Relaxation of Chromatin Structure Induced by Ethidium Binding. Involvement of the Intercalation Process

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Abstract
The effects of the binding of ethidium on the structure of [mouse Ehrlich ascites cell] chromatin, using micrococcal nuclease as a structural probe, were studied. This binding induces 2 structural changes of chromatin isolated or in the nuclei. An unfolding of the overall structure occurs which results in an activation of the rate of degradation by the nuclease. A disorganization of the core particle structure occurs which has the effect of unwrapping the DNA from the histone core, this disruption can go on so far as to leave only 90 base pairs. By comparing the bindings of ethidium and tetramethylethidium, one could conclude that the 1st type of structural change is due to an electrostatic effect and does not depend upon intercalation. The 2nd one is due to the intercalation process and to the change of topological constraints on the DNA that such a process involves.