LASER PHOTOFOOTPRINTING OF D(C).D(G).D(G) INTRAMOLECULAR TRIPLEX

Supercoiling-induced structural transition of the d(C24GC21) . d(G21CG 24) sequence in plasmid DNA in the presence of Mg2+ at neutral pH resu lts in alterations of efficiencies of not only single-quantum (pyrimid ine[6-4]pyrimidone adducts) but also two-quantum (alkali-sensitive les ions of dG residues) photomodifications of nucleoside residues within this sequence. The generation of both types of photoreactions was achi eved by the application of high-intensity laser UV radiation (intensit y approximately 10(11) W/m2, pulse duration approximately 10(-8) s, la mbda = 266 nm) for irradiation of a plasmid DNA. The modification exte nt sufficient for analysis of photoreaction efficiency distributions a long both strands of the insert (photofootprinting) was obtained by th e action of a single nanosecond pulse of laser UV radiation. The patte rn of a laser photofootprinting is consistent with the d(C) . d(G) . d (G) triplex formation in the presence of Mg2+ within the insert and sh ows some details of this triplex structure.