Gibson M M, Bagga D A, Miller C G, Maguire M E
Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106.
Mol Microbiol. 1991 Nov;5(11):2753-62. doi: 10.1111/j.1365-2958.1991.tb01984.x.
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 3min, 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 Co(2+)-resistance loci to assess their influence on the CorA Mg2+ transport system. The Vmax and Km values for 28Mg2+ or for 57Co2+ and 63Ni2+ 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 28Mg2+ 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.
鼠伤寒沙门氏菌的CorA镁离子转运系统介导镁离子的流入和流出。corA位点（83.5分钟处）的突变赋予对钴离子的抗性。利用转座子诱变，发现了另外三个钴离子抗性位点（corB、corC和corD），并分别定位在鼠伤寒沙门氏菌染色体的55、15和3分钟处。未获得与大肠杆菌中报道的位于95分钟处的corB位点相对应的突变。corB、corC和corD突变赋予的钴离子抗性水平介于野生型和corA突变之间。构建了在三个钴离子抗性位点中每个位点都含有转座子插入突变组合的同基因菌株，以评估它们对CorA镁离子转运系统的影响。与野生型值相比，CorA系统转运的镁离子类似物28Mg2+或57Co2+和63Ni2+流入的Vmax和Km值变化不到两倍，无论存在何种突变。然而，虽然在携带corB、corC或corD中一个或两个突变等位基因的菌株中，通过CorA系统的28Mg2+流出减少了三倍，但在corA或corBCD菌株中流出完全被消除。因此，虽然corA基因产物对于介导镁离子流入是必要且充分的，但镁离子流出需要corB、corC或corD位点中至少一个的野生型等位基因的存在。
