An Escherichia coli mutant lacking the cold shock-induced palmitoleoyltransferase of lipid A biosynthesis -: Absence of unsaturated acyl chains and antibiotic hypersensitivity at 12°C

An acyltransferase induced by cold shock in Escherichia coli, designated LpxP, incorporates a palmitoleoyl moiety into nascent lipid A in place of the secondary laurate chain normally added by LpxL(HtrB) (Carty, S. M., Sreekumar, K. R., and Raetz, C. R. H. (1999) J. Biol. Chem. 274, 9677-9685). To determine whether the palmitoleoyl residue alters the properties of the outer membrane and imparts physiological benefits at low growth temperatures, we constructed a chromosomal insertion mutation in lpxP, the structural gene for the transferase. Membranes from the lpxP mutant MKV11 grown at 12 degreesC lacked the cold-induced palmitoleoyltransferase present in membranes of cold-shocked wild type cells but retained normal levels of the constitutive lauroyltransferase encoded by lpxL. When examined by mass spectrometry, about two-thirds of the lipid A molecules isolated from wild type E. coli grown at 12 degreesC contained palmitoleate in place of laurate, whereas the lipid A of cold-adapted MKV11 contained only laurate in amounts comparable with those seen in wild type cells grown at 30 degreesC or above. To probe the integrity of the outer membrane, MKV11 and an isogenic wild type strain were grown at 30 or 12 degreesC and then tested for their susceptibility to antibiotics. MKV11 exhibited a 10-fold increase in sensitivity to rifampicin and vancomycin at 12 degreesC compared with wild type cells but showed identical resistance when grown at 30 degreesC. We suggest that the palmitoleoyltransferase may confer a selective advantage upon E. coli cells growing at lower temperatures by making the outer membrane a more effective barrier to harmful chemicals.