Characterization of the DNA triplex formed by d(TGGGTGGGTGGTTGGGTGGG) and a critical R.Y sequence located in the promoter of the murine Ki-ras proto-oncogene.

The binding of the G-rich oligonucleotide d(TGGGTGGGTGGTTGGGTGGG) to a critical homopurine-homopyrimidine sequence located in the promoter of the murine Ki-ras proto-oncogene has been investigated. The duplex and the oligonucleotide form a triple helix as evidenced by band-shift electrophoresis, hydroxyapatite (HA) chromatography, UV-melting and circular dichroism (CD) experiments. Upon thermal denaturation in 50 mM Tris-acetate, pH 7.4, 50 mM NaCl, 10 mM MgCl2, 0.1 mM spermine the triplex exhibits two cooperative transitions: one of these is attributed to the triplex-to-duplex transformation, the other to the duplex-to-coil transformation. The thermodynamic parameters of triplex formation have been determined by a van't Hoff analysis of the UV-melting curves which provided values of delta H = 79 +/- 8 kcal/mol, delta S = 224 +/- 22 e.u., delta G298 = 12.2 +/- 1.2 kcal/mol. These data are compared with those reported for the YRY triplex motif.