Effect of Cycloheximide on the Nucleolar RNA Synthesis in Rat Liver

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Abstract
Although administration of cycloheximide to rats caused a rapid decrease (T1/2 75 min) in RNA polymerase I (or A) activity in the nucleolus, the activity in the nucleus or in the whole cell decreased at a much slower rate (T1/2 6 h). A concomitant increase in RNA polymerase I activity in the extranucleolar nuclear fraction indicates a rapid leakage of this enzyme from the nucleolus into the nucleoplasm upon cessation of protein synthesis. The RNA synthetic activity of isolated nucleoli as well as in vivo 45 S RNA synthesis decreased much more rapidly (T1/2 35 min); i.e. twice as fast as the RNA polymerase I activity in the nucleolus. RNA synthesis with isolated nucleoli in the presence of heparin followed precisely the same pattern, the activity always being approximately 80% of that without heparin. The results strongly suggest that isolated nucleoli essentially do not initiate transcription de novo and that cycloheximide causes a rapid decrease in the amount of template-bound enzyme which determines the nucleolar activity in vivo and in vitro. Polymerase-extracted nucleoli as well as nucleolar chromatin prepared with various washing procedures were able to use exogenously added RNA polymerase I. The template activity of the nucleolus as determined by this procedure decreased greatly after cycloheximide treatment. Since the elongation rate of preinitiated RNA chains was the same in the control and the cycloheximide-treated nucleoli, the data suggest that exogenously added enzyme cannot initiate on the drug-treated nucleolar template as efficiently as on the control nucleolar template.

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