High cleavage activity and stability of hammerhead ribozymes with a uniform 2'-amino pyrimidine modification

The uniform 2'-pyrimidine modifications have been found to inhibit ribozyme cleavage activity. However, in the present study we show that a good cleavage activity can be achieved for the 2'-amino modified ribozymes when their sequences were designed to contain only a few pyrimidines in helix I. In particular, ribozymes with no pyrimidines in helix I cleaved their target RNAs with almost the same efficacy as their unmodified versions. Interestingly, selective 2'-amino modification at positions 2.1 and 2.2 reduced the ribozyme cleavage activity by 8-fold, suggesting that the 2'-amino groups at these two positions may interfere with the formation of the ribozyme active conformation. In addition, uniformly modified ribozymes showed a remarkable stability in serum. Taken together these results should facilitate the design of stable ribozymes with sustained cleavage activity.