Variety of molecular conformation of plasmid pUC18 DNA and solenoidally supercoiled DNA

The plasmid pUC18 DNA isolated from Escherichia coli HB101 were analyzed by two-dimensional agarose gel electrophoresis and hybridization. The results show that the DNA sample can be separated into six groups of different structural components. The plectonemically and solenoidally supercoiled pUC18 DNA coexist in it. These two different conformations of supercoiled DNA are interchangeable with the circumstances (ionic strength and type, etc.). The amount of solenoidally supercoiled pUC18 DNA in the samples can be changed by treatment of DNA topoisome rases. Under an electron microscope, the solenoidal supercoiling DNA has a round shape with an average diameter of 45 nm. The facts suggest that solenoidaUy supercoiled DNA be a structural entity independent of histones. The polymorphism of DNA structure may be important to packing of DNA in vivo.     

质粒pUC18 DNA分子构象的多样性与线圈型超螺旋DNA

从大肠杆菌HB101细胞制备的质粒pUC18 DNA可被双向琼脂糖凝胶电泳分离成6组电泳行为不同的结构成分.超螺旋pUC18 DNA的两种结构形式:互缠型超螺旋与线圈型超螺旋构象同时存在于这种DNA样品中.在离子强度等环境条件改变时这两种DNA构象可以发生互变.DNA拓扑异构酶能改变线圈型DNA在样品中的含量.线圈型pUC18 DNA的电子显微镜表观为面包圈形,直径约45nm.实验事实表明线圈型超螺旋DNA是不依赖组蛋白而独立存在的一种结构实体.