Tandem riboswitches control genes

Riboswitches are RNA molecules that control gene expression by binding metabolites. Most known riboswitches are simple on/off switches that respond to a single metabolite to regulate a gene, although more complex riboswitches have been found that have two binding sites for one metabolite. Now, Ronald R. Breaker and coworkers at Yale University show that a messenger RNA from the bacterium Bacillus clausii has two riboswitches that bind different metabolites, S-adenosylmethionine and coenzyme B₁₂ (Science 2006, 314, 300). If either of the metabolites binds, gene expression is shut off. The two riboswitches act independently, with binding at one site causing no structural changes in the other. The dissociation constant for each binding site is unaffected by the amount of the other ligand, and the sites appear not to bind cooperatively. Thus, the tandem riboswitch effectively functions as a Boolean NOR logic gate, with either of two inputs causing gene repression.