Rajagopal P, Feigon J
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024.
Nature. 1989 Jun 22;339(6226):637-40. doi: 10.1038/339637a0.
Interest in triple and quadruple DNA helices built from homopurine and homopyrimidine strands has recently intensified principally because such structures may occur in vivo but also because of the potential use of triplexes both in forming highly sequence-specific complexes for use in chromosome mapping and in repressing transcription. From fibre diffraction data, models for triplex structures with poly(U).poly(A).poly(U) and poly(dT).poly(dA).poly(dT) have been proposed, in which the purine and one pyrimidine strand are Watson-Crick paired in an A' helix, and the other pyrimidine strand is Hoogsteen base-paired parallel to the purine strand along the major groove. A similar base-pairing scheme involving G and C would require protonation of C for Hoogsteen base-pair formation, and models for such triplexes have been proposed by analogy to the single-sequence fibre diffraction data. To date, however, there have been no single crystal or NMR structural data on DNA triplexes, and no direct observation of the protonated C in such a context. We present here the first NMR evidence for triplex formation in DNA from the homopurine d(G-A) and homopyrimidine d(T-C) oligonucleotides, and report direct observation of imino protons from protonated cytosines in the triplex.
近期，人们对由同型嘌呤和同型嘧啶链构成的三链和四链DNA螺旋的兴趣显著增强，这主要是因为此类结构可能在体内出现，还因为三链体在形成用于染色体图谱绘制的高度序列特异性复合物以及抑制转录方面具有潜在用途。根据纤维衍射数据，已提出了聚（U）·聚（A）·聚（U）和聚（dT）·聚（dA）·聚（dT）三链体结构的模型，其中嘌呤链和一条嘧啶链在A'螺旋中以沃森-克里克方式配对，另一条嘧啶链则沿着大沟与嘌呤链平行进行霍格施泰因碱基配对。涉及G和C的类似碱基配对方案需要C质子化以形成霍格施泰因碱基对，并且已根据单序列纤维衍射数据类推提出了此类三链体的模型。然而，迄今为止，尚无关于DNA三链体的单晶或核磁共振结构数据，也没有在这种情况下对质子化C的直接观察。我们在此展示了来自同型嘌呤d（G-A）和同型嘧啶d（T-C）寡核苷酸的DNA中形成三链体的首个核磁共振证据，并报告了对三链体中质子化胞嘧啶的亚氨基质子的直接观察结果。
