The phosphatase activity is the target for Mg2+ regulation of the sensor protein PhoQ in Salmonella

The PhoP/PhoQ two component system controls the expression of essential virulence traits in the pathogenic bacterium Salmonella enterica serovar Typhimurium. Environmental deprivation of Mg2+ activates the PhoP/PhoQ signal transduction cascade, which re suits in an increased expression of genes necessary for survival inside the host. It was previously demonstrated that the interaction of Mg2+ with the periplasmic domain of PhoQ promotes a conformational change in the sensor protein that leads to the down-regulation of PhoP-activated genes. We have now examined the regulatory effect of Mg2+ On the putative activities of the membrane-bound PhoQ. We demonstrated that Mg2+ promotes a phospho-PhoP phosphatase activity in the sensor protein. This activity depends on the intactness of the conserved Ris-277, suggesting that the phosphatase active site overlaps the H box. The integrity of the N-terminal domain of PhoQ was essential for the induction of the phosphatase activity, because Mg2+ did not stimulate the release of inorganic phosphate from phospho-PhoP in a fusion protein that lacks this sensing domain. These findings reveal that the sensor PhoQ harbors a phospho-PhoP phosphatase activity, and that this phosphatase activity is the target of the extracellular Mg2+-triggered regulation of the PhoP/PhoQ system.