Conformational and Thermodynamic Impact of Bulky Aminofluorene Adduction on Simulated Trans lesion DNA Synthesis
Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, University of Rhode Island, Kingston, RI 02881, United States.Chemical Research in Toxicology (Impact Factor: 3.53). 03/2011; 24(4):597-605. DOI: 10.1021/tx2000587
We report a systematic spectroscopic investigation on the conformational evolution during primer extension of a bulky fluoroaminofluorene-modified dG adduct (FAF-dG) in chemically simulated translesion synthesis. FAF-dG was paired either with dC or dA (dC-match and dA-mismatch series, respectively). Dynamic (19)F NMR/CD results showed that the FAF-adduct exists in a syn/anti equilibrium and that its conformational characteristics are modulated by the identity of an inserted nucleotide at the lesion site and the extent of primer elongation. At the pre-insertion site, the adduct adopted preferentially a syn conformation where FAF stacked with preceding bases. Insertion of the correct nucleotide dC at the lesion site and subsequent elongation resulted in a gradual transition to the anti conformation. By contrast, the syn conformer was persistent along with primer extension in the dA-mismatch series. In the dC-match series, FAF-induced thermal (T(m)) and thermodynamic (-ΔG°(37 °C)) stabilities were significantly reduced relative to those of the controls. However, the corresponding T(m) and -ΔG°(37 °C) values were increased in the FAF-modified mismatched dA series. The lesion impact persisted up to three 5'-nucleotides from the lesion. Occupation of the minor groove of the W-conformer with the bulky carcinogenic fluorene moiety not only would limit the DNA mobility but also would impose a serious difficulty for the active site of a polymerase throughout the replication process. Our spectroscopic results are consistent with reported data on AF, which showed dramatic (~10(4)-fold) differences in the nucleotide insertion rates between the dC-match and dA-mismatch series. The results emphasize the importance of adduct-induced steric constraints for determining the replication fidelity of a polymerase.
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ABSTRACT: M06-2X/6-31G(d,p) is used to calculate the structure of all natural deoxydinucleoside monophosphates with G in the 5' or 3' position, the anti or syn conformation, and each natural (A, C, G, T) base in the corresponding flanking position. When the ortho or para C8-phenoxyl-2'-deoxyguanosine (C8-phenoxyl-dG) adduct replaces G in each model, there is little change in the relative base-base orientation or backbone conformation. However, the orientation of the C8-phenoxyl group can be characterized according to the position (5' versus 3'), conformation (anti versus syn), and isomer (ortho versus para) of damage. Although the degree of coplanarity between the phenoxyl ring and G base in the ortho adduct is highly affected by the sequence since the hydroxyl group can interact with neighboring bases, the para adduct generally does not exhibit discrete interactions with flanking bases. For both adducts, steric clashes between the phenoxyl group and the backbone or flanking base destabilize the anti conformation preferred by the natural nucleotide and thereby result in a clear preference for the syn conformation regardless of the sequence or position. This contrasts the conclusions drawn from smaller (nucleoside, nucleotide) models previously used in the literature, which stresses the importance of using models that address the steric constraints present due to the surrounding environment. Since replication errors for other C8-dG bulky adducts have been linked to a preference for the syn conformation, our findings provide insight into the possible mutagenicity of phenolic adducts.
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ABSTRACT: The DNA sequence effect is an important structural factor for determining the extent and nature of carcinogen-induced mutational and repair outcomes. In this study, we used two 16-mer template sequences, TG*A [d(5'-CTTCTTG*ACCTCATTC-3')] and CG*A [d(5'-CTTCTCG*ACCTCATTC-3')], to study the impact of the 5'-flanking nucleotide (T vs C) on aminofluorene (AF)-induced stacked (S)/major groove (B)/wedge (W) conformational heterogeneity during a simulated translesion synthesis. In addition, we probed the sequence effect on nucleotide insertion efficiencies catalyzed by the Klenow fragment (exonuclease-deficient) of DNA polymerase I. Our (19)F NMR/ICD/DSC results showed that AF in the CG*A duplex sequence adopts a greater population of S-conformer than the TG*A sequence. We found that the S conformer of CG*A thermodynamically favors insertion of A over C at the lesion site (n). Significant stalling occurred at both the prelesion (n - 1) and lesion (n) sites; however, the effect was more persistent for the S conformer of CG*A than TG*A at the lesion site (n). Kinetics show that relative nucleotide insertion frequencies (f(ins)) were greater for TG*A than the S conformer of CG*A for either dCTP or dATP at the lesion site (n), and the insertion rate was significantly reduced at immediate upstream base pairs (n, n + 1). Taken together, the results provide insight into how the mutagenic AF could exhibit an S/B/W equilibrium in the active site of a polymerase, causing different mutations. This work represents a novel structure-function relationship in which adduct structure is directly linked to nucleotide insertion efficiency in a conformation-specific manner during translesion DNA synthesis.
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ABSTRACT: Bulky DNA adducts are formed through the covalent attachment of aryl groups to the DNA nucleobases. Many of these adducts are known to possess conformational heterogeneity, which is responsible for the variety of mutagenic outcomes associated with these lesions. The present contribution reviews several conformational and mutagenic themes that are prevalent among the DNA adducts formed at the C8-site of the guanine nucleobase. The most important conclusions obtained (to date) from experiments are summarized including the anti/syn conformational preference of the adducts, their potential to inflict DNA mutations and mismatch stabilization, and their interactions with DNA polymerases and repair enzymes. Additionally, the unique role that computer calculations can play in understanding the structural properties of these adducts are highlighted.
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