Mg2+ Sensing by the Mg2+ Sensor PhoQ of Salmonella enterica

Abstract

The PhoP/PhoQ two-component regulatory system governs the adaptation to low Mg2+ environments and virulence in several Gram-negative species. During growth in low Mg2+, the sensor PhoQ modifies the activity of the response regulator PhoP promoting gene transcription, whereas growth in high Mg2+ represses transcription of PhoP-activated genes. The PhoQ protein harbors a periplasmic domain of 146 amino acid residues that binds Mg2+in vitro and is required for Mg2+-mediated repression in vivo. Here, we identify periplasmic mutants of the Salmonella PhoQ protein that allow transcription of PhoP-activated genes even under high Mg2+ concentrations. When expressed in a strain harboring a PhoP variant that is phosphorylated from acetyl phosphate, some of the mutants failed to repress PhoP-promoted transcription in high Mg2+, whereas others displayed a wild-type ability to do so. Mutant PhoQ proteins that allowed expression of PhoP-activated genes in high Mg2+ displayed a pattern of iron-mediated cleavage in vitro that was different from that displayed by wild-type PhoQ, indicative of altered Mg2+ binding. A PhoQ protein with the conserved histidine residue (H277) substituted by alanine could not promote transcription of PhoP-activated genes in low Mg2+ but could turn off expression in response to high Mg2+. Our studies demonstrate that residues G93, W97, H120 and T156 are required for a wild-type response to Mg2+, and suggest that Mg2+ binding to the periplasmic domain regulates several activities in the PhoQ protein.