The Tetrahymena ribozyme has been shown to catalyze an RNA polymerase-like reaction in which an RNA primer is extended by the sequential addition of pN nucleotides derived from GpN dinucleotides, where N = A, C, or U. Here, we show that this reaction is influenced by the presence of a template; bases that can form Watson-Crick base pairs with a template add as much as 25-fold more efficiently than mismatched bases. A mutant enzyme with an altered guanosine binding site can catalyze template-directed primer extension with all four bases when supplied with dinucleotides of the form 2-aminopurine-pN.

We have captured an 8.7 A conformational change that takes place in the cleavage site of the hammerhead ribozyme during self-cleavage, using X-ray crystallography combined with physical and chemical trapping techniques. This rearrangement brings the hammerhead ribozyme from the ground state into a conformation that is poised to form the transition state geometry required for hammerhead RNA self-cleavage. Use of a 5’-C-methylated ribose adjacent to the cleavage site permits this ordinarily transient conformational change to be kinetically trapped and observed crystallographically after initiating the hammerhead ribozyme reaction in the crystal. Cleavage of the corresponding unmodified hammerhead ribozyme in the crystal under otherwise identical conditions is faster than in solution, indicating that we have indeed trapped a catalytically relevant intermediate form of this RNA enzyme