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Recruitment of factors linking transcription and processing of pre-rRNA to NOR chromatin is UBF-dependent and occurs independent of transcription in human cells

Biomedical Research Centre, Ninewells Hospital and Medical School, University of Dundee, Dundee, DD1 9SY Scotland, United Kingdom

Efficient ribosome biogenesis requires coordination of a highly complex series of events. Early events include pre-RNA transcription, processing, and modification. Analysis in yeast has demonstrated that t-UTPs, components of the U3 snoRNA-containing pre-rRNA processing complex, are required for efficient transcription of ribosomal genes (rDNA) by RNA polymerase I (pol I). Here, we characterize human t-UTPs and establish that their ability to link transcription and pre-rRNA processing is evolutionarily conserved. The pol I transcription factor UBF binds extensively across rDNA throughout the cell cycle, resulting in a specialized form of chromatin that is the hallmark of active nucleolar organizer regions (NORs). Transcriptionally silent pseudo-NORs are ectopic, chromosomally integrated, artificial arrays that mimic this specialized chromatin structure. Pseudo-NORs sequester t-UTPs and factors linking transcription with pre-rRNA modification (Nopp140 and Treacle). Recruitment is independent of transcription, the underlying DNA sequence, and location within the nucleolus. Previously, we have demonstrated that pseudo-NORs sequester every component of the pol I transcription machinery. Taken together, these results highlight the importance of the specialized chromatin structure at active NORs in coordinating early events in ribosome biogenesis and nucleolar formation.

[Keywords: Nucleolus; rDNA transcription; pre-rRNA processing; nucleolar organizer region (NOR); t-UTPs; UBF]

Received April 10, 2007; revised version accepted July 6, 2007.

E-MAIL brian.mcstay{at}cancer.org.uk; FAX 01382 669993.

Supplemental material is available at http://www.genesdev.org.

Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.436707

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