Abstract
DNA fragments were constructed in which acetylaminofluorene adducts were introduced specifically at each one of the three different guanines of the 6-base-pair sequence -GGCGCC-. Incubation of the DNA with the UvrABC excision nuclease in vitro revealed major differences in the efficiency of adduct excision depending on the site of modification. Oligonucleotide excision of adducts bound to the second guanine was only 15% as efficient as excision of adducts at the first guanine, whereas the excision efficiency for adducts bound to the third guanine was intermediary. However, recognition of DNA damage appeared to occur with nearly 100% efficiency at all three adduct positions, as judged from DNase I footprint analysis of the DNA/protein binding complexes. Hence, it appears that the structural elements for DNA damage recognition by the UvrABC enzyme are different from the signals for excision. Furthermore, the repair pattern observed is not inversely correlated with the potential of these adducts to induce mutations since mutation analysis of single-adduct DNA has shown that only adducts at the third guanine are strongly premutagenic. We conclude that the effectiveness of excision repair depends on the context of the DNA sequence and that ineffectively repaired adduct sites are not necessarily mutational hot spots.