The mitotically phosphorylated form of the transcription termination factor TTF-1 is associated with the repressed rDNA transcription machinery.
The transcription termination factor TTF-1 exerts two functions in ribosomal gene (rDNA) transcription: facilitating initiation and mediating termination of transcription. Using HeLa cells, we show that TTF-1 protein is colocalized with the active transcription machinery in the nucleolus and also with the inactive machinery present in certain mitotic nucleolar organizer regions (NORs) when rDNA transcription is repressed. We also show that TTF-1 is specifically phosphorylated during mitosis in a manner dependent on the cdc2-cyclin B kinase pathway and on an okadaic acid-sensitive phosphatase. Interestingly, the mitotically phosphorylated form of TTF-1 appearing at the G(2)/M transition phase was more easily solubilized than was the interphase form. This indicates that the chromatin-binding affinity of TTF-1 appears to be different in mitotic chromosomes compared to the interphase nucleolus. Correlated with this, the other DNA-binding factor, UBF, which interferes with chromatin conformation in the rDNA promoter, was more strongly bound to rDNA during mitosis than at interphase. The reorganization of the mitotic rDNA promoter might be induced by phosphorylation of certain components of the rDNA transcription machinery and participate in silencing of rDNA during mitosis.
Sirri
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Institut Jacques Monod, UMR 7592, Paris, France.
Roussel
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Hernandez-Verdun
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D000918:Antibody Specificity; D001323:Autoantibodies / Q000276:immunology / Q000494:pharmacology; D017489:CDC28 Protein Kinase, S cerevisiae / Q000037:antagonists & inhibitors / Q000378:metabolism; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology; D002875:Chromosomes / Q000235:genetics; D004275:DNA, Ribosomal / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000032:analysis / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology; D006367:HeLa Cells; D006801:Humans; D007399:Interphase / Q000502:physiology; D008938:Mitosis / Q000502:physiology*; D009687:Nuclear Proteins / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D009697:Nucleolus Organizer Region / Q000737:chemistry / Q000201:enzymology; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins; D011687:Purines / Q000494:pharmacology; D012318:RNA Polymerase I / Q000032:analysis; D000077546:Roscovitine; D000074482:Thyroid Nuclear Factor 1; D014157:Transcription Factors / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D015533:Transcriptional Activation / Q000502:physiology*
D001323:Autoantibodies; D004275:DNA, Ribosomal; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; C000614412:NKX2-1 protein, human; D009687:Nuclear Proteins; D035463:Pol1 Transcription Initiation Complex Proteins; D011687:Purines; D000074482:Thyroid Nuclear Factor 1; D014157:Transcription Factors; C063844:transcription factor UBF; D000077546:Roscovitine; D017489:CDC28 Protein Kinase, S cerevisiae; D012318:RNA Polymerase I