Lee J S, Johnson D A, Morgan A R
Nucleic Acids Res. 1979 Jul 11;6(9):3073-91. doi: 10.1093/nar/6.9.3073.
(Pyrimidine)n . (purine)n DNAs of repeating sequences form a distinctive complex on lowering the pH below 6. Previously this complex was thought to be tetra-stranded. The present work is inconsistent with this view, and four lines of evidence show that the complex consists of a triplex together with a poly d(purine) possessing secondary structure. Formula: (see text). (a) S1 nuclease digestion leads to degradation of 50% of the poly d(purine) content of the pH 5-induced complex. (b) Buoyant density studies demonstrate that there is no interaction between the triplex and added free poly d(purine) and also that the complex formed from duplex DNA contained poly d(purine) which is free to form a triplex on addition of an appropriate poly d(pyrimidine) in the correct stoichiometry. (c) The hyperchromic shifts of the triplex and poly d(purine), upon melting, are mutually independent. (d) The circular dichroism spectrum of the complex is simply the weighted average of a triplex together with a free poly d(purine). The triplexes have tm's approximately 20 degrees higher than the corresponding duplexes under comparable conditions and they are extremely resistant to various deoxyribonucleases; properties which may prove useful for their isolation from natural sources.
（嘧啶）n.（嘌呤）n重复序列的DNA在pH值降至6以下时会形成一种独特的复合物。此前人们认为这种复合物是四链的。目前的研究结果与这一观点不一致，四条证据表明该复合物由一个三链体和一个具有二级结构的聚d（嘌呤）组成。化学式：（见正文）。（a）S1核酸酶消化导致pH 5诱导复合物中50%的聚d（嘌呤）含量降解。（b）浮力密度研究表明，三链体与添加的游离聚d（嘌呤）之间没有相互作用，并且由双链DNA形成的复合物中含有的聚d（嘌呤）在以正确的化学计量比添加适当的聚d（嘧啶）时可自由形成三链体。（c）三链体和聚d（嘌呤）在解链时的增色效应相互独立。（d）复合物的圆二色光谱只是三链体和游离聚d（嘌呤）的加权平均值。在可比条件下，三链体的解链温度比相应的双链体高约20摄氏度，并且它们对各种脱氧核糖核酸酶具有极强的抗性；这些特性可能有助于从天然来源中分离它们。
