Expression of the Escherichia coli kdpABC operon, which is responsible for a high-affinity potassium-uptake system, is regulated in response to a change in the medium osmolarity. In this study, we clarified the structure and function of the kdpABC promoter including its regulatory sequence at the molecular level. The canonical -35 and -10 regions determined for the promoter were not fully functional, i.e. in addition to them, a cis-acting sequence located upstream of the -35 region was essential for full activation of the promoter. This upstream sequence was demonstrated to be the target site for the trans-acting activator, KdpE.