Functional group requirements in the probable active site of the VS ribozyme

The VS ribozyme catalyses the site-specific cleavage of a phosphodiester linkage by a transesterification reaction that entails the attack of the neighbouring 2'-oxygen with departure of the 5'-oxygen. We have previously suggested that the A730 loop is an important component of the active site of the ribozyme, and that A756 is especially important in the cleavage reaction. Functional group modification experiments reported here indicate that the base of A756 is more important than its ribose for catalysis. A number of changes to the base, including complete ablation, lead to cleavage rates that are reduced 1000-fold, while removal of the 2'-hydroxyl group from the ribose results in tenfold slower cleavage. 2-Aminopurine fluorescence experiments indicate that this 2'-hydroxyl group is important for the structure of the A730 loop. Catalytic activity is especially sensitive to changes involving the exocyclic amine of A756; by contrast, the cleavage activity is only weakly sensitive to modification at the 7-position of the purine nucleus. These results suggest that the Watson-Crick edge of the adenine base is important in ribozyme function. We sought to test the possibility of a direct role of the nucleobase in the chemistry of the cleavage reaction. Addition of imidazole base in the medium failed to restore the activity of a ribozyme from which the nucleobase of A756 was removed. However, no restoration was obtained with exogenous adenine base either, indicating that the cavity that might result from ablation of the base was closed.