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

Gel sequencing experiments with the 5′- and 3′-end-labeled oligonucleotides d(A3GA4GA5GA6GA3G) 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.