Repression of RNA polymerase I transcription by the tumor suppressor p53

The tumor suppressor protein p53 is frequently inactivated in tumors. It fu nctions as a transcriptional activator as well as a repressor for a number of viral and cellular promoters transcribed by RNA polymerase Il (Pol II) a nd by RNA Pol III, Moreover, it appears that p53 also suppresses RNA Pol I transcription. In this study, we examined the molecular mechanism of Pol I transcriptional inhibition by p53. We show that wild-type, but not mutant, p53 can repress Pol I transcription from a human rRNA gene promoter in cotr ansfection assays, Furthermore, we show that recombinant p53 inhibits rRNA transcription in a cell-free transcription system. In agreement with these results, p53-null epithelial cells display an increased Pol I transcription al activity compared to that of epithelial cells that express p.53. However , both cell lines display comparable Pol I factor protein levels. Our bioch emical analysis shows that p53 prevents the interaction between SL1 and UBF . Protein-protein interaction assays indicate that p53 binds to SL1, and th is interaction is mostly mediated by direct contacts with TATA-binding prot ein and TAF(1)110. Moreover, template commitment assays show that while the formation of a UBF-SL1 complex can partially relieve the inhibition of tra nscription, only the assembly of a UBF-SL1-Pol I initiation complex on the rDNA promoter confers substantial protection against p53 inhibition, In sum mary, our results suggest that p53 represses RNA Pol I transcription by dir ectly interfering with the assembly of a productive transcriptional machine ry on the rRNA promoter.