The cell cycle regulatory factor TAF1 stimulates ribosomal DNA transcription by binding to the activator UBF

Control of ribosome biogenesis is a potential mechanism for the regulation of cell size during growth [1, 2], and a key step in regulating ribosome production is ribosomal RNA synthesis by RNA polymerase I (Pol I) [3, 4]. In humans, Pol I transcription requires the upstream binding factor UBF and the selectivity factor SL1 to assemble coordinately on the promoter [5-7]. UBF is an HMG box-containing factor that binds to the rDNA promoter and activates Pol I transcription through its acidic carboxy-terminal tail [8, 9]. Using UBF (284-670) as bait in a yeast two-hybrid screen, we have identified an interaction between UBF and TAF1, a factor involved in the transcription of cell cycle and growth regulatory genes [10]. Coimmunoprecipitation and protein-protein interaction assays confirmed that TAF1 binds to UBF. Confocal microscopy showed that TAF1 colocalizes with UBF in Hela cells, and cell fractionation experiments provided further evidence that a portion of TAF1 is localized in the nucleolus, the organelle devoted to ribosomal DNA transcription. Cotransfection and in vitro transcription assays showed that TAF1 stimulates Pol I transcription in a dosage-dependent manner. Thus, TAF1 may be involved in the coordinate expression of Pol I and Pol II-transcribed genes required for protein biosynthesis and cell cycle progression.