BASE-PAIR CONFORMATION-DEPENDENT EXCISION OF BENZO[A]PYRENE DIOL EPOXIDE-GUANINE ADDUCTS BY HUMAN NUCLEOTIDE EXCISION-REPAIR ENZYMES

Human nucleotide excision repair processes carcinogen-DNA adducts at h ighly variable rates, even at adjacent sites along individual genes, H ere, we identify conformational determinants of fast or slow repair by testing excision of N-2-guanine adducts formed by benzo[a]pyrene diol epoxide (BPDE), a potent and ubiquitous mutagen that induces mainly G .C-->T. A transversions and frameshift deletions. We found that human nucleotide excision repair processes the predominant (+)-trans-BPDE-N- 2-dG adduct 15 times less efficiently than a standard acetylaminofluor ene-C-8-dG lesion in the same sequence, No difference was observed bet ween (+)-trans- and (-)-trans-BPDE-N-2-dG, but excision was enhanced a bout 10-fold by changing the adduct configurations to either (+)-cis- or (-)-cis-BPDE-N-2-dG. Conversely, excision of (+)-cis- and (-)-cis- but not (+)-trans-BPDE-N-2-dG was reduced about 10-fold when the compl ementary cytosine was replaced by adenine, and excision of these BPDE lesions was essentially abolished when the complementary deoxyribonucl eotide was missing, Thus, a set of chemically identical BPDE adducts y ielded a greater-than-100-fold range of repair rates, demonstrating th at nucleotide excision repair activity is entirely dictated by local D NA conformation, In particular, this unique comparison between structu rally highly defined substrates shows that fast excision of BPDE-N-2-d G lesions is correlated with displacement of both the modified guanine and its partner base in the complementary strand from their normal in trahelical positions, The very slow excision of carcinogen-DNA adducts located opposite deletion sites reveals a cellular strategy that mini mizes the fixation of frameshifts after mutagenic translesion synthesi s.