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 tandem ly repeated HMG homology domains which are required for DNA binding. W e have used highly purified RNA polymerase I (Pol I) and upstream bind ing 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, affini ty chromatography and protein blotting, that UBF physically associates with Pol I. Mutational analysis reveals that the HMG boxes play an im portant role in this specific interaction. UBF binds to mouse and yeas t 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 recognize d by UBF. No specific interaction was observed with Pol II. Unexpected ly, UBF was found to associate also with a unique subunit of yeast Pol III. This apparent specific interaction of UBF with the two classes o f RNA polymerases may reflect functionally important interactions of H MG box-containing transcription factors with the transcriptional appar atus.