Effect of structural modifications on the activity of the leadzyme.

The structure/function properties of functional groups in the leadzyme have been studied by assaying the activity of analog ribozymes generated by the systematic substitution of modified nucleotides in the internal loop region of the ribozyme. Guanosine analogs introduced at positions 4 and 7 occupied by guanosine in the wild-type molecule severely diminished cleavage. The substitution of deoxycytidine for cytidine at the cleavage site completely eliminated the activity of the leadzyme, as expected if the adjacent 2'-OH were the nucleophile in the cleavage reaction. On the other hand, substitution of an abasic nucleotide for adenosine at position 8 did not affect the activity of the ribozyme. An analysis of the activity of these analogs gives rise to the proposal of a triple-base pair motif implicating C1, G4, and G7.