Complex formation between ethidium bromide and nucleic acids

The interaction between ethidium bromide and nucleic acids shows a pronounced metachromatic effect which has been used to obtain quantitative data on the process of complex formation. Ethidium binds strongly to both DNA and RNA at sites which appear to be saturated when one drug molecule is bound for every 4 or 5 nucleotides. After the primary sites have been filled, a secondary binding process can occur, leading to the precipitation of a complex containing one drug molecule bound per nucleotide. The strong primary binding to DNA is not influenced by the base-composition or by denaturation of the DNA, but is sensitive to changes in the salt concentration, particularly when magnesium ions are present. Addition of magnesium chloride causes a marked reduction in the strength of the interaction without significantly affecting the number of sites available to bind the drug. The process of complex formation is shown to be reversible in solution by demonstrating an exchange reaction between free and bound ethidium. Complexes between ethidium and DNA can be dissociated by using a cation-exchange resin. The binding of ethidium to DNA follows a similar pattern to that found with proflavine, suggesting that the forces involved in the binding of the two drugs may be similar.