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A more recent version of this article appeared on February 1, 2007
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Submitted on March 29, 2006
Revised on November 3, 2006
Accepted on November 6, 2006

Departments of *Cell and Molecular Biology and
Medicine, Division of Pulmonary and Critical Care Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL 60611;
University of Florida, Health Science Center, Gainesville, FL 32610
Monitoring Editor: A. Gregory Matera
Pre-rRNA synthesis and processing are key steps in ribosome biogenesis. While 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 upon 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, while 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.
These authors contributed equally to this work.
Address correspondence to:
S. Huang (s-huang2{at}northwestern.edu)
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