NMR solution structure of the Spn preQ1-II riboswitch in complex with preQ1. (A) Superposition of the 18 lowest-energy structures. Superposition is on P2, L1, P3, and L2 residues (all heavy atoms). (Inset) Superposition of P4 stem residues only. (B) Stick representation of the lowest-energy structure. (C) Stick representation of the binding pocket with the G-C base pair below and two U-A-U triples above C8-preQ1-U19-A50, rotated 90° from B (side view). (D) Sphere representation of the lowest-energy structure, rotated 180° from B (back view).