SOLUTION STRUCTURE OF THE AMINOFLUORENE [AF]-EXTERNAL CONFORMER OF THE ANTI-[AF]-C-8-DG ADDUCT OPPOSITE DC IN A DNA DUPLEX

The Escherichia coli genome contains a C-G(1)-G(2)-C-G(3)-C-C NarI hot spot sequence for -2 deletion mutations at G(3) by aromatic amine car cinogens 2-acetylaminofluorene (AAF) and 2-aminofluorene (AF) that for m covalent adducts at the C-8-position of the guanine ring, Each of th e three guanines are positioned in different sequence contexts (C-G(1) -G, G-G(2)-C, and C-G(3)-C) which provides an opportunity to investiga te the potential sequence dependent interconversion between AF-interca lated and AF-external conformers of the [AF]dG adduct positioned oppos ite dC within the NarI sequence at the duplex level. We have prepared and purified DNA duplexes containing the [AF]dG adduct positioned in C -[AF]G-G, G-[AF]G-C, and C-[AF]G-C NarI sequence contexts and observe the ratio of AF-intercalated to AF-external conformers to be 30:70, 10 :90, and 50:50, respectively. We have applied a combined NMR-molecular mechanics approach to define the structure of the AF-external conform er in the G-[AF]G-C NarI sequence context where it is the predominant conformation (90%) in solution. The modified guanine of the [AF]dG add uct aligns through Watson-Crick pairing with its partner cytosine and is stacked into the helix between flanking Watson-Crick dG.dC base pai rs, The AF-external conformer with its anti[AF]dG residue causes minim al perturbations in the DNA duplex at and adjacent to the lesion site with the covalently linked fluorenyl ring readily accommodated in the major groove and tilted toward the 5'-end of the modified strand of th e helix. This paper on the structure of the AF-external conformer with an anti-[AF]dG adduct together with the preceding paper in this issue on the structure of the AF-intercalated conformer with a syn-[AF]dG a dduct defines for the first time the capacity of the mutagenic [AF]dG lesion to adopt interconverting syn and anti alignments with the equil ibrium shifting between the conformers depending on nearest neighbor a nd next-nearest neighbor sequences, Perhaps, recognition of the [AF]dG lesion by the repair machinery would be able to discriminate between the AF-intercalated conformer with its base displacement-fluorenyl rin g insertion perturbation of the helix and the AF-external conformer wh ere the DNA helix is essentially unperturbed at the lesion site and th e fluorenyl ring is positioned with directionality in the major groove .