Modulation of KdpD phosphatase implicated in the physiological expression of the Kdp ATPase of Escherichia coli

The KdpD sensor kinase and the KdpE response regulator control the expressi on of the kdpFABC operon, encoding the KdpFABC high-affinity K+ transport s ystem of Escherichia coli. Low turgor pressure has been postulated to be th e environmental stimulus to express KdpFABC. KdpD has autokinase, phosphotr ansferase and, like many sensor kinases, response regulator (phospho-KdpE) specific phosphatase activity. To determine which of these activities are a ltered in response to the environmental stimulus, we isolated and analysed six kdpD mutants that cause constitutive expression of KdpFABC. In three of the mutants, phosphatase activity was undetectable and, in two, phosphatas e was reduced. Kinase activity was unaffected in four of the mutants, but e levated in one. In one mutant, a pseudorevertant of a kdpD null mutation, k inase and phosphatase were both reduced to 20% of the wild-type level. Thes e findings suggest that initiation of signal transduction by KdpD is mediat ed by the inhibition of the phospho-KdpE-specific phosphatase activity of K dpD, leading to an accumulation of phospho-KdpE, which in turn activates th e expression of the KdpFABC system. The data also suggest that levels of ac tivity in vitro may differ from what occurs in vivo, because in vitro condi tions cannot replicate those in vivo.