Riboswitches are metabolite binding domains within certain messenger RNAs that serve as precision sensors for their corresponding targets. Allosteric rearrangement of mRNA structure is mediated by ligand binding, and this results in modulation of gene expression. We have identified a class of riboswitches that selectively recognizes guanine and becomes saturated at concentrations as low as 5 nM. In Bacillus subtilis, this mRNA motif is located on at least five separate transcriptional units that together encode 17 genes that are mostly involved in purine transport and purine nucleotide biosynthesis. Our findings provide further examples of mRNAs that sense metabolites and that control gene expression without the need for protein factors. Furthermore, it is now apparent that riboswitches contribute to the regulation of numerous fundamental metabolic pathways in certain bacteria.