TI  - Activity of RNA polymerase I transcription factor <prot>UBF</prot> blocked by <prot>Rb</prot> gene
      product.
PG  - 177-80
AB  - The protein encoded by the <prot>retinoblastoma susceptibility gene</prot> (<prot>Rb</prot>)
      functions as a tumour suppressor and negative growth regulator. As
      actively growing cells require the ongoing synthesis of ribosomal RNA, we
      considered that <prot>Rb</prot> might interact with the ribosomal DNA transcription
      apparatus. Here we report that (1) there is an accumulation of <prot>Rb</prot> protein
      in the nucleoli of differentiated U937 cells which correlates with
      inhibition of rDNA transcription; (2) addition of <prot>Rb</prot> to an in vitro
      transcription system inhibits transcription by RNA polymerase I; (3) this
      inhibition requires a functional <prot>Rb</prot> pocket; and (4)  <prot>Rb</prot>  specifically
      inhibits the activity of the RNA polymerase I transcription factor  <prot>UBF</prot>
      (<prot>upstream binding factor</prot>)  in vitro. This last observation was confirmed by
      affinity chromatography and immunoprecipitation, which demonstrated an
      interaction between  <prot>Rb</prot>  and  <prot>UBF</prot> . These results indicate that there is an
      additional mechanism by which <prot>Rb</prot> suppresses cell growth, namely that <prot>Rb</prot>
      directly represses transcription of the rRNA genes.
AD  - Sigfried and Janet Weis Center for Research, Geisinger Clinic, Danville,
