Mixed deoxyribo- and ribo-oligonucleotides with catalytic activity.

The RNA of viroids and virusoids in plants, and the RNA transcripts of some tandemly repeated DNA sequences in the newt, can undergo self-catalysed cleavage to generate RNA with 5'-OH and 2',3'-cyclic-phosphate termini. These catalytic RNAs, or ribozymes, form a stem-loop secondary structure called a 'hammerhead' in which the catalytic (ribozyme) and substrate sequences are brought close together. Catalytically active mimics of hammerhead ribozymes can be readily made using oligoribonucleotides. Consequently, hammerhead analogues in which certain ribonucleotides are replaced by different ones have been constructed both to identify consensus residues required for cleavage activity and to determine the details of the cleavage mechanism. But these ribonucleotide-replacements tend to alter the conformation of the hammerhead by changing hydrogen-bonding and stacking potential at the position of substitution. We have now constructed structurally less-disrupted hammerhead analogues in which deoxyribonucleotides, which lack 2'-OH groups, are substituted for ribonucleotides. These mixed RNA-DNA polymers were synthesized using a strategy for the chemical synthesis of RNA that is compatible with DNA synthesis. Analysis of the cleavage products of several of these hammerhead analogues confirms the involvement in the reaction of the 2'-OH adjacent to the cleavage site in the substrate, and demonstrates that some 2'-OH groups in the catalytic region strongly affect activity. The results also indicate that the three-dimensional structure producing nucleic acid-type catalysis is not restricted to RNA.