Regulation of magnesium ion transport in <i>Escherichia coli</i> : insights into the role of the 5’ upstream region in <i>corA</i> expression

In <i>Escherichia coli</i>, transport of magnesium ions across the cellular membrane relies on MgtA and CorA transporters. While the expression of <i>mgtA</i> is controlled by the two-component system PhoQ/PhoP and 5' upstream region elements, <i>corA</i> expression is considered to be constitutive and not to depend on cellular factors. Importantly, the 5' upstream region of <i>corA</i> is predicted to fold into structures highly similar to the magnesium-sensing <i>mgtA</i> 5' upstream region. Here using biochemical and genetic assays, we show that the intracellular concentration of magnesium ions affects <i>corA</i> expression. Similarly to <i>mgtA</i>, we find that the effect of magnesium ions on <i>corA</i> expression is mediated by the 5' upstream region. We demonstrate that the RNA structure is important for regulation and that the Rho transcription factor is involved in the modulation of transcription termination. Consistent with previous studies, we find that translation of <i>corL</i>, a short ORF located within the 5' upstream region, is important for <i>corA</i> regulation. Our data indicate that the efficiency of <i>corL</i> translation is inversely proportional to <i>corA</i> expression, similar to what has been described for <i>mgtA</i> and <i>corA</i> in <i>Salmonella enterica</i>. Using a novel assay to control the import of magnesium ions, we show that while the expression of <i>mgtA</i> is regulated by both extra- and intracellular magnesium ions, <i>corA</i> is regulated by variations in intracellular magnesium ions. Our results support a model in which the expression of <i>corA</i> is regulated by the 5' upstream region that senses variations of intracellular magnesium ions.