2 MAJOR TERTIARY FOLDING TRANSITIONS OF THE TETRAHYMENA CATALYTIC RNA

The L-21 Tetrahymena ribozyme, an RNA molecule with sequence-specific endoribonuclease activity derived from a self-splicing group I intron, provides a model system for studying the RNA folding problem. A 160 n ucleotide, independently folding domain of tertiary structure (the P4- P6 domain) comprises about half of the ribozyme. We now apply Fe(II)-E DTA cleavage to mutants of the ribozyme to explore the role of individ ual structural elements in tertiary folding of the RNA at equilibrium. Deletion of peripheral elements near the 3' end of the ribozyme desta bilizes a region of the catalytic core (P3-P7) without altering the fo lding of the P4-P6 domain. Three different mutations within the P4-P6 domain that destabilize its folding also shift the folding of the P3-P 7 region of the catalytic core to higher MgCl2 concentrations. We conc lude that the role of the extended P4-P6 domain and of the 3'-terminal peripheral elements is at least in part to stabilize the catalytic co re. The organization of RNA into independently folding domains of tert iary structure may be common in large RNAs, including ribosomal RNAs. Furthermore, the observation of domain-domain interactions in a cataly tic RNA supports the feasibility of a primitive spliceosome without an y proteins.