PROTECTION AGAINST UV-INDUCED PYRIMIDINE DIMERIZATION IN DNA BY TRIPLEX FORMATION

CYCLOBUTANE and [6-4]-pyrimidine dimers are major photo-products of ultraviolet-irradiated DNA. The yield of these photo-products is dependent on the sequence1,2 and structure3 of the DNA. By analysing the photofootprints of fragments produced by cleavage of the DNA chain near [6-4]-pyrimidine dimers4, we show here that a homopurine-homopyrimidine insert (with either d(TC)x or d(C)n) in plasmid pUC19 is, as expected, a good target for UV-induced pyrimidine-dimer formation. But we find that dimeriz-ation is virtually completely suppressed when the pyrimidine oligonucleotides dfTC)y or d(C) m are added to DNA carrying d(TC)x- or d(C)n-containing inserts, respectively. This effect is dependent on the type of oligonucleotide used and is site-specific. The protection occurs under acidic conditions that favour the formation of intermolecular triplexes between the homopurine-homopyrimidine inserts and homologous oligopyrimidines5. We therefore conclude that triplex formation effectively protects the DNA duplex from UV-induced damage (pyrimidine dimerization). This observation makes the photofootprinting assay6 a very promising method for studying intermolecular and intramolecular triplexes (?-form DNA) both in vitro and in vivo. © 1990 Nature Publishing Group.