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J. Biol. Chem., Vol. 280, Issue 33, 29551-29558, August 19, 2005

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TBP-TAF Complex SL1 Directs RNA Polymerase I Pre-initiation Complex Formation and Stabilizes Upstream Binding Factor at the rDNA Promoter^*

J. Karsten Friedrich, Kostya I. Panov, Pavel Cabart, Jackie Russell, and Joost C. B. M. Zomerdijk¶

From the Division of Gene Regulation and Expression, School of Life Sciences, Wellcome Trust Biocentre, University of Dundee, Dundee DD1 5EH, Scotland, United Kingdom

Knowledge of the role of components of the RNA polymerase I transcription machinery is paramount to understanding regulation of rDNA expression. We describe key findings for the roles of essential transcription factor SL1 and activator upstream binding factor (UBF). We demonstrate that human SL1 can direct accurate Pol I transcription in the absence of UBF and can interact with the rDNA promoter independently and stably, consistent with studies of rodent SL1 but contrary to previous reports of human SL1. UBF itself does not bind stably to rDNA but rapidly associates and dissociates. We show that SL1 significantly reduces the rate of dissociation of UBF from the rDNA promoter. Our findings challenge the idea that UBF activates transcription through recruitment of SL1 at the rDNA promoter and suggest that the rate of pre-initiation complex (PIC) formation is primarily determined by the rate of association of SL1, rather than UBF, with the promoter. Therefore, we propose that SL1 directs PIC formation, functioning in core promoter binding, RNA polymerase I recruitment, and UBF stabilization and that SL1-promoter complex formation is a necessary prerequisite to the assembly of functional and stable PICs that include the UBF activator in mammalian cells.

Received for publication, February 10, 2005 , and in revised form, June 20, 2005.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains one supplemental figure.

Recipient of a Medical Research Council Ph.D. studentship. Present address: Cancer Research UK Molecular Oncology Unit, Barts and The London School of Medicine and Dentistry, John Vane Science Centre, Charterhouse Square, London EC1M 6BQ, UK.

Present address: Dept. of Pathology, Albert Einstein College of Medicine, Jack and Pearl Resnick Campus, 1300 Morris Park Ave., Bronx, NY 10461.

^¶ A Wellcome Trust Senior Research Fellow in the Basic Biomedical Sciences. To whom correspondence should be addressed. Tel.: 44-1382-344242; Fax: 44-1382-348072; E-mail: j.zomerdijk{at}dundee.ac.uk.

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