"Magnesium controls the phosphatase activity of the two-component sensor protein PhoQ in Salmonella enterica"

E. G. VÉSCOVI, M. E. CASTELLI, F. C. SONCINI

Los Angeles, USA

Congreso; 100th General Meeting, American Society for Microbiology; 2000

American Society for Microbiology

Magnesium Controls the Phosphatase activity of the Two-Component Sensor Protein PhoQ in Salmoenlla enterica.  E. García Véscovi, M. E. Castelli, F.C. Soncini Rosario National University, School of Biochemistry, Rosario, Argentina   The PhoP/PhoQ two component system controls the expression of essential virulence traits in the pathogenic bacterium Salmonella enterica serovar typhimurium. Environmental deprivation of magnesium activates the PhoP/PhoQ signal transduction cascade that results in an increased expression of genes necessary for survival inside the host. It was previously demonstrated that the interaction of magnesium 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 magnesium on the putative activities of the membrane bound PhoQ. We demonstrated that Mg promotes a phosphorylated PhoP (PhoP-P) phosphatase activity in the sensor protein. This activity depends on the intactness of the conserved His-277, suggesting that the phosphatase active site overlaps the active site of the kinase (the H box). The integrity of the N-terminal domain of PhoQ was essential for the induction of the phosphatase activity, since Mg did not stimulate the release of inorganic phosphate from PhoP-P in a fusion protein that lacks this sensing domain. We set up an in vivo experiment using a strain that harbours a PhoP mutant protein (PhoP*) that accepts phosphate from endogenous acetyl phosphate. This mutant is capable of inducing the expression of PhoP-activated genes in a PhoQ-independent manner. In this strain PhoQ down regulated the PhoP-activated genes in the presence of millimolar concentration of Mg in the culture medium. These findings reveal that the sensor PhoQ harbours a PhoP-P phosphatase activity, and that this phosphatase activity is the target of the extracellular Mg triggered regulation of the PhoP/PhoQ system.