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A systematic spectroscopic and computational study was conducted in order to probe the influence of base sequences on stacked (S) versus B-type (B) conformational heterogeneity induced by the major dG adduct derived from the model carcinogen 7-fluoro-2-aminofluorene (FAF). We prepared and characterized eight 12-mer DNA duplexes (-AG*N- series, d[CTTCTAG*NCCTC]; -CG*N- series, d[CTTCTCG*NCCTC]), in which the central guanines (G*) were site-specifically modified with FAF with varying flanking bases (N = G, A, C, T). S/B heterogeneity was examined by CD, UV, and dynamic 19F NMR spectroscopy. All the modified duplexes studied followed a typical dynamic exchange between the S and B conformers in a sequence dependent manner. Specifically, purine bases at the 3'-flanking site promoted the S conformation (G > A > C > T). Simulation analysis showed that the S/B energy barriers were in the 14-16 kcal/mol range. The correlation times (tau = 1/kappa) were found to be in the millisecond range at 20 degrees C. The van der Waals energy force field calculations indicated the importance of the stacking interaction between the carcinogen and neighboring base pairs. Quantum mechanics calculations showed the existence of correlations between the total interaction energies (including electrostatic and solvation effects) and the S/B population ratios. The S/B equilibrium seems to modulate the efficiency of Escherichia coli UvrABC-based nucleotide excision repair in a conformation-specific manner: i.e., greater repair susceptibility for the S over B conformation and for the -AG*N- over the -CG*N- series. The results indicate a novel structure-function relationship, which provides insights into how bulky DNA adducts are accommodated by UvrABC proteins.

We report a systematic and quantitative structure-function relationship study of the major N-[deoxyguanosin-8-yl]-2-aminofluorene adduct (AF) derived from the prototype carcinogen 2-aminofluorene and its derivatives. The AF adduct is known to exist in two distinct conformational motifs, depending upon the location of the hydrophobic fluorine moiety: major groove binding "B type" (B) conformation (AF-dG anti ) and base-displaced "stacked" (S) conformation (AF-dG syn ). The AF-induced S/B conformational heterogeneity is sequence-dependent and follows a typical two-site dynamic chemical exchange. The population of S conformation decreases in the order of 3'-G > A » C > T, indicating the importance of the purine flanking bases in promoting the stacking structure. Line-shape analysis showed that the S/B interconversion energy barriers (ΔG ‡ ) are in the narrow 14-16 kcal/mol range. The energy differences of the two conformers are relatively small (

We report novel induced circular dichroism (ICD) characteristics for probing the conformational heterogeneity induced by the arylamine carcinogen 2-aminofluorene, namely, B type (B), stacked (S), and wedged (W) conformers. CD experiments were conducted with five different aminofluorene-modified DNA duplexes (I-V). An intense positive ICD was observed for the W conformeric I in the 290-360 nm range (ICD(290)(-)(360nm)). This was in contrast to the negative ICD(290)(-)(360nm) exhibited by the mostly B conformeric V (17% S/83% B). Duplex IV, which adopts an approximately equal mixture of S (53%) and B (47%), exhibited low ellipticities along the baseline. The magnitude of the positive ICD for I was significantly greater than that observed for II (70% S/30% B). While the ICD(290)(-)(360nm) of the W conformeric III showed no changes in intensity with increasing temperature from 10 to 35 degrees C, dramatic changes were observed for I across the same temperature range. Dynamic (19)F NMR results revealed that I exists in an 85:15 mixture of W and S/B conformers. The dramatic intensity changes observed for I are consistent with the presence of a W/B heterogeneity because of its susceptibility to result in a large difference on the magnitude of the ICD(290)(-)(360nm). In conclusion, the sign and magnitude of the ICD(290)(-)(360)(nm) are sensitive conformational markers for studying arylamine-induced conformational heterogeneity. The temperature-dependent ICD(290)(-)(360nm) data, coupled with (19)F NMR spectroscopy, provide valuable information about conformational distribution and dynamics, which are important factors that affect mutational outcomes.

