MAGNESIUM TRANSPORT IN SALMONELLA-TYPHIMURIUM - MGTA ENCODES A P-TYPE ATPASE AND IS REGULATED BY MG2+ IN A MANNER SIMILAR TO THAT OF THE MGTB P-TYPE ATPASE

Salmonella typhimurium has three distinct Mg2+ transport systems: CorA, MgtA, and MgtB, each encoded by its respective gene, corA and mgtB have been previously sequenced and characterized. This report details the sequence and properties of mgtA. Like mgtB, mgtA encodes a P-type ATPase. The mgtA gene encodes a slightly smaller protein than does mgtB, with a predicted molecular mass of about 95 kDa, running at 91 kDa on protein gels, which compares with values of 101 and 102 kDa, respectively, for the MgtB protein. The deduced amino acid sequence of MgtA is only 50% identical to that of MgtB, with a further 25% conservative amino acid substitutions, surprisingly low for such otherwise functionally similar proteins from the same organism. Codon usage for each gene is normal for S. typhimurium, however, indicating that neither gene is the result of a recent acquisition from another organism. A single open reading frame at mgtA encodes MgtA, in contrast to mgtB, which is shown to be an operon encoding (5' to 3') the 22.5-kDa MgtC and the MgtB proteins. Genetic constructs were used to show that deletion of MgtC does not alter the expression or transport properties of MgtB, making the role of the companion MgtC protein unclear. (The S. typhimurium homolog of treR, which encodes a putative repressor for trehalose uptake, is encoded by a gene adjacent to mgtA, and its sequence is also reported.) Finally, extremely strong Mg2+ regulation of the mgtA and mgtB promoters but not of the cord or treR promoters was demonstrated by cloning the appropriate DNA sequences with luxAB and measuring enhancement of light production as a function of extracellular Mg2+ concentration. Lowering the extracellular Mg2+ concentration from 10 mM to 1 or 10 mu M elicited a transcriptional response of several thousandfold from both the mgtA and mgtB promoters.