A method is described for the accurate determination of the superhelical density (σ) of highly underwound circular DNA molecules. Using this method, duplex DNA bound by RecA protein in the presence of ATP at pH 7.5 is found to be underwound by 39.6% (σ = -0.396), corresponding to a helical periodicity of 17.4 base-pairs per turn. The underwinding is increased to 41% (17.9 base-pairs per turn) in the presence of low levels of ATPγS, in good agreement with the 18.6 base-pairs per turn reported previously. In spite of the extensive underwinding, the distribution of DNA topoisomers produced by RecA protein binding is small. This indicates a high degree of structural uniformity among RecA-double-stranded DNA complexes in the presence of ATP.
All Science Journal Classification (ASJC) codes
- Structural Biology
- Molecular Biology