Sriram M, Yang D, Gao Y G, Wang A H
Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign 61801.
Ann N Y Acad Sci. 1994 Jul 29;726:18-43; discussion 43-4. doi: 10.1111/j.1749-6632.1994.tb52794.x.
O6-ethyl-guanine (e6G) is a relatively persistent alkylation lesion caused by the exposure of DNA to carcinogen N-ethyl-N-nitrosourea. We have studied the structural consequences of the e6G incorporation in DNA by X-ray crystallography and NMR. We have obtained crystals of the modified DNA dodecamer d(CGC[e6G]AATTCGCG) complexed to several minor groove binding drugs including Hoechst 33258, Hoechst 33342, netropsin, and SN6999. The space group of the crystals from those complexes is P2(1)2(1)2(1). However the crystal structure of the SN6999 complex is not isomorphous to that from the other three complexes. In all four refined crystal structures the drugs bind in the narrow minor groove at or close to the central AATT region of the dodecamer B-DNA duplex. The DNA conformation is influenced by the binding of drugs. The eight independent e6G:C base pairs have a conformation ranging from one with three-centered hydrogen bonds between the bases to a wobble conformation with two hydrogen bonds. The ethyl group of the eight e6G bases is mostly in the proximal orientation to N7. Our 1D and 2D-NMR studies of the same (free) dodecamer reveal that the e6G:C base pairs in the duplex are likely to adopt a wobble conformation in solution. Those results suggest that the e6G:C base pair has a dynamic equilibrium among various conformations, which may present an ambiguous signal to cells. In contrast, the e6G:T base pair adopts a Watson-Crick-like conformation. This may be a plausible explanation of why thymine is found preferentially incorporated across the e6G during replication.
O6-乙基鸟嘌呤（e6G）是DNA暴露于致癌物N-乙基-N-亚硝基脲后产生的一种相对持久的烷基化损伤。我们通过X射线晶体学和核磁共振研究了e6G掺入DNA后的结构后果。我们获得了与几种小沟结合药物（包括Hoechst 33258、Hoechst 33342、纺锤菌素和SN6999）复合的修饰DNA十二聚体d（CGC[e6G]AATTCGCG）的晶体。这些复合物晶体的空间群为P2(1)2(1)2(1)。然而，SN6999复合物的晶体结构与其他三种复合物的晶体结构不同晶型。在所有四种精修的晶体结构中，药物结合在十二聚体B-DNA双链体中心AATT区域或其附近的狭窄小沟中。DNA构象受药物结合的影响。八个独立的e6G:C碱基对的构象范围从碱基之间具有三中心氢键的构象到具有两个氢键的摆动构象。八个e6G碱基的乙基大多朝向N7近端。我们对相同（游离）十二聚体的一维和二维核磁共振研究表明，双链体中的e6G:C碱基对在溶液中可能采用摆动构象。这些结果表明，e6G:C碱基对在各种构象之间存在动态平衡，这可能会向细胞呈现模糊的信号。相比之下，e6G:T碱基对采用类似沃森-克里克的构象。这可能是对复制过程中胸腺嘧啶优先掺入e6G对面这一现象的合理解释。
