Ail and PagC-Related Proteins in the Entomopathogenic Bacteria of <i>Photorhabdus</i> Genus

<div><p>Among pathogenic <i>Enterobacteriaceae</i>, the proteins of the Ail/OmpX/PagC family form a steadily growing family of outer membrane proteins with diverse biological properties, potentially involved in virulence such as human serum resistance, adhesion and entry into eukaryotic culture cells. We studied the proteins Ail/OmpX/PagC in the bacterial <i>Photorhabdus</i> genus. The <i>Photorhabdus</i> bacteria form symbiotic complexes with nematodes of <i>Heterorhabditis</i> species, associations which are pathogenic to insect larvae. Our phylogenetic analysis indicated that in <i>Photorhabdus asymbiotica</i> and <i>Photorhabdus luminescens</i> only Ail and PagC proteins are encoded. The genomic analysis revealed that the <i>Photorhabdus ail</i> and <i>pagC</i> genes were present in a unique copy, except two <i>ail</i> paralogs from <i>P. luminescens</i>. These genes, referred to as <i>ail1</i>_Pl and <i>ail2</i>_Pl, probably resulted from a recent tandem duplication. Surprisingly, only <i>ail1</i>_Pl expression was directly controlled by PhoPQ and low external Mg²⁺ conditions. In <i>P. luminescens</i>, the magnesium-sensing two-component regulatory system PhoPQ regulates the outer membrane barrier and is required for pathogenicity against insects. In order to characterize Ail functions in <i>Photorhabdus</i>, we showed that only <i>ail2</i>_Pl and <i>pagC</i>_Pl had the ability, when expressed into <i>Escherichia coli</i>, to confer resistance to complement in human serum. However no effect in resistance to antimicrobial peptides was found. Thus, the role of Ail and PagC proteins in <i>Photorhabdus</i> life cycle is discussed.</p>