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Vol. 18, Issue 2, 394-403, February 2007

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Pol I Transcription and Pre-rRNA Processing Are Coordinated in a Transcription-dependent Manner in Mammalian Cells

K. Kopp^*,, J. Z. Gasiorowski^,, D. Chen^*, R. Gilmore^*, J. T. Norton^*, C. Wang^*, D. J. Leary^*, E.K.L. Chan, D. A. Dean, and S. Huang^*

*Departments of Cell and Molecular Biology and Medicine, Division of Pulmonary and Critical Care Medicine, Feinberg School of Medicine Northwestern University, Chicago, IL 60611; and Health Science Center, University of Florida, Gainesville, FL 32610

Submitted March 29, 2006; Revised November 3, 2006; Accepted November 6, 2006
Monitoring Editor: A. Gregory Matera

Pre-rRNA synthesis and processing are key steps in ribosome biogenesis. Although recent evidence in yeast suggests that these two processes are coupled, the nature of their association is unclear. In this report, we analyze the coordination between rDNA transcription and pre-rRNA processing in mammalian cells. We found that pol I transcription factor UBF interacts with pre-rRNA processing factors as analyzed by immunoprecipitations, and the association depends on active rRNA synthesis. In addition, injections of plasmids containing the human rDNA promoter and varying lengths of 18S rDNA into HeLa nuclei show that pol I transcription machinery can be recruited to rDNA promoters regardless of the product that is transcribed, whereas subgroups of pre-rRNA processing factors are recruited to plasmids only when specific pre-rRNA fragments are produced. Our observations suggest a model for sequential recruitment of pol I transcription factors and pre-rRNA processing factors to elongating pre-rRNA on an as-needed basis rather than corecruitment to sites of active transcription.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

These authors contributed equally to this work.

Address correspondence to: Sui Huang (s-huang2{at}northwestern.edu)

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