Initiation of nucleolar assembly is independent of RNA polymerase I transcription.
This report examines the distribution of an RNA polymerase I transcription factor (upstream binding factor; UBF), pre-rRNA processing factors (nucleolin and fibrillarin), and pre-rRNAs throughout mitosis and postmitotic nucleologenesis in HeLa cells. The results demonstrate that nucleolin, fibrillarin, and pre-rRNAs synthesized at G2/M phase of the previous cell cycle are directly recruited to UBF-associated nucleolar organizer regions (NORs) early in telophase before chromosome decondensation. Unlike the fusion of prenucleolar bodies to the nucleoli, this early recruitment of processing factors and pre-rRNAs is independent of RNA polymerase I transcription. In the absence of polymerase I transcription, the initial localization of nucleolin, fibrillarin, and pre-rRNAs to UBF-associated NORs generates segregated mininucleoli that are similar to the larger ones observed in interphase cells grown under the same conditions. Pre-rRNAs are juxtaposed to UBF-nucleolin-fibrillarin caps that may represent the segregated nucleoli observed by electron microscopy. These findings lead to a revised model of nucleologenesis. We propose that nucleolar formation at the end of mitosis results from direct recruitment of processing factors and pre-rRNAs to UBF-associated NORs before or at the onset of rDNA transcription. This is followed by fusion of prepackaged prenucleolar bodies into the nucleolus. Pre-ribosomal ribonucleoproteins synthesized in the previous cell cycle may contribute to postmitotic nucleologenesis.
Dousset
T
T
Institut Jacques Monod, Paris, France.
Wang
C
C
Verheggen
C
C
Chen
D
D
Hernandez-Verdun
D
D
Huang
S
S
D002453:Cell Cycle / Q000187:drug effects; D002466:Cell Nucleolus / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D002868:Chromosomal Proteins, Non-Histone / Q000187:drug effects / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000187:drug effects / Q000378:metabolism; D003609:Dactinomycin / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D009697:Nucleolus Organizer Region / Q000187:drug effects / Q000378:metabolism*; D010750:Phosphoproteins / Q000187:drug effects / Q000378:metabolism; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I / Q000187:drug effects / Q000378:metabolism*; D012322:RNA Precursors / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000187:drug effects / Q000378:metabolism; D012261:Ribonucleoproteins / Q000187:drug effects / Q000378:metabolism; D014157:Transcription Factors / Q000187:drug effects / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology*; D000097796:Nucleolin
D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; D003609:Dactinomycin; D019384:Nucleic Acid Synthesis Inhibitors; D010750:Phosphoproteins; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I; D012322:RNA Precursors; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; D014157:Transcription Factors; D000097796:Nucleolin; C047458:fibrillarin; C063844:transcription factor UBF