HUMAN DNA TOPOISOMERASE I-MEDIATED CLEAVAGES STIMULATED BY ULTRAVIOLET LIGHT-INDUCED DNA-DAMAGE

DNA topoisomerases have been proposed as the proteins involved in the formation of the DNA-protein cross-links detected after ultraviolet li ght (UV) irradiation of cellular DNA. This possibility has been invest igated by studying the effects of UV-induced DNA damage on human DNA t opoisomerase I action, UV lesions impaired the enzyme's ability to rel ax negatively supercoiled DNA. Decreased relaxation activity correlate d with the stimulation of cleavable complexes, Accumulation of cleavab le complexes resulted from blockage of the rejoining step of the cleav age-religation reaction, Mapping of cleavage sites on the pAT153 genom e indicated UV-induced cleavage at discrete positions corresponding to sites stimulated also by the topoisomerase I inhibitor camptothecin, except for one, Subsequent analysis at nucleotide level within the seq uence encompassing the UV-specific cleavage site revealed the precise positions of sites stimulated by camptothecin with respect to those sp ecific for UV irradiation, Interestingly, one of the UV-stimulated cle avage sites was formed within a sequence that did not contain dimerize d pyrimidines, suggesting transmission of the distortion, caused by ph otodamage to DNA, into the neighboring sequences, These results suppor t the proposal that DNA structural alterations induced by UV lesions c an be sufficient stimulus to induce cross-linking of topoisomerase I t o cellular DNA.