Alternate strand recognition of double-helical DNA by (T,G)-containing oligonucleotides in the presence of a triple helix-specific ligand.

Triple helix formation requires a polypurine- polypyrimidine sequence in the target DNA. Recent works have shown that this constraint can be circumvented by using alternate strand triplex-forming oligonucleotides. We have previously demonstrated that (T,G)-containing triplex- forming oligonucleotides may adopt a parallel or an antiparallel orientation with respect to an oligopurine target, depending upon the sequence and, in particular, upon the number of 5'-GpT-3' and 5'-TpG-3' steps [Sun et al. (1991) C.R. Acad. Sci. Paris Ser III, 313, 585-590]. A single (T,G)-containing oligonucleotide can therefore interact with two oligopurine stretches which alternate on the two strands of the target DNA. The (T,G) switch oligonucleotide contains a 5'-part targeted to one of the oligopurine sequences in a parallel orientation followed by a 3'-part that adopts an antiparallel orientation with respect to the second oligopurine sequence. We show that a limitation to the stability of such a triplex may arise from the instability of the antiparallel part, composed of reverse-Hoogsteen C.GxG and T.AxT base triplets. Using DNase I footprinting and ultraviolet absorption experiments, we report that a benzo[e]pyridoindole derivative [(3-methoxy- 7H-8-methyl-11-[(3'-amino-propyl) amino] benzo[e]pyrido [4,3-b]indole (BePI)], a drug interacting more tightly with a triplex than with a duplex DNA, strongly stabilizes triplexes with reverse-Hoogsteen C.GxG and T.AxT triplets thus allowing a stabilization of the triplex-forming switch (T,G) oligonucleotide on alternating oligopurine- oligopyrimidine 5'-(Pu)14(Py)14-3' duplex sequences. These results lead to an extension of the range of oligonucleotide sequences for alternate strand recognition of duplex DNA.