Effects of actinomycin D on base composition and nearest neighbor frequency of nucleolar RNA of the Walker tumor and liver.

The base compositions of nucleolar and extranucleolar nuclear IlNA's of the tumor and the liver were determined in controls and rats treated with actinomycin D in a dose of 150 Mg/kg of body weight. A decrease in content of cytosine and an increase in A + U/G + C ratio in the nucleolar RNA of the liver were found, but in the tumor the effects were not marked. Using ATP-U-32P2 or GTP-a-32P, the nearest neighbor frequencies of nucleotides to AMP or GMP moieties of nucleolar RNA from liver and Walker tumor of the control and actinomycin D-treated rats were determined and found to be different in the liver and the Walker tumor. The changes in nearest neighbor frequency are consistent with the suppression of synthesis of high molecular weight, GC-rich RNA. As shown by autoradiographic studies, the greater suppressive effects of actinomycin D on nucleolar RNA synthesis than on extranucleolar nuclear RNA synthesis are apparently related to the localization of actinomycin D in the extranucleolar chromatin in the form of a ring containing the nucleoli.