Isolation and molecular characterization of the locked-on mutant of Mg2+ sensor PhoQ in Escherichia coli

A Mg**2+ sensor mutant (PhoQD179L(A)) in which D179 of PhoQ was changed into L or A was isolated and characterized in Escherickia coli. PhoQ-PhoP regulon genes, phoPQ, mgtA and mgrB transcriptions were repressed at a high Mg**2+ concentration in WQ3007 (phoQ-defective strain)/pHO119, but not in WQ3007/ pHO179L(A). The in vitro autophosphorylation activity of membrane-bound PhoQ was repressed by Mg**2+ (10 dim), but that of membrane-bound PhoQD179L(A) was not. Furthermore, the phosphotransfer from membrane-bound PhoQ to PhoP was also repressed by Mg**2+, but was not observed in membrane-bound PhoQD179L(A). These results suggest that PhoQ- D179L(A) is a locked-on mutant that is defective in extracellular Mg**2+-sensing and that the D179 amino acid residue of PhoQ plays an essential role in signal transfer between the Mg**2+-sensory and histidine kinase domain of PhoQ.