Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci

Abstract

Salmonella typhimurium has three distinct Mg²⁺ transport systems, the constitutive high-capacity CorA transporter and two P-type ATPases, MgtA and MgtB, whose transcription is repressed by normal concentrations of Mg²⁺ in the growth medium. The latter Mg²⁺-transporting ATPase is part of a two-gene operon, mgtCB, with mgtC encoding a 23 kDa protein of unknown function. Transcriptional regulation using fusions of the promoter regions of mgtA and mgtCB to luxAB showed a biphasic time and Mg²⁺ concentration dependence. Between 1 and 6 h after transfer to nitrogen minimal medium containing defined concentrations of Mg²⁺, transcription increased about 200-fold for mgtCB and up to 400-fold for mgtA, each with a half-maximal dependence on Mg²⁺ of 0.5 mM. Continued incubation revealed a second phase of increased transcription, up to 2000-fold for mgtCB and up to 10000-fold for mgtA. This secondary increase occurred between 6 and 9 h after transfer to defined medium for mgtCB but between 12 and 24 h for mgtA and had a distinct half-maximal dependence for Mg²⁺ 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 Mg²⁺ was not dependent on a functional stationary-phase sigma factor encoded by rpoS, but it was dependent on the presence of a functional phoPQ two-component regulatory system. Whereas mgtCB was completely dependent on regulation via phoPQ, the secondary Sate Mg²⁺-dependent phase of mgtA transcription was still evident in strains carrying a mutation in either phoP or phoQ, albeit substantially diminished. Several divalent cations blocked the early phase of the increase in transcription elicited by the decrease in Mg²⁺ concentration, including cations that inhibit Mg²⁺ uptake (Co²⁺, Ni²⁺ and Mn²⁺) and those which do not (Ca²⁺ and Zn²⁺). 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 mgtA and mgtCB loci exist.