IMPORTANCE OF SPECIFIC PURINE AMINO AND HYDROXYL-GROUPS FOR EFFICIENT CLEAVAGE BY A HAMMERHEAD RIBOZYME

Eight modified ribozymes of 19 residues have been prepared with individual purine amino or hydroxyl groups excised. The modified ribozymes were chemically synthesized with the substitution of a single 2'-deoxyadenosine, 2'-deoxyguanosine, inosine, or purine riboside for residues G10, A11, G13, or A14. Five of the modified ribozymes cleaved the 24-mer substrate with little change in rate as monitored by simple first-order kinetics. However, deletion of the 2-amino group at G10 (replacement with inosine) or deletion of either of the 2'-hydroxyls at G10 or G13 (replacement with 2'-deoxyguanosine) resulted in ribozymes with a drastic decrease in cleavage efficiency. Increasing the concentration of the Mg2+ cofactor from 10 mM to 50 mM significantly enhanced cleavage efficiency by these three derivatives. Steady-state kinetic assays for these three ribozymes indicated that the modifications result in both an increase in K(m) and a decrease in k(cat). These results suggest that the exocyclic amino group at G10 and the hydroxyls at G10 and G13 are important both for ribozyme-substrate binding and for the Mg2+-catalyzed cleavage reaction.