Riboswitches are defined sequences (aptamers) within an mRNA that bind a ligand, such as a metabolite, in a way that changes the mRNA's structure and affects expression of the mRNA. It has been thought that riboswitches had only two states, on or off. A new NMR study indicates this view to be too simplistic. Using a riboswitch found in organisms that exist in environments over a range of temperatures, Harald Schwalbe and colleagues found that the adenine-sensing riboswitch encoded by the add gene of the human pathogen Vibrio vulnificus can exist in three states: on, off and a third position that allows tighter control of expression by simultaneously monitoring both temperature and ligand concentration.
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