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NORs and their transcription competence during the cell cycle
E Smirnov1, M Kalmárová, K Koberna
1Institute of Cellular Biology and Pathology, 1st Faculty of Medicine, Charles University, Prague, Czech Republic. esmir@lfl.cuni.cz
Folia Biologica
|November 9, 2006
Summary
Competent nucleolar organizing regions (NORs) in human cells, identified by UBF protein, are transcriptionally active during interphase. This activity was confirmed by visualizing individual NORs using premature chromosome condensation, revealing conserved competence patterns across cell cycles.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- Ribosomal genes in human cells are organized in nucleolar organizing regions (NORs) on acrocentric chromosomes.
- Certain NORs, termed 'competent' NORs, contain RNA polymerase I transcription machinery components like UBF during mitosis.
- The interphase transcriptional status of individual NORs has remained unclear due to visualization challenges.
Purpose of the Study:
- To determine the interphase transcriptional activity of competent NORs.
- To investigate the stability of NOR competence across cell cycles.
- To visualize and characterize individual NORs in interphase cells.
Main Methods:
- UBF immunofluorescence and silver staining to detect competent NORs.
- Fluorescence in situ hybridization (FISH) for rDNA and chromosome visualization.
- Roscovitine stimulation and bromo-uridine incorporation assays.
- Premature chromosome condensation (PCC) induced by calyculin A.
Main Results:
- Competent NORs identified by UBF showed conserved patterns of competence across cell cycles.
- UBF-loaded NORs were the only ones incorporating bromo-uridine, indicating active transcription.
- PCC allowed visualization of individual NORs in interphase, confirming mitotic competence patterns.
- Competent NORs are transcriptionally active during interphase.
Conclusions:
- Competent NORs are consistently transcriptionally active during interphase.
- NOR competence is a stable characteristic, conserved across cell cycles.
- The study provides direct evidence for interphase ribosomal gene transcription at specific NORs.
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