DEGREE OF SUPERHELICITY OF COVALENTLY CLOSED CYCLIC DNAS FROM ESCHERICHIA COLI

The degree of superhelicity of four covalently closed double-stranded cyclic DNA's from Escherichia coli has been measured by determining the sedimentation coefficients of the DNA's at 20 °C in 3 m-CsCl-0.01 m-sodium EDTA (pH 8), as a function of ethidium bromide concentration in the sedimentation medium. The number of superhelical turns is -8.5 for an E. coli 15 plasmid DNA (1.45 megadalton), -19 for φ 174 RFI (3.4 megadalton), -100 for λ b2b5c DNA (26 megadalton), -136 for λcI857 DNA (33 megadalton) from sensitive cells, and -155 for λcI857 DNA from superinfected immune cells. The superhelical densities (number of superhelical turns per ten base pairs) of these species are -0.039, -0.037, -0.026, -0.027 and -0.031, respectively. Although there is a 20-fold range in the number of superhelical turns for these DNA's, the superhelical density is constant within a factor of 1.5. For λb2b5c DNA, the degree of superhelicity of the intracellularly closed species is identical for samples isolated from cells infected in the presence or absence of 30 μg of chloramphenicol/ml. It appears that closed cyclic λb2b5c DNA isolated from cells infected at 39 °C is slightly less twisted than the same DNA isolated from cells infected at 29 °C. © 1969.