Functional states of RNA polymerase in the macronucleus of Tetrahymena pyriformis and their dependence on culture growth.

RNA polymerase In in macronuclei of late log-phase cells of the ciliate Tetrahymena pyriformis is found to be present in 2 functional states, the one being actively engaged in transcription of ribosomal genes, the other one in a "free" state, i.e. free to transcribe added DNA or poly d(AT). Stimulation of RNA synthesis induced by dilution of stationary-phase cells into fresh medium is correlated with an increase in activity of template engaged and a decrease of free RNA polymerase I. In contrast, RNA polymerase II shows no pronounced changes in activity and is not available to transcribe poly d(AT). The data favour the assumption that factors other than the amount of RNA polymerase I molecules available control transcription of the ribosomal genes.