Activity of RNA polymerase I transcription factor UBF blocked by Rb gene product

Abstract

THEprotein encoded by the retinoblastoma susceptibility gene (Rb) functions as a tumour suppressor and negative growth regulator¹. As actively growing cells require the ongoing synthesis of riboso-mal RNA, we considered that Rb might interact with the ribosomal DNA transcription apparatus. Here we report that (1) there is an accumulation of Rb protein in the nucleoli of differentiated U937 cells which correlates with inhibition of rDNA transcription; (2) addition of Rb to an in vitro transcription system inhibits transcription by RNA polymerase I; (3) this inhibition requires a functional Rb pocket²; and (4) Rb specifically inhibits the activity of the RNA polymerase I transcription factor UBF (upstream binding factor) in vitro. This last observation was confirmed by affinity chromato-graphy and immunoprecipitation, which demonstrated an interaction between Rb and UBF. These results indicate that there is an additional mechanism by which Rb suppresses cell growth, namely that Rb directly represses transcription of the rRNA genes.