Member of the two-component regulatory system PhoP/PhoQ which regulates the expression of genes involved in virulence, adaptation to acidic and low Mg(2+) environments and resistance to host defense antimicrobial peptides (PubMed:2544889, PubMed:9573193, PubMed:25972862). Essential for intramacrophage survival of S.typhimurium. In low periplasmic Mg(2+), PhoQ phosphorylates PhoP, resulting in the expression of PhoP-activated genes (PAG) and repression of PhoP-repressed genes (PRG). In high periplasmic Mg(2+), PhoQ dephosphorylates phospho-PhoP, resulting in the repression of PAG and may lead to expression of some PRG. Essential for transcription of spiC inside macrophages by controlling the expression of the two-component regulatory system SsrB/SpiR (SsrA) and Pir at transcriptional and post-transcriptional levels respectively. Promotes expression of the two-component regulatory system PmrA/PmrB via activation of pmrD gene. Is required to attenuate bacterial growth within fibroblast cells and to enhance bacterial resistance to bile in intestinal cells. Negatively regulates prgH, which is required for invasion of epithelial cells. PhoP uses multiple mechanisms to promote transcription and activates promoters for PAG at low (uM range) Mg(2+) concentrations (Probable). Involved in acid tolerance. Monomer in the inactive, unphosphorylated state and dimer in the active, phosphorylated state. The phoP/phoQ operon is positively autoregulated by both PhoP and PhoQ in a Mg(2+)-dependent manner. Repressed by RcsB via sigma factor RpoS. Phosphorylated and dephosphorylated by PhoQ. 10,000-fold reduction in virulence in male BALB/c mice (for strain ATCC 14028). Decreased tolerance to acid stress. PhoP/PhoQ-signaling cascade, which activates virulence membrane proteins (PagC, PagO, PagD, PagK, PgtE and PhoN), is induced by cationic antimicrobial peptides (CAMP) (polymyxin, alpha-helical peptide C18G and sheet peptide protegrin-1) at sublethal concentrations.