The Kdp system of Escherichia coli, a transport ATPase with high affinity for potassium, is expressed when turgor pressure is low. Expression requires KdpD, a 99-kDa membrane protein, and KdpE, a 25-kDa soluble cytoplasmic protein. The sequences of KdpD and KdpE show they are members of the sensor-effector class of regulatory proteins: the C-terminal half of KdpD is homologous to sensors such as EnvZ and PhoR, and KdpE is homologous to effectors such as OmpR and PhoB. The predicted structure of KdpD suggests that it is anchored to the membrane by four membrane-spanning segments near its middle, with both C- and N-terminal portions in the cytoplasm. Subcellular fractionation confirms the expected location of the protein in the inner membrane. The N-terminal region has no homology to known proteins and is the site of mutations that make Kdp expression partially constitutive; this portion may serve to sense turgor pressure. Since several other sensor-effectors have been shown to mediate control through phosphorylation, this mechanism is proposed to control expression of Kdp.