Inhibition of restriction endonuclease cleavage via triple helix formation by homopyrimidine oligonucleotides.

A 17-mer homopyrimidine oligonucleotide was designed to bind to the major groove of SV40 DNA at a 17 base pair homopurine-homopyrimidine sequence via Hoogsteen base pairing. This sequence contains the recognition site for the class II-S restriction enzyme Ksp 632-I. The oligonucleotide was shown to inhibit enzymatic cleavage under conditions that allow for triple helix formation. Inhibition is sequence-specific and occurs in the micromolar concentration range. Triple helix formation by oligonucleotides opens new possibilities for sequence-specific regulation of gene expression.