An RNA internal loop acts as a hinge to facilitate ribozyme folding and catalysis.

RNA molecules commonly consist of helical regions separated by internal loops, and in many cases these internal loops have been found to assume stable structures. We have examined the function and dynamics of an internal loop, J5/5a, that joins the two halves of the P4-P6 domain of the Tetrahymena self-splicing group I intron. P4-P6 RNAs with mutations in the J5/5a region showed nondenaturing gel electrophoretic mobilities and levels of Fe(II)-EDTA cleavage protection intermediate between those of wild-type RNA and a mutant incapable of folding into the native P4-P6 tertiary structure. Mutants with the least structured J5/5a loops behaved the most like wild-type P4-P6, and required smaller amounts of Mg2+ to rescue folding. The activity of reconstituted introns containing mutant P4-P6 RNAs correlated similarly with the nature of the J5/5a mutation. Our results suggest that, in solution, the P4-P6 RNA is in a two-state equilibrium between folded and unfolded states. We conclude that this internal loop mainly acts as a flexible hinge, allowing the coaxially stacked helical regions on either side of it to interact via specific tertiary contacts. To a lesser extent, the specific bases within the loop contribute to folding. Furthermore, it is crucial that the junction remain unstructured in the unfolded state. These conclusions cannot be derived from a simple examination of the P4-P6 crystal structure (Cate JH et al., 1996, Science 273:1678-1685), showing once again that structure determination must be supplemented with mutational and thermodynamic analysis to provide a complete picture of a folded macromolecule.