cells grown at 12 °C. The appearance of palmitoleate at 12 °C is accompanied by a decline in laurate from ~18% to ~5.5%. We now report that wild-type E. coli shifted from 30 °C to 12 °C acquire a novel palmitoleoyl- acyl carrier protein (ACP)-dependent acyltransferase that acts on the key lipid A precursor Kdo2-lipid IV(A). The palmitoleoyl transferase is induced more than 30-fold upon cold shock, as judged by assaying extracts of cells shifted to 12 °C. The induced activity is maximal after 2 h of cold shock, and then gradually declines but does not disappear. Strains harboring an insertion mutation in the lpxL(htrB) gene, which encodes the enzyme that normally transfers laurate from lauroyl-ACP to Kdo2-lipid IV(A) (Clementz, T., Bednarski, J. J., and Raetz, C. R. H. (1996) J. Biol. Chem. 271, 12095- 12102) are not defective in the cold-induced palmitoleoyl transferase. Recently, a gene displaying 54% identity and 73% similarity at the protein level to lpxL was found in the genome of E. coli. This lpxL homologue, designated lpxP, encodes the cold shock-induced palmitoleoyl transferase. Extracts of cells containing lpxP on the multicopy plasmid pSK57 exhibit a 10-fold increase in the specific activity of the cold-induced palmitoleoyl transferase compared with cells lacking the plasmid. The elevated specific activity of the palmitoleoyl transferase under conditions of cold shock is attributed to greatly increased levels of lpxP mRNA. The replacement of laurate with palmitoleate in lipid A may reflect the desirability of maintaining the optimal outer membrane fluidity at 12 °C.