Thermal unfolding of a group I ribozyme: the low-temperature transition is primarily disruption of tertiary structure

Little is known about the folding pathways of RNA. A particularly interesting RNA is L-21 Sca I, a linear form of the self-splicing intron from the precursor of the Tetrahymena thermophila large subunit (LSU) rRNA. Thermal unfolding of L-21 Sca I is studied by UV absorption and chemical mapping in 50 mM Na+ and 10 mM free Mg2+ at pH 7.5. UV melting experiments identify two major transitions with maxima at 65 and 73 degrees C. Chemical mapping at the beginning and middle of the first transition suggests it primarily involves disruption of tertiary structure. Phylogenetic comparisons suggest a potential tertiary interaction between loops L2.1 and L9.1a. Chemical mapping and melting experiments on a truncated form of the intron lacking P9.1a, L-21 Nhe I, are consistent with this hypothesis. The results indicate that increasing temperature disrupts tertiary interactions before disrupting secondary structure. This suggests tertiary interactions are weaker than secondary interactions in this case. These results support an important assumption for RNA structure prediction: that secondary structure dominates the free energy of folding.