Remodeling of Lipid A in Pseudomonas syringae pv. phaseolicola In Vitro

Pseudomonas species infect a variety of organisms, including mammals and plants. Mammalian pathogens of the Pseudomonas family modify their lipid A during host entry to evade immune responses and to create an effective barrier against different environments, for example by removal of primary acyl chains, addition of phosphoethanolamine (P-EtN) to primary phosphates, and hydroxylation of secondary acyl chains. For Pseudomonas syringae pv. phaseolicola (Pph) 1448A, an economically important pathogen of beans, we observed similar lipid A modifications by mass spectrometric analysis. Therefore, we investigated predicted proteomes of various plant-associated Pseudomonas spp. for putative lipid A-modifying proteins using the well-studied mammalian pathogen Pseudomonas aeruginosa as a reference. We generated isogenic mutant strains of candidate genes and analyzed their lipid A. We show that the function of PagL, LpxO, and EptA is generally conserved in Pph 1448A. PagL-mediated de-acylation occurs at the distal glucosamine, whereas LpxO hydroxylates the secondary acyl chain on the distal glucosamine. The addition of P-EtN catalyzed by EptA occurs at both phosphates of lipid A. Our study characterizes lipid A modifications in vitro and provides a useful set of mutant strains relevant for further functional studies on lipid A modifications in Pph 1448A.