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A more recent version of this article appeared on April 1, 2007
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Submitted on September 22, 2006
Revised on January 3, 2007
Accepted on January 22, 2007
*Department of Biotechnology and Biosciences, University of Milano-Bicocca, I-20126 Milano, Italy; Department of Animal Biology, Laboratory of Cell Biology and Neurobiology, and Institute of Molecular Genetics, Consiglio Nazionale delle Ricerche, University of Pavia, 27100 Pavia, Italy
Monitoring Editor: Karsten Weis
Mammalian cleavage factor I (CF Im) is an essential factor that is required for the first step in pre mRNA 3' end processing. Here, we characterize CF Im68 subnuclear distribution and mobility. Fluorescence microscopy reveals that in addition to paraspeckles CF Im68 accumulates in structures that partially overlap with nuclear speckles. Analysis of synchronized cells shows that CF Im68 distribution in speckles and paraspeckles varies during the cell cycle. At an ultrastructural level, CF Im68 is associated with perichromatin fibrils, the sites of active transcription, and concentrates in interchromatin granules-associated zones. We show that CFIm68 colocalizes with BrU, RNA polymerase II, and the splicing factor SC35. On inhibition of transcription endogenous CF Im68 no longer associates with perichromatin fibrils but can still be detected in interchromatin granules-associated zones. These observations support the idea that not only splicing but also 3' end processing occurs cotranscriptionally. Finally, fluorescence recovery after photobleaching (FRAP) analysis reveals that the CF Im68 fraction associated with paraspeckles moves at a rate similar to the more dispersed molecules in the nucleoplasm demonstrating the dynamic nature of this compartment. These findings suggest that paraspeckles are a functional compartment involved in RNA metabolism in the cell nucleus.
Present address: Institute of Cell and Molecular Biology, University of Edinburgh, King’s Buildings, Edinburgh EH9 3JR, United Kingdom.
Address correspondence to:
Silvia M.L. Barabino (silvia.barabino{at}unimib.it)
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