AN UNUSUALLY STABLE PURINE(PURINE-PYRIMIDINE) SHORT TRIPLEX - THE 3RDSTRAND STABILIZES DOUBLE-STRANDED DNA

Classical models for DNA triple helix formation assume the stabilizati on of these structures through the formation of Hoogsteen hydrogen bon ds. This assumes that G-rich duplex DNA is more stable than tripler DN A. We report the results of co-migration assay, dimethyl sulfate footp rint, and UV spectroscopic melting studies that reveal that at least i n some cases of short (13-mer) purine(purine-pyrimidine) tripler the s tability of double-stranded DNA is increased by the binding of the thi rd strand. Under conditions which are usually considered as physiologi cal (10 mM MgCl2, 150 mM Na+ or K+) and with a rate of heating/cooling of 1 degrees C/min, there is a good reversibility of the melting prof iles which is consistent with a high rate of tripler formation. Other factors than Hoogsteen hydrogen bonds should therefore be involved in tripler stabilization. We suggest that oligonucleotides with similar p roperties could be efficient agents for artificial gene regulation.