The Salmonellae PhoQ sensor: mechanisms of detection of phagosome signals

Sensor histidine kinases are widely used by prokaryotes to regulate gene expression in response to environmental signals, and such sensing is essential for virulence for plants and animals. PhoQ is one such sensor kinase that is conserved across a variety of Gram-negative pathogens and functions as part of a two-component system with its transcriptional regulator PhoP. Salmonella typhimurium PhoPQ is one of the most studied two-component systems and has been demonstrated to regulate hundreds of genes encoding the majority of virulence properties including intracellular survival, invasion, lipid A structure, resistance to antimicrobial peptides, and phagosome alteration. PhoQ is activated within acidified phagosomes and host tissues. PhoQ binds divalent cations to form bridges with the membrane that maintain the PhoQ repressed state. Antimicrobial peptides and acidic pH activate PhoQ and these signals, present within phagosomes and intestinal tissues, may serve as specific signatures of the host environment for Gram-negative pathogens. This review will focus on recent progress in the molecular details of PhoQ structure and sensing of defined signals. In addition, Pseudomonas aeruginosa and Salmonella typhimurium PhoQ are compared, providing evidence that a subset of PhoQ proteins of environmental organisms evolved more limited sensing capability using a different structural sensing mechanism.