Although the U3 small nucleolar RNA (snoRNA), a member of the box C/D class of snoRNAs, was identified with the spliceosomal small nuclear RNAs (snRNAs) over 30 years ago1,2, its function and its associated protein components have remained more elusive. The U3 snoRNA is ubiquitous in eukaryotes and is required for nucleolar processing of pre-18S ribosomal RNA in all organisms where it has been tested3,4. Biochemical and genetic analyses suggest that U3–pre-rRNA base-pairing interactions mediate endonucleolytic pre-rRNA cleavages3. Here we have purified a large ribonucleoprotein (RNP) complex from Saccharomyces cerevisiae that contains the U3 snoRNA and 28 proteins. Seventeen new proteins (Utp1–17) and Rrp5 were present, as were ten known components. The Utp proteins are nucleolar and specifically associated with the U3 snoRNA. Depletion of the Utp proteins impedes production of the 18S rRNA, indicating that they are part of the active pre-rRNA processing complex. On the basis of its large size (80S; calculated relative molecular mass of at least 2,200,000) and function, this complex may correspond to the terminal knobs present at the 5′ ends of nascent pre-rRNAs. We have termed this large RNP the small subunit (SSU) processome.

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We thank A. Djikeng, G. Dreyfuss, P. Gordon, J. Laney, T. Serio and T. Stone for assistance and advice. We also thank P. Glazer, M. Snyder and J. Steitz for critical reading of the manuscript. F.D. was supported by the Anna Fuller Fund for Molecular Oncology. This work was supported by federal grants from the NIH to D.F.H and S.J.B. and the NSF to A.L.B., and by The Patterson Trust to S.J.B.

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Author notes

    • Donald F. Hunt

    Present address: Department of Pathology, University of Virginia, Charlottesville, Virginia, 22904, USA

    • François Dragon

    Present address: Université du Québec à Montréal, Montréal, H36 3P8, Canada


  1. Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut, 06520-8040, USA

    • François Dragon
    • , Patricia A. Compagnone-Post
    • , Brianna M. Mitchell
    •  & Susan J. Baserga
  2. Department of Genetics, Yale University School of Medicine, New Haven, Connecticut, 06520-8040, USA

    • Jennifer E. G. Gallagher
    • , Kara A. Porwancher
    • , Karen A. Wehner
    •  & Susan J. Baserga
  3. Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut, 06520-8040, USA

    • Steven Wormsley
  4. ProteoMS, LLC, Charlottesville, Virginia, 22903, USA

    • Robert E. Settlage
  5. Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22904, USA

    • Jeffrey Shabanowitz
    •  & Donald F. Hunt
  6. Department of Microbiology, University of Virginia, Charlottesville, Virginia, 22904, USA

    • Yvonne Osheim
    •  & Ann L. Beyer


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The authors declare that they have no competing financial interests.

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Correspondence to Susan J. Baserga.

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