Noncanonical and canonical receptors in group I introns. The two tetraloops in the Twort ribozyme (A), the GUAA (L2) and the GAAA (L9) dock into an A-minor motif in P8 and a half receptor in P5, respectively (shown in their secondary and tertiary forms in B and C, respectively). These two noncanonical tetraloop receptor interactions are colored blue to indicate their affiliations to the slow cluster. The Azoarcus (D) and the Tetrahymena (F) ribozymes, containing the canonical GAAA tetraloops and their cognate 11-nt receptors (represented in its secondary and tertiary structure forms in E), are shown. The L9–P5 and L2–P8 tetraloop–receptor interactions in the Azoarcus are colored red and green, respectively, to indicate their cluster affiliations. Similarly, the L5b–J6a/6b tetraloop–receptor interaction in the Tetrahymena ribozyme is shown in green to indicate its cluster affiliation.
