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Molecular and Cellular Biology, August 2000, p. 5930-5938, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Repression of RNA Polymerase I Transcription by the Tumor Suppressor p53

Weiguo Zhai and Lucio Comai*

Department of Molecular Microbiology and Immunology, Keck School of Medicine, University of Southern California, Los Angeles, California 90033

Received 11 January 2000/Returned for modification 28 February 2000/Accepted 11 May 2000

The tumor suppressor protein p53 is frequently inactivated in tumors. It functions as a transcriptional activator as well as a repressor for a number of viral and cellular promoters transcribed by RNA polymerase II (Pol II) and 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 cotransfection 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 transcriptional activity compared to that of epithelial cells that express p53. However, both cell lines display comparable Pol I factor protein levels. Our biochemical analysis shows that p53 prevents the interaction between SL1 and UBF. Protein-protein interaction assays indicate that p53 binds to SL1, and this interaction is mostly mediated by direct contacts with TATA-binding protein and TAFI110. Moreover, template commitment assays show that while the formation of a UBF-SL1 complex can partially relieve the inhibition of transcription, only the assembly of a UBF-SL1-Pol I initiation complex on the rDNA promoter confers substantial protection against p53 inhibition. In summary, our results suggest that p53 represses RNA Pol I transcription by directly interfering with the assembly of a productive transcriptional machinery on the rRNA promoter.

* Corresponding author. Mailing address: Department of Molecular Microbiology and Immunology, University of Southern California, Keck School of Medicine, HMR 509, 2011 Zonal Ave., Los Angeles, CA 90033. Phone: (323) 442-3950. Fax: (323) 442-1721. E-mail: comai{at}hsc.usc.edu.

Molecular and Cellular Biology, August 2000, p. 5930-5938, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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