The Diverse Roles of the Global Transcriptional Regulator PhoP in the Lifecycle of .

, the causative agent of plague, has a complex infectious cycle that alternates between mammalian hosts (rodents and humans) and insect vectors (fleas). Consequently, it must adapt to a wide range of host environments to achieve successful propagation. PhoP is a response regulator of the PhoP/PhoQ two-component signal transduction system that plays a critical role in the pathogen's adaptation to hostile conditions. PhoP is activated in response to various host-associated stress signals detected by the sensor kinase PhoQ and mediates changes in global gene expression profiles that lead to cellular responses. PhoP is required for resistance to antimicrobial peptides, as well as growth under low Mg and other stress conditions, and controls a number of metabolic pathways, including an alternate carbon catabolism. Loss of function in causes severe defects in survival inside mammalian macrophages and neutrophils in vitro, and a mild attenuation in murine plague models in vivo, suggesting its role in pathogenesis. A mutant also exhibits reduced ability to form biofilm and to block fleas in vivo, indicating that the gene is also important for establishing a transmissible infection in this vector. Additionally, promotes the survival of inside the soil-dwelling amoeba , a potential reservoir while the pathogen is quiescent. In this review, we summarize our current knowledge on the mechanisms of PhoP-mediated gene regulation in and examine the significance of the roles played by the PhoP regulon at each stage of the life cycle.