The influx of Mg2+ in Salmonella typhimurium LT-2 was studied by both kinetic and genetic techniques. Wild-type cells grown in a high MgSO4 concentration (10 mM) exhibited a K(m) of 15 μM for Mg2+ influx, with a V(max) of 0.25 nmol of Mg2+ per min per 108 cells. The apparent K(m) decreased to 3 μM, and the V(max) increased 60% after growth in a low MgSO4 concentration (10 μM). Co2+ was a simple competitive inhibitor (K(i) = 30 μM) of Mg2+ influx in cells grown in high Mg2+ concentrations but blocked only a portion of the Mg2+ influx in cells grown in low Mg2+ concentrations. Co2+ influx exhibited kinetics similar to those of Mg2+ influx (K(m) = 30 μM; V(max) = 0.5 nmol of Co2+ per min per 108 cells) but was not affected by growth conditions. Co2+ influx was competitively inhibited by both Mg2+ and Mn2+. Mutations affecting Mg2+ uptake were isolated by selection for spontaneous resistance to toxic levels of Co2+. One class of mutants designated corA mapped at 84 min near metE with the following gene order: corA, metE, zie-3161::Tn10, pepQ. A second class designated corB mapped at 98 min near pyrB. Mg2+ influx was decreased in a corA mutant strain (relative to that of the wild type) when grown in high Mg2+ concentrations but was restored when grown in low Mg2+ concentrations. Co2+ transport was completely abolished by the corA mutation under all growth conditions. Recombinant plasmids carrying the corA region from either Escherichia coli K-12 or S. typhimurium complemented the corA mutation in S. typhimurium, restoring uptake of both Co2+ and Mg2+ and conferring sensitivity to Co2+. The S. typhimurium corA gene was localized to a restriction fragment of approximately 1.5 kilobases.