Mechanisms of activation of the bacterial sensor kinase PhoQ

S. typhimurium is an intracellular pathogen that survives inside host macrophages, where the bacteria encounter numerous antimicrobial mechanisms. The two-component regulatory system PhoPQ is required for Salmonella virulence and functions to sense and respond appropriately to the host environment to promote virulence. In response to an activating signal, the inner membrane sensor kinase PhoQ undergoes intermolecular autophosphorylation within the dimer and then transfers the phosphate to the response regulator PhoP. PhoP controls transcription of many genes involved in virulence. The system responds in vitro to changes in the concentrations of divalent cations, being repressed by millimolar concentrations of Ca 2+, Mg2+ and Mn2+. As the concentration of Mg2+ in the phagosome during infection has been measured at approximately 1 mM, other signals must be important for activation in this environment. Indeed, antimicrobial peptides, which act as host innate defense molecules, interact directly with PhoQ at sub-inhibitory concentrations to activate PhoPQ. In this work, evidence is presented that acidic pH, another potential signature of the phagosome environment, also activates PhoQ by directly affecting conformational change in the periplasmic sensing domain. As both peptides and acidic pH are present inside phagosomes, together they represent identifying features of the mammalian intracellular environment. Though PhoQ in Salmonella appears to sense host signatures, PhoQ homologues are also present in non-pathogenic bacteria. For example, the environmental organism and opportunistic pathogen Pseudomonas aeruginosa has the phoPQ genes. Data presented here demonstrate that Pseudomonas PhoQ is repressed by divalent cations, is not activated by antimicrobial peptides and exhibits an intermediate activation by acidic pH. Salmonella expressing Pseudomonas-PhoQ were at a competitive disadvantage for systemic virulence for mice, demonstrating that the ability to sense antimicrobial peptides and acidic pH are required for PhoQ to fully mediate a productive infection. Finally, a series of structural studies suggest induced flexibility in discrete regions of the protein as the basis for activation of PhoQ by these structurally distinct signals. These results demonstrate that S. typhimurium PhoQ is capable of directly sensing acidic pH, in addition to antimicrobial peptides, via specific structural mechanisms to enhance survival within a host.