The complete sequence of the kdp gene region of Clostridium acetobutylicum has been determined. This part of the chromosome comprises two small open reading frames (orfZ and orfY), putatively encoding hydrophobic peptides, and the genes kdpA, kdpB, kdpC, and kdpX, followed by an operon encoding a pair of sensor-effector regulatory proteins (KdpD and KdpE). Except for orfZ, orfY, and kdpX, all genes showed significant homology to the kdp genes of Escherichia coli, encoding a high-affinity potassium transport ATPase and its regulators. The complete genome sequence of Synechocystis sp. strain PCC 6803 and a recently published part of the Mycobacterium tuberculosis genome indicate the existence of a kdp system in these organisms as well, but all three systems comprise neither a second orf upstream of kdpA nor an additional kdpX gene. Expression of the clostridial kdp genes, including the unique kdpX gene, was found to be inducible by low potassium concentrations. A transcription start point could be mapped upstream of orfZ. A promoter upstream of kdpD was active only under noninducing conditions. Lowering the potassium content of the medium led to formation of a common transcript (orfZYkdpABCXDE), with a putative internal RNase E recognition site, which could be responsible for the instability of the common transcript. Except for the two small peptides, all gene products could be detected in in vitro transcription-translation experiments.