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

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Abstract
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 ΔH = 79 ± 8 kcal/mol, δS = 224 22 e.u., ΔG 298 = 12.2 ± 1.2 kcal/mol. These data are compared with those reported for the YRY triplex motif.