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 Escherichia 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 mM), 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 PhoQD179L(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.