Influencing the B-Z switch in supercoiled DNA

The effect of Z-binding ligands on the supercoiling threshold in the supercoil-induced B-Z transition has been examined from the point of view of a two-state model. Expressions have been derived for the determination of the shift in critical supercoil density in terms of the physical parameters of the DNA-ligand system. Representative calculations indicate that the stabilizing action of Z-binding ligands on the Z conformation in closed circular DNA depends largely on the binding characteristics of the ligand. Application of the theoretical data has been demonstrated using the experimental results reported by Lafer et al. (J. Biol. Chem. 261 (1986) 6438).