Nature of the So-called Fibrillar Component in the Segregated Nucleolus after Treatment of Actinomycin D

Abstract

Nucleolar segregation which appears after treatment with actinomycin D has been explained as that of the granular from the fibrillar RNP component. In the present study, the nucleolus was segregated into two distinct components, namely the granular and the so-called fibrillar components. Based on the following three findings, the so-called fibrillar component was suggested to contain mainly proteinous material, but not RNP fibrils. Morphologically, the fibrillar component was composed of condensed masses of fine fibrils but the so-called fibrillar component was distributed homogenously. Gradual disappearance of the condensed fibrillar component was observed at the early stiges of the effect of actinomycin D. The fibrillar component contains mainly 45S RNA, which is the precursor of ribosomal RNA. Since actinomycin D inhibits synthesis of this RNA and the treated nucleoli contained little 45S RNA, the fibrillar component should diminish and ultimately disappear in the nucleolus. By enzymatic digestion studies, the so-called fibrillar component was removed by pepsin, but not by RNase or DNase.