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 Salmonella enterica serovar Typhimurium is controlled by its 5′ 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.