Member of the two-component regulatory system PhoP/PhoQ involved in adaptation to low Mg(2+) environments and the control of acid resistance genes. In low periplasmic Mg(2+), PhoQ functions as a membrane-associated protein kinase that undergoes autophosphorylation and subsequently transfers the phosphate to PhoP, resulting in the expression of PhoP-activated genes (PAG) and repression of PhoP-repressed genes (PRG). In high periplasmic Mg(2+), acts as a protein phosphatase that dephosphorylates phospho-PhoP, resulting in the repression of PAG and may lead to expression of some PRG (By similarity). PhoP-regulated transcription is redox-sensitive, being activated when the periplasm becomes more reducing (deletion of dsbA/dsbB, or treatment with dithiothreitol). MgrB acts between DsbA/DsbB and PhoP/PhoQ in this pathway; the 2 periplasmic Cys residues of MgrB are required for its action on PhoQ, which then acts on PhoP. Mediates magnesium influx to the cytosol by activation of mgtA. Promotes expression of the two-component regulatory system rstA/rstB and transcription of the hemL, mgrB, nagA, slyB, vboR and yrbL genes. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Acetyl-CoA acts as a non-competitive inhibitor of the PhoQ autokinase activity. Feedback inhibited by MgrB, which seems to bind PhoQ, altering its activity and that of downstream effector PhoP. Homodimer; probably dimerizes via the cytoplasmic domain (Probable). Probably interacts with MgrB in the periplasm, altering its activity and that of downstream effector PhoP. The phoP/phoQ operon is positively autoregulated by both PhoP and PhoQ in a Mg(2+)-dependent manner, inhibited at high Mg(2+) concentrations (PubMed:10464230). Induced by dsbA disruption and dithiothreitol (PubMed:22267510). There is a close linkage between the PhoP/PhoQ and Rcs signaling systems, and both signaling systems respond to certain external divalent cations (zinc and magnesium). Two-component regulatory system EvgA/EvgS interacts with PhoP/PhoQ via signal transduction mediated by phospho-EvgA.