An unusually stable purine(purine-pyrimidine) short triplex. The third strand stabilizes double-stranded DNA

Classical models for DNA triple helix formation assume the stabilization of these structures through the formation of Hoogsteen hydrogen bonds. This assumes that G-rich duplex DNA is more stable than triplex DNA. We report the results of co-migration assay, dimethyl sulfate footprint, and UV spectroscopic melting studies that reveal that at least in some cases of short (13-mer) purine(purine-pyrimidine) triplex the stability of double-stranded DNA is increased by the binding of the third strand. Under conditions which are usually considered as physiological (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 profiles which is consistent with a high rate of triplex formation. Other factors than Hoogsteen hydrogen bonds should therefore be involved in triplex stabilization. We suggest that oligonucleotides with similar properties could be efficient agents for artificial gene regulation.