Metal sensing by RNA in bacteria: exception or rule?

Until 2006, it was believed that bacteria control their intracellular metal ion concentrations exclusively through metal-sensing proteins. However, the detection of the first riboswitch that uses a small ion, Mg2+, as ligand to regulate gene expression in Salmonella challenged this assumption. Now, the discovery and in-depth characterization of a second metal-ion-sensing riboswitch that seems to be ubiquitously present in Gram-positive bacteria suggest that metal sensing by RNAs may represent a widespread mechanism.