Growth-dependent regulation of rRNA synthesis is mediated by a transcription initiation factor (TIF-IA).

Mouse RNA polymerase I requires at least two chromatographically distinct transcription factors (designated TIF-IA and TIF-IB) to initiate transcription accurately and efficiently in vitro. In this paper we describe the partial purification of TIF-IA by a four-step fractionation procedure. The amount or activity of TIF-IA fluctuates in response to the physiological state of the cells. Extracts from quiescent cells are incapable of specific transcription and do not contain detectable levels of TIF-IA. Transcriptionally inactive extracts can be restored by the addition of TIF-IA preparations that have been highly purified from exponentially growing cells. During the fractionating procedure TIF-IA co-purifies with RNA polymerase I, suggesting that it is functionally associated with the transcribing enzyme. We suggest that only those enzyme molecules that are associated with TIF-IA are capable to interact with TIF-IB and to initiate transcription.