The early effects of a low dose of actinomycin D (AMD) on the ultrastructure of ribosomal transcriptional complexes (RNP) have been studied in Chinese hamster ovary (CHO) cells. After various periods of treatment with a concentration of AMD which inhibits preferentially ribosomal RNA biosynthesis, either whole nuclei or nucleoli were isolated and spread for electron microscopic observation according to a procedure adapted from Miller's technique. Even after the shortest period of AMD action, striking modifications could be detected in the actively transcribed ribosomal matrices. After a 1-min treatment, most of the elongating RNP fibrils were still attached to the DNP strand but abnormally large "gaps" appeared in most of the ribosomal matrix units. After 2 min of action the spacing of the RNP fibrils on the matrix unit became irregular, occasionally simultaneous with a shortening of the length of the RNP-covered segment. Some bound RNA plymerase molecules were devoid of an associated RNP fibril. The visualization of tandem repeat units became exceptional. However, when they were detected, adjacent matrices displayed different patterns of alteration. On and after 5 min of treatment, the frequency of detection of ribosomal matrix units decreased considerably. The persistent ribosomal complexes corresponded generally to the presence of a few residual RNP fibrils. © 1979 Academic Press, Inc.
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