Purine riboswitches are RNA structures that regulate protein biosynthesis in response to purines. In general, riboswitches are metabolite-binding domains within certain messenger RNAs that act 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.
Purine riboswitches are a class of riboswitches that selectively recognises guanine and adenine, the riboswitch 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 synthesis. As some members of this family are also specific for adenine or deoxyguanosine, this family are termed purine riboswitches. The purine riboswitch is the only riboswitch so far that has been mutated to respond to non-natural ligands, opening up possibilities to use riboswitches as novel gene-expression tools.