Cpx two-component signal transduction in Escherichia coli: Excessive CpxR-P levels underlie CpxA* phenotypes

In Escherichia coli, the CpxA-CpxR two-component signal transduction system and the sigma(E) and sigma(32) response pathways jointly regulate gene exp ression in adaptation to adverse conditions. These include envelope protein distress, heat shock, oxidative stress, high pH and entry into stationary phase. Certain mutant versions of the CpxA sensor protein (CpxA* proteins) exhibit an elevated ratio of kinase to phosphatase activity on CpxR, the co gnate response regulator. As a result, CpxA* strains display numerous pheno types, many of which cannot be easily related to currently known functions of the CpxA-CpxR pathway. It is unclear whether CpxA* phenotypes are caused solely by hyperphosphorylation of CpxR. We here report that all of the tes ted CpxA* phenotypes depend on elevated levels of CpsR-P and not on cross-s ignalling of CpxA* to noncognate response regulators.