Isolated plant nuclei can be used for fundamental studies on the transcription apparatus. Total RNA polymerase activity can be measured using plant nuclei, and by using different α-amanitin concentrations in the enzyme assay, the individual RNA polymerase I, II, and in activities can be measured (1-5). The assay procedure involves the incubation of nuclei in the presence of the four substrates for RNA synthesis: ATP, GTP, CTP, and UTP. If one of these precursors is supplied as a radio-labeled molecule, transcription can be detected as incorporation of radioactivity into acid-insoluble material. Following incubation, transcription products are precipitated with TCA, collected and washed on glass fiber filter discs, and counted by liquid scintillation counting.