The Kdp system is a three-subunit member of the Ej—E2 family of transport ATPases. There is sequence homology of the 72 kDa KdpB protein, the largest subunit of Kdp, with the other members of this family. The predicted structure of the 21 kDa KdpC subunit resembles that of the |3 subunit of the Na + ,K + -ATPase, suggesting that these subunits may have a similar function. The 59 kDa KdpA subunit has no known homologue; it is very hydrophobic and is predicted to cross the membrane 10-12 times. Genetic studies implicate this subunit in the binding of K + . As the binding site must be close to the beginning of the transmembrane channel, we suggest that KdpA also forms most or all of the latter. KdpA may have evolved from a K + /H + antiporter that was recruited by the KdpB precursor to achieve the high affinity and specificity for K + , and the activation of transport by low turgor pressure characteristic of Kdp. Turgor pressure controls the expression of Kdp. This action is dependent on the 70 kDa KdpD and 23 kDa KdpE proteins. We are in the process of sequencing these genes. KdpE is homologous to the smaller protein of other members of a family of pairs of regulatory proteins implicated in control of a variety of bacterial processes such as porin synthesis, phosphate regulon expression, nitrogen metabolism, chemotaxis and nodule formation.