Examples of the diversity of RNA three-dimensional structures in riboswitches. The phosphodiester backbone in each case is shown as a solid tube; bound metabolites are shown in red. (a) The S-adenosylmethionine-II riboswitch; (b) a guanine-binding riboswitch; (c) a lysine-binding riboswitch; and (d) a tetrahydrofolate-binding riboswitch, bound here to two molecules of folinic acid.