Influence of Cleavage Site on Global Folding of an RNA-Cleaving DNAzyme
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8-17 is a DNAzyme with metal-dependent endoribonuclease activity. Recently, a variant termed 8-17NG was reported as the first nucleic acid enzyme capable of cleaving all 16 dinucleotide junctions of RNA with rate enhancements ranging from 1000- to 1,000,000,000-fold over background activity. We attributed this broad-ranging cleavage efficiency to global folding of the DNAzyme. We sought to examine the influence of dinucleotides at the cleavage site of 8-17NG on global folding by using three-color (3c) FRET. By comparing the folding of 8-17NG with all 16 possible dinucleotide junctions, we found all examined DNAzyme-substrate constructs adopted a two-step folding process in the presence of Mn(2+), which was consistent with previous metal-induced folding studies of 8-17. Interestingly, Mn(2+) titration experiments also suggest that the second folding step is dependent on dinucleotide identity: purine-purine junctions allowed 8-17NG to fold at lower concentrations than pyrimidine-pyrimidine linkages. This finding was corroborated by RNA cleavage assays, in which the largest improvement in cleavage yield was observed in pyrimidine-pyrimidine junctions when [Mn(2+)] was increased. Taken together, these results support the previously observed hierarchy of 8-17 activity for different cleavage sites. Complemented by earlier sequence and structure-function studies, this investigation allowed for the first detailed examination of crucial relationships between the structural influence and junction preferences of nucleic acid-catalyzed RNA cleavage reactions.
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