Ethidium bromide as a cooperative effector of a DNA structure.

A salt-induced cooperative conformational transition of a synthetic DNA, poly(dG-dC), is reversed by addition of ethidium bromide. Binding of the dye at high salt concentrations is highly cooperative. Circular dichroism spectra of the complex and the kinetic data support a model for this cooperative binding that is formally equivalent to the "allosteric" one proposed for oligomeric proteins by Monod et al. Thus, double-helical DNA of at least one defined sequence can undergo a cooperative conformational change in solution, with simple salts and drug molecules as antagonistic effectors. Such transitions may be involved in regulatory phenomena operating directly at the level of nucleic acid structure.