ALKALI-LABILE PHOTOLESIONS MAPPING TO PURINE SITES IN ULTRAVIOLET-IRRADIATED DNA

Gel sequencing experiments with the 5'- and 3'-end-labeled oligonucleotides d(A(3)GA(4)GA(5)GA(6)GA(3)G) and d(AT)(10) have demonstrated that dimeric adenine photoproducts and thymine-adenine photoadducts constitute alkali-labile lesions in UV-irradiated DNA. On treatment with hot piperidine, DNA strand breakage occurs predominantly at the sites of 5'-adenines in the dimeric photoproducts and of 3'-adenines in the thymine-adenine photoadducts. With 5'-end-labeled oligonucleotides of mixed sequence, major UV-induced loci for alkaline cleavage map to purine bases flanked on their 5'-side by two pyrimidines. This behavior does not arise from enhanced photoreactivity of purines in this sequence context as has been inferred from photofootprinting studies. Instead, as shown by 3'-labeling and selective substitution with 5-methylcytosine, it results from the anomalous electrophoretic mobility of 5'-end-labeled fragments produced by alkaline cleavage of DNA at adjacent pyrimidine (6-4) pyrimidone photoproducts.