Riboswitches are structured RNAs typically located in the 5′ untranslated regions of bacterial mRNAs that bind metabolites and control gene expression. Most riboswitches sense one metabolite and function as simple genetic switches. However, we found that the 5′ region of the Bacillus clausii metE messenger RNA includes two riboswitches that respond to S -adenosylmethionine and coenzyme B 12 . This tandem arrangement yields a composite gene control system that functions as a two-input Boolean NOR logic gate. These findings and the discovery of additional tandem riboswitch architectures reveal how simple RNA elements can be assembled to make sophisticated genetic decisions without involving protein factors.