Transfer of palmitate from phospholipids to lipid A in outer membranes of Gram‐negative bacteria

Regulated covalent modifications of lipid A are implicated in virulence of pathogenic Gram‐negative bacteria. The Salmonella typhimurium PhoP/PhoQ‐activated gene pagP is required both for biosynthesis of hepta‐acylated lipid A species containing palmitate and for resistance to cationic anti‐microbial peptides. Palmitoylated lipid A can also function as an endotoxin antagonist. We now show that pagP and its Escherichia coli homolog (crcA) encode an unusual enzyme of lipid A biosynthesis localized in the outer membrane. PagP transfers a palmitate residue from the sn‐1 position of a phospholipid to the N‐linked hydroxymyristate on the proximal unit of lipid A (or its precursors). PagP bearing a C‐terminal His6‐tag accumulated in outer membranes during overproduction, was purified with full activity and was shown by cross‐linking to behave as a homodimer. PagP is the first example of an outer membrane enzyme involved in lipid A biosynthesis. Additional pagP homologs are encoded in the genomes of Yersinia and Bordetella species. PagP may provide an adaptive response toward both Mg2+ limitation and host innate immune defenses.