Formation of purine photoproducts in a defined human DNA sequence.

The formation of DNA base damages by broad spectrum ultraviolet irradiation (250-400 nm) was investigated using a defined sequence of human DNA. The irradiated, 92 base pair, 3'-end of the human alphoid segment was incubated with an enzyme fraction purified from bacteriophage T4-infected E. coli. As previously reported, analysis of reaction products by sequencing gels showed enzymic incision of purine-containing photoproducts as well as pyrimidine cyclobutane photodimers. The purine-incising activity does not require metal ions and was unaffected by beta-mercaptoethanol or dithiothreitol. The formation of the purine photoproducts is independent of buffer; these lesions are produced by irradiation of DNA in Tris, Hepes or phosphate buffers. They are produced at biologically significant wavelengths between 260 to 300 nm. Only low levels were detected above or below this range. The formation of purine photoproducts is dose dependent with similar yields at some specific loci to pyrimidine dimers. These results suggest that purine-containing photoproducts could be of consequence in ultraviolet carcinogenesis.