Toward the Combinatorial Selection of Chemically Modified DNAzyme RNase A Mimics Active Against all-RNA Substrates

The convenient use of SELEX and related combinatorial methods of in vitro selection provides a formidable gateway for the generation of DNA enzymes, especially in the context of improving their potential as gene therapeutic agents. Here, we report on the selection of DNAzyme 12–91, a modified nucleic acid catalyst adorned with imidazole, ammonium, and guanidinium groups that provide for efficient M²⁺-independent cleavage of an all-RNA target sequence (<i>k</i>_obs = 0.06 min^–1). While Dz12–91 was selected for intramolecular cleavage of an all-RNA target, it surprisingly cleaves a target containing a lone ribocytosine unit with even greater efficiency (<i>k</i>_obs = 0.27 min^–1) than Dz9–86 (<i>k</i>_obs = 0.13 min^–1). The sequence composition of Dz12–91 bears a marked resemblance to that of Dz9–86 (<i>k</i>_obs = 0.0014 min^–1 with an all-RNA substrate) that was selected from the same library to cleave a target containing a single ribonucleotide. However, small alterations in the sequence composition have a profound impact on the substrate preference and catalytic properties. Indeed, Dz12–91 displays the highest known rate enhancement for the M²⁺-independent cleavage of all-RNA targets. Hence, Dz12–91 represents a step toward the generation of potentially therapeutically active DNAzymes and further underscores the usefulness of modified triphosphates in selection experiments