The transcriptional control of the kdpFABC (K ؉ transport) operon of Salmonella typhimurium was characterized with a lacZ fusion. The kdpFABC operon of this organism was induced by K ؉ limitation and high osmolality, and osmotic induction was antagonized by a high concentration of K ؉ . In the trkA (sapG) kdp ؉ mutant background, high concentrations of K ؉ inhibited growth, along with repressing the kdp operon. This result, which has not been reported for Escherichia coli, is inconsistent with the model in which the signal for the induction of the kdp operon is turgor loss.In enteric bacteria, K ϩ is taken up by two major permeases, Trk and Kdp, and by a minor permease, Kup (13,18). In Escherichia coli, the activity of these three systems has been shown to be stimulated by osmotic stress (2). Stimulation of Kdp is effected largely by transcriptional induction of the kdpFABC operon. The kdpFABC operon is also induced by K ϩ limitation. Transcriptional control of the kdpFABC operon is mediated by the KdpD and KdpE proteins (2), which belong to the family of two-component regulatory proteins. It has been proposed that the regulatory signal for the expression of the kdp operon is turgor (4), but whether the osmotic-stress-and the K ϩ -dependent regulations are mediated by a single mechanism or by two distinct mechanisms is controversial (1, 2, 6, 7).The regulation of the kdp operon in Salmonella typhimurium has not been studied. Because of difficulties in obtaining kdp mutations in this organism, it has been suggested that there may be an important difference between the properties of the Kdp system in E. coli and in S. typhimurium (17). In this report, we describe the isolation of a kdp-lacZ fusion in S. typhimurium.Strain construction. The medium used for the isolation of the kdp-lacZ mutations was K0 medium (5), which has a K ϩ concentration of 0.1 mM (introduced as a trace contaminant in other chemicals). Approximately 5,000 derivatives of the wildtype S. typhimurium strain LT2 carrying random MudI1734 (Km lac) insertions (10) were replica plated to K0 plates containing 10 mM glucose, kanamycin, and 40 mg of X-Gal (5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside) per liter (12) and to plates containing the same ingredients plus 10 mM KCl. We identified one derivative, strain TL2626, which formed a very small, dark blue colony on the former medium and a normal-sized, white colony on the latter; the mutation in this strain was designated kdp-101::MudI1734.P22 transductions (3) demonstrated that the kdp-101:: MudI1734 insertion was 96% linked and was located upstream of a kdp::Tn10 insertion (obtained from E. Groisman) (data not shown). In Southern blot analyses, a fragment containing the kdpFABC operon of E. coli exhibited different patterns of hybridization to EcoRI, EcoRV, PvuII, and HincII fragments of the DNA from strain TL2626 than to those from strain LT2 (data not shown). The kdp-101::MudI1734 insertion was complemented by a plasmid which expressed the kdpB ϩ C ϩ genes of E. coli (obtained from W. Epstein)...