Riboswitch folding: one at a time and step by step.

Riboswitches are mRNAs that regulate gene expression upon specific binding of a small metabolite (vitamin cofactors, nucleobases, amino acids, carbohydrates or even metal ions). Riboswitches must fold into very intricate 3D structures to accomplish their function. Single-molecule studies are very powerful techniques that allow the characterization and elucidation of complex folding paths as well as the identification of intermediate conformational states that an RNA must traverse to acquire its native, functional structure. We review some recent reports on the folding of the adenine-binding riboswitch as studied by fluorescence resonance energy transfer (FRET) and force-measuring optical tweezers (FMOT) techniques.