The NarI recognition sequence (5'-G1G2CG3CN-3') is the most vulnerable hot spot for frameshift mutagenesis induced by the carcinogen 2-aminofluorene and its analogues in Escherichia coli. Lesioning of the guanine in the G3 position induces an especially high frequency of -2 deletion mutations; vulnerability to these mutations is modulated by the nature of the nucleotide in the N position (C approximately A > G > T). The objective of the present study was to probe the structural basis of this N-mediated influence on the propensity of the G3 lesion to form a slipped mutagenic intermediate (SMI) during translesion synthesis. We studied NarI-based fully paired [(5'-CTCG1G2CG3*CNATC-3')(5'-GATNCGGCCGAG-3'), N = dC or dT] and -2 deletion [(5'-CTCG1G2CG3*CNATC-3')(5'-GATNGCCGAG-3'), N = dC or dT] duplexes, in which G* was either AF [N-(2'-deoxyguanosin-8-yl)-2-aminofluorene] or the 19F probe FAF [N-(2'-deoxyguanosin-8-yl)-7-fluoro-2-aminofluorene]. The latter sequences mimic the bulged SMI for -2 deletion mutations. Dynamic 19F NMR, circular dichroism, and UV melting results indicated that the NarI-dC/-2 deletion duplex adopts exclusively an intercalated conformer, whereas the NarI-dT/-2 deletion duplex exists as multiple conformers. The data support the presence of a putative equilibrium between a carcinogen-intercalated and a carcinogen-exposed SMI for the dT/-2 duplex. A similar dT-induced conformational heterogeneity was observed for the fully paired duplexes in which all three guanines were individually modified by AF or FAF. The frequency of the carcinogen stacked S-conformation was found to be highest (69-75%) at the G3 hot spot in NarI-dC duplexes. Taken together, our results support the hypothesis that the conformational stability of the SMI is a critical determinant for the efficacy of -2 frameshift mutagenesis in the NarI sequence. We also provide evidence for AF/FAF conformational compatibility in the NarI sequences.

Adduct-induced conformational heterogeneity complicates the understanding of how DNA adducts exert mutation. A case in point is the N -deacetylated AF lesion [ N -(2′-deoxyguanosin-8-yl)-2-aminofluorene], the major adduct derived from the strong liver carcinogen N -acetyl-2-aminofluorene. Three conformational families have been previously characterized and are dependent on the positioning of the aminofluorene rings: B is in the “ B -DNA” major groove, S is “stacked” into the helix with base-displacement, and W is “wedged” into the minor groove. Here, we conducted 19 F NMR, CD, T m , and modeling experiments at various primer positions with respect to a template modified by a fluorine tagged AF-adduct (FAF). In the first set, the FAF-G was paired with C and in the second set it was paired with A. The FAF-G:C oligonucleotides were found to preferentially adopt the B or S-conformers while the FAF-G:A mismatch ones preferred the B and W-conformers. The conformational preferences of both series were dependent on temperature and complementary strand length; the largest differences in conformation were displayed at lower temperatures. The CD and T m results are in general agreement with the NMR data. Molecular modeling indicated that the aminofluorene moiety in the minor groove of the W-conformer would impose a steric clash with the tight-packing amino acid residues on the DNA binding area of the Bacillus fragment (BF), a replicative DNA polymerase. In the case of the B-type conformer, the carcinogenic moiety resides in the solvent-exposed major groove throughout the replication/translocation process. The present dynamic NMR results, combined with previous primer extension kinetic data by Miller & Grollman, support a model in which adduct-induced conformational heterogeneities at positions remote from the replication fork affect polymerase function through a long-range DNA–protein interaction.

Circular dichroism (CD) and UV-melting experiments were conducted with 16 oligodeoxynucleotides modified by the carcinogen 2-aminofluorene, whose sequence around the lesion was varied systematically [d(CTTCTNG[AF]NCCTC), N = G, A, C, T], to gain insight into the factors that determine the equilibrium between base-displaced stacked (S) and external B-type (B) duplex conformers. Differing stabilities among the duplexes can be attributed to different populations of S and B conformers. The AF modification always resulted in sequence-dependent thermal (T(m)) and thermodynamic (-DeltaG degrees ) destabilization. The population of B-type conformers derived from eight selected duplexes (i.e. -AG*N- and -CG*N-) was inversely proportional to the -DeltaG degrees and T(m) values, which highlights the importance of carcinogen/base stacking in duplex stabilization even in the face of disrupted Watson-Crick base pairing in S-conformation. CD studies showed that the extent of the adduct-induced negative ellipticities in the 290-350 nm range is correlated linearly with -DeltaG degrees and T(m), but inversely with the population of B-type conformations. Taken together, these results revealed a unique interplay between the extent of carcinogenic interaction with neighboring base pairs and the thermodynamic properties of the AF-modified duplexes. The sequence-dependent S/B heterogeneities have important implications in understanding how arylamine-DNA adducts are recognized in nucleotide excision repair.