Comparative proteomic analysis reveals new components of the PhoP regulon and highlights a role for PhoP in the regulation of genes encoding the F1F0 ATP synthase in Edwardsiella tarda

Edwardsiella tarda is an important cause of haemorrhagic septicaemia in fish and also of gastro- and extra-intestinal infections in humans. We have recently demonstrated that the PhoP-PhoQ two-component regulatory system plays important roles in both virulence and stress tolerance in E. tarda. In this study, the proteomes of the WT and phoP mutant strains were compared to define components of the PhoP regulon in E. tarda EIB202. Overall, 18 proteins whose expression levels exhibited a twofold or greater change were identified; 13 of these proteins were found to require the presence of PhoP for full expression, while five were expressed at a higher level in the phoP mutant background. Identified proteins represented diverse functional categories, including energy production, amino acid metabolism and oxidative stress defence. Quantitative real-time PCR analysis of the mRNA levels for the identified proteins confirmed the proteomics data. Interestingly, the beta subunit of the F1F0 ATP synthase, playing an important role in growth and virulence of E tarda, was listed as one of the proteins whose expression was greatly dependent on PhoP. The F1F0 ATP synthase was encoded in a gene cluster (atp/BEFHAGDC) and the nine genes were transcribed as an operon. PhoP positively regulated the transcription of the nine ATP synthase genes and exerted this effect through direct binding to the promoter of atpl. Overall, the results provide new insights into the PhoP regulon and unravel a novel role for PhoP in the regulation of the F1F0 ATP synthase.