The importance of peripheral sequences in determining the metal selectivity of an in vitro-selected Co(2+) -dependent DNAzyme.

DNAzymes are catalytically active DNA molecules that use metal cofactors for their enzymatic functions. While a growing number of DNAzymes with diverse functions and metal selectivities have been reported, the relationships between metal ion selectivity, conserved sequences and structures responsible for selectivity remain to be elucidated. To address this issue, we report biochemical assays of a family of previously reported in vitro selected DNAzymes. This family includes the clone 11 DNAzyme, which was isolated by positive and negative selection, and the clone 18 DNAzyme, which was isolated by positive selection alone. The clone 11 DNAzyme has a higher selectivity for Co(2+) over Pb(2+) compared with clone 18. The reasons for this difference are explored here through phylogenetic comparison, mutational analysis and stepwise truncation. A novel DNAzyme truncation method incorporated a nick in the middle of the DNAzyme to allow for truncation close to the nicked site while preserving peripheral sequences at both ends of the DNAzyme. The results demonstrate that peripheral sequences within the substrate binding arms, most notably the stem loop, loop II, are sufficient to restore its selectivity for Co(2+) over Pb(2+) to levels observed in clone 11. A comparison of these sequences' secondary structures and Co(2+) selectivities suggested that metastable structures affect metal ion selectivity. The Co(2+) selectivity of the clone 11 DNAzyme showed that the metal ion binding and selectivities of small, in vitro selected DNAzymes may be more complex than previously appreciated, and that clone 11 may be more similar to larger ribozymes than to other small DNAzymes in its structural complexity and behavior. These factors should be taken into account when metal-ion selectivity is required in rationally designed DNAzymes and DNAzyme-based biosensors.

In vitro selection of metal ion-selective DNAzymes.

The discovery of DNAzymes that can catalyze a wide range of reactions in the presence of metal ions is important on both fundamental and practical levels; it advances our understanding of metal-nucleic acid interactions and allows for the design of highly sensitive and selective metal ion sensors. A crucial factor in this success is a technique known as in vitro selection, which can rapidly select metal-specific RNA-cleaving DNAzymes. In vitro selection is an iterative process where a DNA pool containing a random region is incubated with the target metal ion. Those DNA sequences that catalyze the preferred reaction (the "winners") are amplified and carried on to the next step, where the selection is carried out under more stringent conditions. In this way, the selection pool becomes enriched with DNAzymes that exhibit desirable activity and selectivity. The method described can be applied to isolate DNAzymes selective to many different types of metal ions or different oxidation states of the same metal ion.