UBF-binding site arrays form pseudo-NORs and sequester the RNA polymerase I transcription machinery

Christine Mais, Jane E. Wright, Jose-Luis Prieto, Samantha L. Raggett, Brian McStay

    Research output: Contribution to journalArticlepeer-review

    153 Citations (Scopus)


    Human ribosomal genes (rDNA) are located in nucleolar organizer regions (NORs) on the short arms of acrocentric chromosomes. Metaphase NORs that were transcriptionally active in the previous cell cycle appear as prominent chromosomal features termed secondary constrictions that are achromatic in chromosome banding and positive in silver staining. The architectural RNA polymerase I (pol I) transcription factor UBF binds extensively across rDNA throughout the cell cycle. To determine if UBF binding underpins NOR structure, we integrated large arrays of heterologous UBF-binding sequences at ectopic sites on human chromosomes. These arrays efficiently recruit UBF even to sites outside the nucleolus and, during metaphase, form novel silver stainable secondary constrictions, termed pseudo-NORs, morphologically similar to NORs. We demonstrate for the first time that in addition to UBF the other components of the pol I machinery are found associated with sequences across the entire human rDNA repeat. Remarkably, a significant fraction of these same pol I factors are sequestered by pseudo-NORs independent of both transcription and nucleoli. Because of the heterologous nature of the sequence employed, we infer that sequestration is mediated primarily by protein–protein interactions with UBF. These results suggest that extensive binding of UBF is responsible for formation and maintenance of the secondary constriction at active NORs. Furthermore, we propose that UBF mediates recruitment of the pol I machinery to nucleoli independently of promoter elements.
    Original languageEnglish
    Pages (from-to)50-64
    Number of pages15
    JournalGenes & Development
    Issue number1
    Publication statusPublished - Jan 2005


    • RNA polymerase I
    • Ribosomal DNA
    • Nucleolar organiser regions
    • Nucleolus
    • Upstream binding factor


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