title: A large nucleolar U3 ribonucleoprotein required for 18S ribosomal RNA biogenesis.
abstract: 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 ago, 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 tested. Biochemical and genetic analyses suggest that U3 pre-rRNA base-pairing interactions mediate endonucleolytic pre-rRNA cleavages. 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.
authors.affiliation: Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06520-8040, USA.
authors.forename: François
authors.initials: F
authors.lastname: Dragon
authors.forename: Jennifer E G
authors.initials: JE
authors.lastname: Gallagher
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Compagnone-Post
authors.forename: Brianna M
authors.initials: BM
authors.lastname: Mitchell
authors.forename: Kara A
authors.initials: KA
authors.lastname: Porwancher
authors.forename: Karen A
authors.initials: KA
authors.lastname: Wehner
authors.forename: Steven
authors.initials: S
authors.lastname: Wormsley
authors.forename: Robert E
authors.initials: RE
authors.lastname: Settlage
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Shabanowitz
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Osheim
authors.forename: Ann L
authors.initials: AL
authors.lastname: Beyer
authors.forename: Donald F
authors.initials: DF
authors.lastname: Hunt
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Baserga
mesh_terms: D002466:Cell Nucleolus / Q000235:genetics / Q000378:metabolism; D002843:Chromatin / Q000235:genetics / Q000378:metabolism; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D012322:RNA Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012331:RNA, Fungal / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012337:RNA, Ribosomal, 18S / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020537:RNA, Small Nucleolar / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020718:Ribonucleoproteins, Small Nucleolar / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*
chemical_list: D002843:Chromatin; D046911:Macromolecular Substances; D012322:RNA Precursors; D012331:RNA, Fungal; D012337:RNA, Ribosomal, 18S; D020537:RNA, Small Nucleolar; C117378:RNA, U3 small nucleolar; D020718:Ribonucleoproteins, Small Nucleolar; D029701:Saccharomyces cerevisiae Proteins; C117377:ribonucleoprotein, U3 small nucleolar