A reevaluation of nuclear and nucleolar changes induced in vitro by actinomycin D.

Abstract

Fine structural studies that included the use of various enzyme digestion procedures and autoradiography were carried out on ME-180 tissue culture cells treated with actinomycin D. Particular emphasis was placed on the evolution of the nucleolar changes that resulted from treatment with the antibiotic. Degradation of the nucleoloemal structure was the main effect of the drug. The fibrillar portion was involved first, indicating that it may contain an actively transcribing chromatin and may possibly be the site of synthesis of the 45 S precursor rRNA. The granular portion became involved later and gave rise to two fractions that, for convenience, were designated P1 and P2 fractions. All evidence indicates that P1 granules, which have been shown to reside on fibrils, are precursors to P2 granules, which lack such fibrillar support. It is suspected that P1 granules contain 32 S RNA and P2 granules contain 28 S RNA. Apart from granules, the P2 fraction was shown to contain microspherules that were found to be composed primarily of basic proteins. These microspherules appear to be identical with those observed in cells treated with adenosine. Fibrillar centers were found to persist longest under the action of actinomycin D. Their close association with chromatin suggests a chromosomal origin. Since chromatin is the target structure of actinomycin D, it is postulated that all nucleolar alterations are due to chromatin changes induced by the antibiotic. It was shown that actinomycin D has a condensating effect on chromatin. Therefore, if a dispersed form of chromatin forms the skeleton of the nucleolonema, it would condense and thus cause the disintegration of the nucleolonema.