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Molecular and Cellular Biology, July 2004, p. 6338-6349, Vol. 24, No. 14
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.14.6338-6349.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Multiple Protein-Protein Interactions by RNA Polymerase I-Associated Factor PAF49 and Role of PAF49 in rRNA Transcription

Kazuo Yamamoto,^1,2 Mika Yamamoto,^1, Ken-ichi Hanada,^3, Yasuhisa Nogi,¹ Toshifumi Matsuyama,² and Masami Muramatsu^1,3,4*

Department of Biochemistry, Saitama Medical School, Iruma-gun, Saitama 350-0495,¹ Division of Cytokine Signaling, Department of Molecular Microbiology and Immunology, Nagasaki University Graduate School of Biomedical Sciences, Nagasaki 852-8523,² Department of Biochemistry, Faculty of Medicine, University of Tokyo, Bunkyo-ku, Tokyo 113-0033,³ Research Center for Genomic Medicine, Saitama Medical School, Hidaka, Saitama 350-1241, Japan⁴

Received 9 November 2003/ Returned for modification 16 December 2003/ Accepted 19 April 2004

We previously demonstrated the critical role of RNA polymerase I (Pol I)-associated factor PAF53 in mammalian rRNA transcription. Here, we report the isolation and characterization of another Pol I-associated factor, PAF49. Mouse PAF49 shows striking homology to the human nucleolar protein ASE-1, so that they are considered orthologues. PAF49 and PAF53 were copurified with a subpopulation of Pol I during purification from cell extracts. Physical association of PAF49 with Pol I was confirmed by a coimmunoprecipitation assay. PAF49 was shown to interact with PAF53 through its N-terminal segment. This region of PAF49 also served as the target for TAF_I48, the 48-kDa subunit of selectivity factor SL1. Concomitant with this interaction, the other components of SL1 also coimmunoprecipitated with PAF49. Specific transcription from the mouse rRNA promoter in vitro was severely impaired by anti-PAF49 antibody, which was overcome by addition of recombinant PAF49 protein. Moreover, overexpression of a deletion mutant of PAF49 significantly reduced pre-rRNA synthesis in vivo. Immunolocalization analysis revealed that PAF49 accumulated in the nucleolus of growing cells but dispersed to nucleoplasm in growth-arrested cells. These results strongly suggest that PAF49/ASE-1 plays an important role in rRNA transcription.

Present address: Department of Preventive Medicine, Institute of Tropical Medicine, Nagasaki University, Nagasaki 852-8523, Japan.

Present address: Surgery Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.

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