The Kdp ATPase is a P-type ATPase consisting of three large protein subunits in a complex that probably contains 2 copies of each subunit. A small hydrophobic peptide, encoded in the same operon as the large subunits, may also participate. Kdp has very high affinity for K+ and serves to scavenge this ion when its concentration is low. Kdp responds to turgor pressure at two levels, at the level of activity and of expression. Kdp mediates net uptake when turgor is low, but mediates exchange without net change when turgor is normal. Kdp is expressed only when turgor is low. This control is mediated by a pair of regulatory proteins, members of the class of 'sensor-effector' regulators widely distributed in bacteria. It is suggested that low turgor changes the conformation of the KdpD 'sensor' protein, activating its presumed kinase activity to phosphorylate KdpE, the 'effector' protein, and phospho-KdpE in turn turns on expression of the operon that encodes the Kdp complex.