). Strains carrying one wild-type and two mutant alleles of the three loci were constructed to study the kinetics and specificity of ion transport of each system in isolation. The transport systems had different Km Assay of Mg2+ transport. Uptake using 28Mg2' has been described previously (3). Mg2+ uptake was measured in strains of S. typhimurium that carry one wild-type and two mutant alleles of the three genes corA, mgtA, and mgtB (2). This allowed dissection of the properties of each individual transport system without interference from the others. For clarity, the Mg2+ transport system present is referred to as CorA, MgtA, or MgtB rather than by the longer genotypic designation of defective systems, e.g., as the CorA system rather than as corA+ mgtA mgtB. The phenotype of each of the strains carrying only a single Mg2+ transport system was always verified the day before each transport assay by using a disk sensitivity test to determine the pattern of growth inhibition by divalent cations, a pattern that is characteristic for each strain (2). The apparent Ki for inhibition by other divalent cations was determined from the cation concentration giving 50% inhibition (IC50) (Fig. 1)

Salmonella typhimurium strains lacking the CorA Mg2+ transport system retain Mg2+ transport and the ability to grow in medium containing a low concentration of Mg2+. Mutagenesis of a corA strain followed by ampicillin selection allowed isolation of a strain that required Mg2+-supplemented media for growth. This strain contained mutations in at least two loci in addition to corA, designated mgtA and mgtB (for magnesium transport). Strains with mutations at all three loci (corA, mgtA, and mgtB) exhibited no detectable Mg2+ uptake and required 10 mM Mg2+ in the medium for growth at the wild-type rate. A wild-type allele at any one of the three loci was sufficient to restore both Mg2+ transport and growth on 50 ,uM Mg2+. P22 transduction was used to map the mgt loci. The mgtA mutation was located to approximately 98 map units (cotransducible with pyrB), and mgtB mapped at about 80.5 map units (near gltC). A chromosomal library from S. typhimurium was screened for clones that complemented the Mg2+ requirement of a corA mgtA mgtB mutant. The three classes of plasmids obtained could each independently restore Mg2+ transport to this strain and corresponded to the corA, mgtA, and mgtB loci. Whereas the corA locus of S. typhimurium is analogous to the corA locus previously described for Escherichia coli, neither of the mgt loci described in this report appears analogous to the single mgt locus described in E. coli. Our data in this and the accompanying papers (M. D. Snavely, J. B. Florer, C. G. Miller, and M. E. Maguire, J. Bacteriol. 171:4752-4760, 4761-4766, 1989) indicate that the corA, mgtA, and mgtB loci of S. typhimurium represent three distinct systems that transport Mg2>

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