Riboswitches are highly structured regulatory domains found in the UTRs of numerous mRNAs. They directly sense the levels of cellular metabolites using an aptamer domain and then modulate transcription termination, translation or splicing through their expression platform domain. Caron et al. now show that the Escherichia coli lysine-sensing lysC riboswitch is a dual-acting riboswitch that regulates gene expression at the level of translation and also controls mRNA decay. Binding of lysine to lysC led to the riboswitch adopting an OFF state that prevented translation initiation and also exposed an RNAse E cleavage site in the riboswitch expression platform domain, resulting in mRNA decay. Importantly, both regulatory activities could be inhibited separately, indicating that the two activities can function independently of each other.