The PhoP-PhoQ Two-Component Regulatory System of Photorhabdus luminescens Is Essential for Virulence in Insects

Abstract

ABSTRACT Photorhabdus luminescens is a symbiont of entomopathogenic nematodes. Analysis of the genome sequence of this organism revealed a homologue of PhoP-PhoQ, a two-component system associated with virulence in intracellular bacterial pathogens. This organism was shown to respond to the availability of environmental magnesium. A mutant with a knockout mutation in the regulatory component of this system ( phoP ) had no obvious growth defect. It was, however, more motile and more sensitive to antimicrobial peptides than its wild-type parent. Remarkably, the mutation eliminated virulence in an insect model. No insect mortality was observed after injection of a large number of the phoP bacteria, while very small amounts of parental cells killed insect larvae in less than 48 h. At the molecular level, the PhoPQ system mediated Mg 2+ -dependent modifications in lipopolysaccharides and controlled a locus ( pbgPE ) required for incorporation of 4-aminoarabinose into lipid A. Mg 2+ -regulated gene expression of pbgP1 was absent in the mutant and was restored when phoPQ was complemented in trans . This finding highlights the essential role played by PhoPQ in the virulence of an entomopathogen.