Louvet E., Junéra H.R., Le Panse S. and Hernandez-Verdun D.
Institut Jacques Monod, UMR 7592 CNRS, University of Paris VI and Paris VII, Paris, France Nucleolar organization is the consequence of ribosomal gene (rDNA) transcription followed by processing of the ribosomal RNAs (rRNAs). How the coordination between transcription and processing in the nucleolus is maintained, is presently poorly understood. To address this question, we induced reversible disconnection of the nucleolar processing machinery from the rRNA transcription sites. This was achieved by treatment with 5, 6 dichloro-1--D-ribofuranosylbenzimidazole (DRB), a kinase inhibitor. During DRB treatment rDNA transcription is maintained and transcription sites are unraveled in the so-called nucleolar necklace. In the present study, we demonstrate that in addition, DRB generates distinct masses containing the late nucleolar processing machinery. This disconnection between transcription and processing is corroborated by the fact that 28S rRNA production is impaired. Removing DRB reverses this disconnection, and induces recovery of compact nucleoli and rRNA processing. Observations of nucleolar recovery in living cells indicate that recovery occurs in two successive steps, dynamic relocalization of the processing proteins around transcription sites, followed by compaction of the rDNA. We demonstrate that relocalization of the processing machinery on transcription sites is ATP-dependent and that nucleolar compaction is sensitive to temperature suggesting an energy-dependent process.
