Bacterial gene regulation: metal ion sensing by proteins or RNA.

Until recently, metal sensing in bacteria seemed to be accomplished exclusively by metalloregulatory proteins; however, a surprising new finding is that a metal ion itself can act as a riboswitch ligand to shut down gene expression. Interestingly, this ion is Mg(2+), known to be required for a wide variety of cellular functions and for correct folding of RNAs. It remains to be discovered whether other ion-dependent riboswitches exist, which would open up a new dimension for regulatory RNAs.