Human endonucleolytic incision of DNA 3' and 5' to a site-directed psoralen monoadduct and interstrand cross-link

Human chromatin-associated protein extracts were examined for endonucleolytic activity on a defined 132-base pair DNA substrate containing a single, site-specific 4,5'-8-trimethylpsoralen plus long wavelength ultraviolet light-induced furan side or pyrone side monoadduct or interstrand cross-link. These extracts produced incisions on both the 3' and 5' sides of each of these lesions. The distance between the 3' and 5' incisions at sites of a furan side monoadduct or cross-link was 9 nucleotides, and at sites of a pyrone side monoadduct or cross-link it was 17 nucleotides. Incisions on the 3' side of both types of furan side and pyrone side adducts were similar and were either at the fourth or fifth phosphodiester bond from the adducted thymine, depending upon the adduct. However, greater differences were observed between sites of 5' incision. This incision occurred at the fifth and sixth phosphodiester bonds from the adducted thymine at sites of furan side monoadducts and cross-links, respectively, and at the 13th and 14th phosphodiester bonds at sites of pyrone side monoadducts and cross-links, respectively. Thus, direct analysis of sites of endonucleolytic incision reveals that the location of sites of incision on TMP-adducted substrates depends upon the type of adduct present.