Sequence-specific recognition and cleavage of duplex DNA via triple-helix formation by oligonucleotides covalently linked to a phenanthroline-copper chelate.

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Abstract
Homopyrimidine oligodeoxynucleotides recognize the major groove of the DNA double helix at homempurine-homopyrimidine sequences by forming local triple helices. Phenanthroline was covalently attached to the 5'' end of an 11-mer homopyrimidine oligonucleotide of sequence d(TTTCCTCCTCT). Simian virus 40 DNA, which contains a single target site for this oligonucleotide, was used as a substrate for the phenanthroline-oligonucleotide conjugate. In the presence of copper ions and a reducing agent, a single specific double-strand cleavage site was observed at 20.degree. C by agarose gel electrophoresis. The efficiency of double-strand cleavage was > 70% at 20.degree. C and pH 7.4. Secondary cleavage sites were observed when binding of the oligonucleotide to mismatched sequences was allowed to take place at low temperature. The exact location of the cleavage sites was determined by polyacrylamide gel electrophoresis of denatured fragments by using both simian virus 40 DNA and a synthetic DNA fragment containing the target sequence. The asymmetric distribution of the cleavage sites on the strands revealed that the cleavage reaction took place in the minor groove even though the phenanthroline linker was located in the major groove. Linkers of different lengths were used to tether phenanthroline to the oligonucleotide and their relative efficacies of DNA cleavage were compared. Based on these comparative studies and on model building, it is proposed that the phenanthroline ring carried by the oligonucleotide intercalates from the major groove and that copper chelation locks the complex in place from within the minor groove where the cleavage reaction occurs.