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How transcribing rRNA genes are organized

Transcription of ribosomal RNA genes (rDNAs) by RNA polymerase I (Pol I) takes place in nucleoli. The transcribing rDNAs are thought to form `Christmas tree' structures in which 150 growing pre-rRNAs branch out from a single rDNA, but where exactly these rDNAs reside in relation to the known nucleolar subcompartments — the granular component, fibrillar centres and the dense fibrillar component (DFC) — has been intensely debated. Dominique Ploton and co-workers have now answered this question by performing ultrastructural studies of isolated, intact nucleoli and using pulse-chase experiments with BrUTP to pinpoint transcribing sites (see p. 3297). They show that BrUTP is initially incorporated into rRNA in the cortex of fibrillar centres but that the elongating rRNAs then enter the surrounding DFC. The authors also performed electron tomography analyses, which reveal that within fibrillar centres Pol I molecules are arranged in discrete clusters as 60-nm-diameter coils. Ploton and co-workers use these findings to generate a novel model for the three-dimensional organization of rDNA, in which each gene is successively folded into a series of loops, rows and cylinders that produce the observed 60 nm x 200 nm coils.

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Three-dimensional organization of active rRNA genes within the nucleolus

Thierry Cheutin, Marie-Françoise O'Donohue, Adrien Beorchia, Marc Vandelaer, Hervé Kaplan, Bruno Deféver, Dominique Ploton, and Marc Thiry
JCS 2002 115: 3297-3307. [Abstract] [Full Text]  

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JCS Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Search for Related Content Social Bookmarking                         What's this?