ACTINOMYCIN BINDING CAPACITY OF DEOXYRIBONUCLEOPROTEIN

Actinomycin D is known to block DNA-dependent RNA synthesis by binding to DNA. The nature and frequency of the actinomycin binding sites in deoxyribonucleoprotein and DNA was studied with the aid of optical titrations. Deoxyribonucleoprotein bound only one molecule of actinomycin per 35-140 nucleotides, whereas DNA preparations bound one actinomycin molecule per 14-20 nucleotides. The actinomycin binding sites available in deoxyribonucleoprotein show properties similar to those in DNA with respect to sensitivity to thermal denaturation, salt and urea treatment. Proteins therefore appear to block potential binding sites in deoxyribonucleoprotein. Ca2+, Mg2+ and Mn2+ ions were found to have a marked inhibitory effect on the in vitro binding of actinomycin to DNA. This suggests that in the chromatin of the intact cell nucleus, actinomycin binding to nucleoprotein can also be greatly modified by the presence of divalent metal ions. Physiological variations in ionbinding to deoxyribonucleoprotein or in the physicochemical state of the deoxyribonucleoprotein in vivo are likely to be eliminated by the procedure used for the isolation of deoxyribonucleoprotein. © 1969.