RECOGNITION OF ALTERNATING OLIGOPURINE OLIGOPYRIMIDINE TRACTS OF DNA BY OLIGONUCLEOTIDES WITH BASE-TO-BASE LINKAGES

A new concept is presented to design and synthesize modified oligonucl eotides in order to extend the range of double-helical DNA sequences t hat can be recognized by oligonucleotides via triple helix formation. The DNA target is composed of adjacent oligopurine oligopyrimidine dom ains where the oligopurine sequences alternate on the two DNA strands. Canonical (C,T)-motif triple helices are formed with each oligopurine . oligopyrimidine domain of the target sequence. The two third-strand oligonucleotides were joined together via an appropriate linker betwe en the two terminal bases with either a 3'-3' or a 5'-5' polarity. Mol ecular modeling was used to predict the optimal length of the linker b ridging two terminal bases. The interaction of DNA with such a modifie d oligonucleotide containing a C-3-U-3 linkage was studied by thermal dissociation, footprinting, and gel retardation experiments. They prov ide experimental evidence that the oligonucleotide does form a switche d triple helix on this extended DNA target sequence. The binding of th e so-called ''switch oligonucleotide'' is enhanced as compared to the two unlinked parental oligonucleotides which form triple helices with each oligopurine oligopyrimidine domain of the target sequence.