Inter-strand photoproducts are produced in high yield within A-DNA exposed to UVC radiation

Far-UV irradiation of DNA leads to the dimerization of pyrimidine bases, resulting in the formation of cyclobutane type dimers and (6±4) photoproducts. In the dry state, an additional thymine dimeric photolesion, the spore photoproduct, is also gener-ated. While most photoproducts are expected to be produced between adjacent pyrimidines, little atten-tion has been paid to lesions involving bases located on different DNA strands. Using HPLC± mass spectrometry analysis of enzymatically digested DNA, we observed that, in the dry state, inter-strand dimeric photoproducts represented 30 % of the total yield of dimeric thymine lesions. The major inter-strand damage was found to be the spore photoproduct. Formation of inter-strand lesions in signi®cant yield could be obtained in solution upon modi®cation of the DNA conformation as the result of the addition of large amounts of ethanol. In both cases, DNA is in the A-form, which is characterized by a high compaction, likely to favor inter-strand photoreactions. Since the latter DNA conformation is also predominant in bacterial spores, the formation and repair of dimeric photo-products involving thymine bases located on different DNA strands may thus be relevant in terms of deleterious effects of UV radiation to the latter microorganisms