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 topoisomerases. Under an electron microscope, the solenoidal supercoiling DNA has a round shape with an average diameter of 45 nm. The facts suggest that solenoidally supercoiled DNA be a structural entity independent of histones. The polymorphism of DNA structure may be important to packing of DNA in vivo.