An RNA sensor for intracellular Mg2+

Most RNA molecules require Mg2+ for their structure and enzymatic properties. Here we report the first example of an RNA serving as sensor for cytoplasmic Mg2+. We establish that expression of the Mg2+ transporter MgtA of Salmoneila enterica serovar Typhimurium is controlled by its 51 untranslated region (5'UTR). We show that the 5'UTR of the mgtA gene can adopt different stem-loop structures depending on the Mg2+ levels, which determine whether transcription reads through into the mgtA coding region or stops within the 5'UTR. We could recapitulate the Mg2+-regulated transcription using a defined in vitro transcription system with RNA polymerase as the only protein component. The initiation of mgtA transcription responds to extracytoplasmic Mg2+ and its elongation into the coding region to cytoplasmic Mg2+, providing a singular example in which the same ligand is sensed in different cellular compartments to regulate disparate steps in gene transcription.