Nucleolar ultrastructure and transcription of preribosomal RNA in Rous Sarcoma Virus-Transformed Chick Embryo Fibroblasts Treated With α-Amanitin

Uninfected and Rous sarcoma virus (RSV)-transformed chick embryo fibroblasts (CEF) were exposed to various concentrations of a-amanitin for different lengths of time. At a concentration of 4 μg α-amanitin/ml. RSV-transformed cells were shown to maintain a normal rate of transcription of all classes of RNA. whereas in uninfected cells transcription was reduced to a very low level. These observations cannot be accounted for by a difference in the penetration rate of a-amanitin through the plasma membrane. Investigation of the ultrastructure showed that the degree of nucleolar fragmentation induced by α-amanitin was comparable in both types of cells. Persistence of transcription with concomitant nucleolar fragmentation in α-amanitin-treated RSV-transformed CEF is not in accord with the hypothesis that nucleolar integrity is required for preribosomal RNA transcription.