超螺旋DNA的碱变构研究

对超螺旋ＤＮＡ（ＤＮＡⅠ）的碱处理产物进行了琼脂糖凝胶电泳，氯化铯－溴化乙锭密度梯度超离心分析，紫外吸收光谱分析和电镜观察。实验结果表明超螺旋ＤＮＡ在碱性环境中的结构改变发生在很窄的ｐＨ范围内（ｐＨ１２．８８─１３．００）．超过ｐＨ临界点的超螺旋ＤＮＡ碱变构产物紫外吸收高于同浓度天然ＤＮＡ紫外吸收的２９％。变构产物在ＣｓＣｌ－ＥＢ密度梯度超离心中的高密度区形成稳定的区带．用透射电镜的观察表明碱变构的超螺旋ｐＢＲ３２２ＤＮＡ具有高电子密度并呈中空颗粒状，以上事实表明，ＤＮＡ在高ｐＨ下可产生一种结构有序的相对稳定的产物．这些结果意味着在碱处理过程中，超螺旋ＤＮＡ在构象上发生了改变，使其分子由扭曲线形变成球形颗粒状。根据实验事实本文对超螺旋ＤＮＡ的碱变构产物（ＤＮＡⅣ）提出一个新的结构模型──压缩模型。这个模型能更合理地解释一些实验现象。

Studies on Alkaline Transconformation of Supercoiled pBR322 DNA

The alkaline transcontormation of supercoiled pBR322 DNA has been studied in detail.The alkaline treated product of pBR322 DNA had been analyzed by agarose gel electrophoresis.It was found that the transconformation of supercoiled PBR322 DNA in the alkaline titration happened in a quite narrow PH range (12. 88─13. 00). The alkaline transconformational DNA (DNA IV) could form a stable band at the high density region in CsCl-EB equillibrium density gradient centrifugation. The U V spectral analysis showed that the alkaline transconformation supercoiled DNA had 29% increasing absorborbance than natural supercoiled DNA at 260 nm, The alkaline treated supercoiled PBR322 DNA (DNA IV) appeared as hollow globoid with several dense joint under electron microscopic visualization. The particles of pBR322 DNA IV were of about 20 nm diameter in EM, which was one of the twentieth to the relaxed circular pBR322DNA theoretically. The facts above demonstrate that supercoiled DNA can change the conformation to relatively stable orderly structure but not randon coil in high PH solutions , and that course should be "Alkaline transconformation". According to our results, we discover a new structural model of the DNA IV──condensation model. It would be able to explain some of the experimental phenomena more reasonably.