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 hybridization and the immunocytochemical detection of the human upstream binding factor (UBF) at the electron microscopic level in HeLa cells. Low doses of AMD (0.001 micrograms/ml, 30 min) selectively block rRNA gene transcription but alter neither nucleolar morphology nor the localization of rDNA with respect to the nucleolar components. Treatment with high doses of AMD (0.05 micrograms/ml, 1 h) resulted in a retraction of the rDNA out of the nucleolus in addition to the well-known blocking of rDNA transcription, total nuclear transcription and nucleolar segregation. Under these conditions accumulations of rDNA were found in patches of chromatin at the nucleolar periphery. We conclude that the blocking of rRNA gene transcription and the changes in nucleolar morphology, both induced by AMD at different doses, are independent phenomena.