REDISTRIBUTION OF RIBOSOMAL DNA AFTER BLOCKING OF TRANSCRIPTION INDUCED BY ACTINOMYCIN-D

We report on the effect of different doses and times of incubation of the cytostatic drug actinomycin D (AMD) on nucleolar morphology, rRNA gene transcription and rDNA gene localization using in situ hybridizat ion and the immunocytochemical detection of the human upstream binding factor (UBF) at the electron microscopic level in HeLa cells. Low dos es of AMD (0.001 mu g/ml, 30 min) selectively block rRNA gene transcri ption but alter neither nucleolar morphology nor the localization of r DNA with respect to the nucleolar components. Treatment with high dose s of AMD (0.05 mu g/ml, 1 h) resulted in a retraction of the rDNA out of the nucleolus in addition to the well-known blocking of rDNA transc ription, total nuclear transcription and nucleolar segregation. Under these conditions accumulations of rDNA were found in patches of chroma tin at the nucleolar periphery. We conclude that the blocking of rRNA gene transcription and the changes in nucleolar morphology, both induc ed by AMD at different doses, are independent phenomena.