has three distinct Mg transport systems, the constitutive high-capacity CorA transporter and two P-type ATPases, MgtA and MgtB, whose transcription is repressed by normal concentrations of Mg in the growth medium. The latter Mg-transporting ATPase is part of a two-gene operon, , with encoding a 23 kDa protein of unknown function. Transcriptional regulation using fusions of the promoter regions of and to showed a biphasic time and Mg concentration dependence. Between 1 and 6 h after transfer to nitrogen minimal medium containing defined concentrations of Mg, transcription increased about 200-fold for and up to 400-fold for , each with a half-maximal dependence on Mg of 0.5 mM. Continued incubation revealed a second phase of increased transcription, up to 2000-fold for and up to 10000-fold for . This secondary increase occurred between 6 and 9 h after transfer to defined medium for but between 12 and 24 h for and had a distinct half-maximal dependence for Mg of 0.01 mM. A concomitant increase of at least 1000-fold in uptake of cation was seen between 8 and 24 h incubation with either system, showing that the transcriptional increase was followed by functional incorporation of large amounts of the newly synthesized transporter into the membrane. Regulation of transcription by Mg was not dependent on a functional stationary-phase sigma factor encoded by , but it was dependent on the presence of a functional two-component regulatory system. Whereas was completely dependent on regulation via , the secondary Sate Mg-dependent phase of transcription was still evident in strains carrying a mutation in either or , albeit substantially diminished. Several divalent cations blocked the early phase of the increase in transcription elicited by the decrease in Mg concentration, including cations that inhibit Mg uptake (Co, Ni and Mn) and those which do not (Ca and Zn). In contrast, the second later phase of the transcriptional increase was not well blocked by any cation except those which inhibit uptake. Overall, the data suggest that at least two distinct mechanisms for transcriptional regulation of the and loci exist.


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