Promoter and Riboswitch Control of the Mg2+ Transporter MgtA from Salmonella enterica

The MgtA protein from Salmonella enterica serovar Typhimurium mediates Mg2+ uptake from the periplasm into the cytoplasm. Here we report that the PhoP/PhoQ two-component regulatory system, which responds to periplasmic Mg2+, governs mgtA transcription initiation at all investigated Mg2+ concentrations and that the Mg2+-sensing 5' leader region of the mgtA gene controls transcription elongation into the mgtA coding region when Salmonella is grown in media with < 50 mu M Mg2+. Overexpression of the Mg2+ transporter CorA, which is believed to increase cytoplasmic Mg2+ levels, decreased mgtA transcription in a manner dependent on a functional mgtA 5' leader.