A buoyant method for the determination of the superhelix density of closed circular DNA

A convenient method for the determination of the superhelix density of a closed circular DNA has been developed and tested with a series of closed DNA's with different superhelix densities and molecular weights. The method is based on the superhelix density-dependent binding affinity of closed DNA for the intercalating dye, ethidium bromide. The superhelix density is calculated from the observed separation between nicked and closed DNA in a cesium chlorideethidium bromide density gradient centrifuged under specified conditions in the preparative ultracentrifuge. The linear relation between the separations and the superhelix density obtained in this study is in good agreement with that predicted by Bauer & Vinograd (1970b).