The PhoP-PhoQ two-component system in gram-negative bacteria regulates virulence, adaptation to Mg²⁺-limiting environments, and various cellular functions. It consists of the inner membrane sensor PhoQ and the cytoplasmic regulator PhoP. PhoP-PhoQ responds to Mg²⁺ and Ca²⁺ levels, distinguishing it from other systems like PhoB-PhoR. The system is crucial for virulence in Salmonella and other pathogens, with PhoP-PhoQ controlling the expression of genes involved in virulence, Mg²⁺ transport, and lipid A modification. PhoP-PhoQ also interacts with other systems, such as PmrA-PmrB, to regulate gene expression in response to environmental signals. The system is involved in resistance to antimicrobial peptides and adaptation to low Mg²⁺ environments. PhoP-PhoQ is essential for intracellular survival and virulence in various gram-negative species, including Salmonella, Shigella, and Yersinia. The system's regulation of gene expression is critical for bacterial survival and pathogenesis, and its study provides insights into bacterial adaptation and virulence mechanisms.The PhoP-PhoQ two-component system in gram-negative bacteria regulates virulence, adaptation to Mg²⁺-limiting environments, and various cellular functions. It consists of the inner membrane sensor PhoQ and the cytoplasmic regulator PhoP. PhoP-PhoQ responds to Mg²⁺ and Ca²⁺ levels, distinguishing it from other systems like PhoB-PhoR. The system is crucial for virulence in Salmonella and other pathogens, with PhoP-PhoQ controlling the expression of genes involved in virulence, Mg²⁺ transport, and lipid A modification. PhoP-PhoQ also interacts with other systems, such as PmrA-PmrB, to regulate gene expression in response to environmental signals. The system is involved in resistance to antimicrobial peptides and adaptation to low Mg²⁺ environments. PhoP-PhoQ is essential for intracellular survival and virulence in various gram-negative species, including Salmonella, Shigella, and Yersinia. The system's regulation of gene expression is critical for bacterial survival and pathogenesis, and its study provides insights into bacterial adaptation and virulence mechanisms.