Fujiwara Y, Iwai S
Department of Bioorganic Chemistry, Biomolecular Engineering Research Institute, Suita, Osaka, Japan.
Biochemistry. 1997 Feb 11;36(6):1544-50. doi: 10.1021/bi9619942.
In order to gain insight into the mechanism of mutations induced by photolesions in DNA, the thermal melting curves of oligonucleotide duplexes containing the (6-4) photoproducts of TT and TC and the Dewar isomer were measured at varying concentrations, and the thermodynamic parameters were determined. The duplexes were designed to be used as models for template-primer systems. In the case of d(CAXYAGCACGAC) x d(GTCGTGCTN), in which XY = T(6-4)T, T(6-4)C, or T(Dewar)T and N = A, G, C, or T, the -deltaG degrees values were the largest when N = G, and the difference between N = G and N = C or T was 1.1, 0.8, and 0.5 kcal/mol at 25 degrees C for XY = T(6-4)T, T(6-4)C, and T(Dewar)T, respectively. These results provide evidence of base pair formation between the 3'-pyrimidone and the opposite guanine, which was proposed previously to explain the T --> C transition induced at the 3' side of the T(6-4)T photoproduct. This interaction was weakened by isomerization to the Dewar photoproduct, which would explain the lower mutation specificity. For d(CAXYAGCACGAC) x d(GTCGTGCTGN), no obvious differences were observed regarding the base opposite the 5'-pyrimidine of the photoproducts. In contrast to the tertiary structure, determined by NMR, of a duplex containing T(6-4)T in its center [Kim, J.-K., & Choi, B.-S. (1995) Eur. J. Biochem. 228, 849-854], base pair formation at the 5'-pyrimidine was not detected for any photoproduct used in this study, even when the base opposite the 3'-pyrimidone of T(6-4)T was changed to adenine.
为了深入了解DNA中光损伤诱导突变的机制，我们在不同浓度下测量了含有TT和TC的（6-4）光产物以及杜瓦异构体的寡核苷酸双链体的热解链曲线，并确定了热力学参数。这些双链体被设计用作模板-引物系统的模型。对于d（CAXYAGCACGAC）×d（GTCGTGCTN），其中XY = T（6-4）T、T（6-4）C或T（杜瓦）T且N = A、G、C或T，当N = G时，-ΔG°值最大，在25℃时，对于XY = T（6-4）T、T（6-4）C和T（杜瓦）T，N = G与N = C或T之间的差异分别为1.1、0.8和0.5 kcal/mol。这些结果为3'-嘧啶酮与对面鸟嘌呤之间形成碱基对提供了证据，之前曾提出这一现象来解释T（6-4）T光产物3'侧诱导的T→C转变。这种相互作用因异构化为杜瓦光产物而减弱，这可以解释较低的突变特异性。对于d（CAXYAGCACGAC）×d（GTCGTGCTGN），在光产物5'-嘧啶对面的碱基方面未观察到明显差异。与中心含有T（6-4）T的双链体通过核磁共振确定的三级结构[Kim, J.-K., & Choi, B.-S. (1995) Eur. J. Biochem. 228, 849 - 854]相反，本研究中使用的任何光产物在5'-嘧啶处均未检测到碱基对形成，即使T（6-4）T的3'-嘧啶酮对面的碱基改为腺嘌呤。
