SIDEBAR 1.
A cartoon illustrating conformational selection by a riboswitch, which is a prominent mode of ligand recognition by proteins49. In state 1 only secondary structures are formed. In states 2–5, tertiary structure has formed but the ligand is not bound. The aptamer exists as an equilibrium of conformational states with both local changes to the ligand-binding pocket, as well as long-range tertiary interactions between different pairing regions. Only conformations with a fully formed, unblocked binding sites are receptive to ligand binding. Upon binding the aptamer adopts a stable conformation (state 6) that effects expression of the associated mRNA.