Wasserman S A, White J H, Cozzarelli N R
Department of Molecular Biology, University of California, Berkeley 94720.
Nature. 1988 Aug 4;334(6181):448-50. doi: 10.1038/334448a0.
DNA in the cell is intertwined at several levels: one polynucleotide strand wraps helically around its complement and the double helix is in turn coiled in space. The higher-order intertwining most often takes the form of supercoiling of the helix axis, but can also be observed as the wrapping of one DNA duplex around another, as in catenation. We have investigated the relationship between intertwining at these three levels, the double helix, supercoiling, and catenation, using an approach that relies on comparative measurements of DNA linking numbers by gel electrophoresis. The method determines both the handedness of DNA catenanes and the change in helical repeat that accompanies catenation-induced supercoiling. For multiply-linked catenated rings of 3.5 kilobase pairs (kb), we conclude that the double helix unwinds by two-thirds of a turn for every right-handed supercoil involved in linking the two circles. Altering the geometry of the catenanes by linking rings of dissimilar size changes the effect of catenation on helical and superhelical parameters. Our experiments used intact DNA rings, but we note that linear DNA molecules, by virtue of their subdivision into closed loops or domains in vivo, can intertwine in the same ways.
细胞中的DNA在多个层面上相互缠绕：一条多核苷酸链螺旋状地缠绕在其互补链上，而双螺旋又在空间中进一步盘绕。更高层次的缠绕通常表现为螺旋轴的超螺旋形式，但也可以观察到一条DNA双链围绕另一条双链缠绕的情况，如连环结构。我们使用一种依赖凝胶电泳对DNA连接数进行比较测量的方法，研究了双螺旋、超螺旋和连环结构这三个层面上的缠绕之间的关系。该方法既能确定DNA连环体的手性，又能确定伴随连环化诱导的超螺旋而发生的螺旋重复变化。对于3.5千碱基对（kb）的多重连接连环环，我们得出结论，每一个参与连接两个环的右手超螺旋会使双螺旋解旋三分之二圈。通过连接不同大小的环来改变连环体的几何形状，会改变连环化对螺旋和超螺旋参数的影响。我们的实验使用的是完整的DNA环，但我们注意到，线性DNA分子由于在体内被细分为闭环或结构域，也能以同样的方式相互缠绕。
