COMPLEMENTARY SETS OF NONCANONICAL BASE-PAIRS MEDIATE RNA HELIX PACKING IN THE GROUP INTRON ACTIVE-SITE

Helix packing is critical for RNA tertiary structure formation, althou gh the rules for helix-helix association within structured RNAs are la rgely unknown. Docking of the substrate helix into the active site of the Tetrahymena group I ribozyme provides a model system to study this question, Using a novel chemogenetic method to analyze RNA structure in atomic detail, we report that complementary sets of noncanonical ba se pairs (a G.U wobble pair and two consecutively stacked sheared A.A pairs) create an RNA helix packing motif that is essential for 5'-spli ce site selection in the group I intron. This is likely to be a genera l motif for helix-helix interaction within the tertiary structures of many large RNAs.