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. (16,41,49). We suggest that phosphoKdpE is a positive effector of Kdp expression and that low turgor pressure causes KdpD to phosphorylate KdpE. MATERUILS AND METHODSDNA sequencing. The 1.7-kb EcoRI fragment of kdpD, cut from plasmid pWE1103 (35), and the 3-kb EcoRI-HindIII fragment carrying kdpE and a part of kdpD, cut from plasmid pDE14 (35), were cloned in both directions in M13 (M13uml8 and M13uml9; International Biotechnologies, Inc.). The resulting phages, M13-JD2, M13-JD3, M13-JD24, and M13-JD34 (Fig. 1) (19). There were three differences between our initial results and his. The GC at positions 5245 and 5246 (Fig. 2) was erroneously recorded as CG in our work; review of the gels shows that GC is correct. The sequence on one strand at positions 7213 and 7214 differed from that of the complementary strand, but the gel with the least band compression suggested that GC as found by Igarashi was correct.

KdpD and KdpE, proteins that control expression of the kdpFABC operon, are members of the class of sensor kinase/response regulator proteins. Using polyclonal antibodies raised against the KdpD protein, we have been able to identify and to localize the chromosome‐encoded KdpD protein in the cytoplasmic membrane of Escherichia coli. Furthermore, it has been possible to detect differences in the expression of the KdpD protein according to the K+ concentration in the growth medium. The phosphorylation capacity of the plasmid‐encoded KdpD protein and the phospho‐transfer to KdpE was investigated. We found that both reactions were strictly dependent on the ionic conditions of the assay medium. Based on optimized conditions, we were able to detect phosphorylation of the chromosome‐encoded KdpD protein. Furthermore, replacement of the conserved histidine (His673), the predicted phosphorylation site in KdpD, by glutamine revealed that phosphorylation of KdpD was no longer possible.

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