Regulated Covalent Modifications of Lipid A

Regulated covalent modifications of lipid A are implicated in virulence of pathogenic Gram-negative bacteria. The Salmonella PhoP/PhoQ-activated gene pagP is required for resistance to cationic antimicrobial peptides and for biosynthesis of hepta-acylated lipid A species containing palmitate. Interestingly, pagP encodes an unusual enzyme of lipid A biosynthesis localized in the outer membrane, whereas all previously characterized lipid A enzymes are cytoplasmic or associated with the inner membrane. PagP is not unique, however, as pagL encodes another outer membrane enzyme in Salmonella that deacylates the 3 position of lipid A.S. typhimurium also synthesizes S-2-hydroxymyristate modified lipid A in a PhoP/PhoQ-dependent manner. We postulated that 2-hydroxylation might be catalyzed by a novel dioxygenase. Using well-characterized dioxygenase sequences as probes, tBLASTn searches revealed unassigned open reading frame(s) with similarity to mammalian aspartyl beta-hydroxylases in bacteria known to make 2-hydroxyacylated lipid A. The S. typhimurium aspartyl beta-hydroxylase homologue (lpxO) was cloned and expressed in Escherichia coli K-12, which does not contain lpxO. Analysis of the resulting construct revealed that lpxO expression induces O(2)-dependent formation of 2-hydroxymyristate-modified lipid A in E. coli. LpxO may be an inner membrane enzyme that catalyzes Fe(2+)/ascorbate/alpha-ketoglutarate dependent hydroxylation of lipid A. We propose that 2-hydroxymyristate released from LPS inside infected animal cells might be converted to 2-hydroxymyristoyl coenzyme A, a potent inhibitor of protein N-myristoyl transferase.