a-b RNA constructs and the imino [15N, 1H] HSQC zoomed into the rG•rU wobble region of the spectra for hp-GU-20 and hp-GU-24. rG•rU mispair resonances are shown in red. c, The Bacillus subtilis guanine binding riboswitch (xpt-G RNA)71 construct and full imino [15N, 1H] HSQC of folded and guanine ligand-bound riboswitch. rG•rU mispair resonances are shown in red. d-f, 15N RD profiles for d, hp-GU-20 e, hp-GU-24 and f, xpt-G riboswitch RNA. Error bars represent experimental uncertainty (one s.d., see Methods).