The HMG box-containing nucleolar transcription factor UBF interacts with a specific subunit of RNA polymerase I

The mammalian transcription activator protein UBF contains five tandemly repeated HMG homology domains which are required for DNA binding. We have used highly purified RNA polymerase I (Pol I) and upstream binding factor (UBF) and investigated whether these two proteins interact in solution. We show by a variety of different experimental approaches, such as immunoprecipitation, glycerol gradient sedimentation, affinity chromatography and protein blotting, that UBF physically associates with Pol I. Mutational analysis reveals that the HMG boxes play an important role in this specific interaction. UBF binds to mouse and yeast Pol I, demonstrating that the interaction of UBF with Pol I has been conserved during evolution. Interestingly, in both species one Pol I-specific subunit (34.5 kDa in yeast and 62 kDa in mouse) was recognized by UBF. No specific interaction was observed with Pol II. Unexpectedly, UBF was found to associate also with a unique subunit of yeast Pol III. This apparent specific interaction of UBF with the two classes of RNA polymerases may reflect functionally important interactions of HMG box-containing transcription factors with the transcriptional apparatus.