Mammalian RNA polymerase I exists as a holoenzyme with associated basal transcription factors.

Transcription initiation of ribosomal RNA genes requires RNA polymerase I (Pol I) and auxiliary factors which either bind directly to the rDNA promoter, e.g. TIF-IB/SL1 and UBF, or are assembled into productive transcription initiation complexes via interaction with Pol I, e.g. TIF-IA, and TIF-IC. Here we show that all components required for specific rDNA transcription initiation are capable of physical interaction with Pol I in the absence of DNA and can be co-immunoprecipitated with antibodies against defined subunits of murine Pol I. Sucrose gradient centrifugation and fractionation on gel filtration columns reveals that approximately 10% of cellular Pol I elutes as a defined complex with an apparent molecular mass of > 2000 kDa. The large Pol I complex contains saturating levels of TIF-IA, TIF-IB and UBF, but limiting amounts of TIF-IC. In support of the existence of a functional complex between Pol I and basal factors, the large complex is transcriptionally active after complementation with TIF-IC. The results suggest that, analogous to class II gene transcription, a pre-assembled complex, the "Pol I holoenzyme", exists that appears to be the initiation-competent form of Pol I.