Kohwi-Shigematsu T, Kohwi Y
La Jolla Cancer Research Foundation, CA 92037.
Nucleic Acids Res. 1991 Aug 11;19(15):4267-71. doi: 10.1093/nar/19.15.4267.
Negative superhelical strain induces the poly(dG)-poly(dC) sequence to adopt two totally different types of triple-helices, either a dG.dG.dC triplex in the presence of Mg(+)+ at both neutral and acidic pHs or a protonated dC+.dG.dC triplex in the absence of Mg(+)+ ions at acidic pH (1). To examine whether there are still other types of non-B DNA structures formed by the same sequence, we constructed supercoiled plasmid DNAs harboring varying lengths of the poly(dG) tract, and the structures adopted by each supercoiled plasmid DNA were studied with a chemical probe, chloroacetaldehyde. The potential of a poly(dG)-poly(dC) sequence to adopt non-B DNA structures depends critically on the length of the tract. Furthermore, in the presence of Mg(+)+ and at a mildly acidic pH, in addition to the expected dG.dG.dC triplex detected for the poly(dG) tracts of 14 to 30 base pairs (bp), new structures were also detected for the tracts longer than 35 bp. The structure formed by a poly(dG) tract of 45 bp revealed chemical reaction patterns consistent with a dG.dG.dC triplex and protonated dC+.dG.dC triple-helices fused together. This structure lacks single-stranded stretches typical of intramolecular triplexes.
负超螺旋应变促使聚（dG）-聚（dC）序列形成两种完全不同类型的三链体，即在中性和酸性pH条件下，在Mg(++)存在时形成dG.dG.dC三链体，或在酸性pH条件下，在无Mg(++)离子时形成质子化的dC+.dG.dC三链体（1）。为了研究该相同序列是否还能形成其他类型的非B型DNA结构，我们构建了含有不同长度聚（dG）片段的超螺旋质粒DNA，并使用化学探针氯乙醛研究了每个超螺旋质粒DNA所采用的结构。聚（dG）-聚（dC）序列形成非B型DNA结构的潜力关键取决于片段的长度。此外，在Mg(++)存在且pH呈轻度酸性的条件下，除了在14至30个碱基对（bp）的聚（dG）片段中检测到预期的dG.dG.dC三链体外，对于长度超过35 bp的片段也检测到了新的结构。由45 bp的聚（dG）片段形成的结构显示出与dG.dG.dC三链体和质子化的dC+.dG.dC三链体融合在一起相一致的化学反应模式。这种结构缺乏分子内三链体典型的单链延伸。
