Conformational equilibria are increasingly recognized as pivotal for biological function. Traditional structural analyses provide as tatic image of conformers in solution that sometimes present conflicting views.From 13 Cand 1 Hchemical exchange saturation transfer experiments,i nc oncert with ligation and selective labeling strategies,w es howt hat in the absence of metabolite,aMg 2+ (0-0.5 mm)-bound apo SAM-II riboswitch RNAe xists in am inor ( % 10 %) partially closed state that rapidly exchanges with ap redominantly ( % 90 %) open form with al ifetime of % 32 ms.T he base and sugar (H6,C6, H1',C1')c hemical shifts of C43 for the dominant conformer are similar to those of afree CMP,but those of the minor apo species are comparable to shifts of CMPs in helical RNAr egions.O ur results suggest that these transient, low populated states stabilized by Mg 2+ will likely enhance rapid ligand recognition and, we anticipate,w ill play potentially ubiquitous roles in RNAs ignaling.