SAM‐II Riboswitch Samples at least Two Conformations in Solution in the Absence of Ligand: Implications for Recognition

Conformational equilibria are increasingly recognized as pivotal for biological function. Traditional structural analyses provide a static image of conformers in solution that sometimes present conflicting views. From ¹³C and ¹H chemical exchange saturation transfer experiments, in concert with ligation and selective labeling strategies, we show that in the absence of metabolite, a Mg²⁺ (0–0.5 mm )‐bound apo SAM‐II riboswitch RNA exists in a minor (≈10 %) partially closed state that rapidly exchanges with a predominantly (≈90 %) open form with a lifetime of ≈32 ms. The base and sugar (H6,C6, H1′,C1′) chemical shifts of C43 for the dominant conformer are similar to those of a free CMP, but those of the minor apo species are comparable to shifts of CMPs in helical RNA regions. Our results suggest that these transient, low populated states stabilized by Mg²⁺ will likely enhance rapid ligand recognition and, we anticipate, will play potentially ubiquitous roles in RNA signaling.