Abstract:

In vitro selection of RNA-cleaving DNAzymes was performed using three heavy lanthanide ions (Ln3+): Ho3+, Er3+ and Tm3+. The resulting sequences were aligned together and about half of the library contained a new family of DNAzyme. These DNAzymes have a simple loop structure, and they are active only with the seven heavy Ln3+. Among the tested non-lanthanide ions, only Y3+ induced cleavage and even Pb2+ failed to cleave, suggesting a very high specificity. A representative DNAzyme, Tm7, has a sigmoidal metal binding curve with a Hill coefficient of 3, indicating that three metal ions are involved in the catalytic step. Its pH-rate profile has a slope of 1, suggesting a single deprotonation step is involved in the rate-limiting step. Tm7 has a cleavage rate of 1.6 min−1 at pH 7.8 with 10 μM Er3+. Phosphorothioate substitution at the cleavage junction completely inhibits the activity, which cannot be rescued by Cd2+ alone, or by a mixture of Er3+ and Cd2+, suggesting that two interacting metal ions are involved in direct bonding to both non-bridging oxygen atoms. A new model involving three lanthanide ions is proposed based on this study. A biosensor is engineered using Tm7 to detect Dy3+ down to 14 nM.