THE P4-P6 DOMAIN DIRECTS HIGHER-ORDER FOLDING OF THE TETRAHYMENA RIBOZYME CORE

The active site of group I self-splicing introns occurs at the interfa ce of two proposed structural domains. In the Tetrahymena intron, half of the catalytic core resides within the independently-folding P4-P6 domain while the other half belongs to a putative domain that includes helices P3, P7, P8, and P9 (P3-P9). To determine whether the P3-P9 re gion of the intron can also fold independently, we used Fe(II)-EDTA an d dimethyl sulfate to probe the solvent accessibility of separate frag ments of the Tetrahymena intron. These RNAs self-assemble into an acti ve complex in trans, enabling analysis of their structural features bo th alone and within the complex. Our results show that while the P3-P9 region of the intron retains its secondary structure, most of the ter tiary interactions within this region do not form stably in the absenc e of the P4-P6 domain. This indicates that the P4-P6 domain induces fo lding in the P3-P9 region, organizing the catalytic cleft between them . Thus the P4-P6 domain provides a scaffold for the folding of the Tet rahymena intron core.