MAGNESIUM TRANSPORT IN SALMONELLA-TYPHIMURIUM - THE INFLUENCE OF NEW MUTATIONS CONFERRING CO2+ RESISTANCE ON THE CORA MG2+ TRANSPORT-SYSTEM

The CorA Mg2+ transport system of Salmonella typhimurium mediates both influx and efflux of Mg2+. Mutations at the corA locus (83.5 min) confer resistance to Co2+. Using transposon mutagenesis, three additional Co2+ resistance loci (corB, corC, and corD) were found and mapped to 55, 15, and 3 min, respectively, on the S. typhimurium chromosome. No mutations corresponding to the reported corB locus at 95 min in Escherichia coli were obtained. The corB, corC, and corD mutations confer levels of Co2+ resistance intermediate between those of the wild-type and corA mutations. Isogenic strains were constructed containing combinations of transposon insertion mutations in each of the three Co2+-resistance loci to assess their influence on the CorA Mg2+ transport system. The V(max) and K(m) values for Mg-28(2+) or for Co-57(2+) and Ni-63(2+) influx, analogues of Mg2+ transported by the CorA system, were changed less than twofold compared with the wild-type values, regardless of the mutation(s) present. However, while efflux of Mg-28(2+) through the CorA system was decreased threefold in strains carrying one or two mutant alleles among corB, corC, or corD, efflux was completely abolished in either a corA or a corBCD strain. Thus, although the corA gene product is necessary and sufficient to mediate Mg2+ influx, Mg2+ efflux requires the presence of a wild-type allele of at least one of the corB, corC or corD loci.