title: Crystal structures of a group II intron lariat primed for reverse splicing.
abstract: The 2'-5' branch of nuclear premessenger introns is believed to have been inherited from self-splicing group II introns, which are retrotransposons of bacterial origin. Our crystal structures at 3.4 and 3.5 angstrom of an excised group II intron in branched ("lariat") form show that the 2'-5' branch organizes a network of active-site tertiary interactions that position the intron terminal 3'-hydroxyl group into a configuration poised to initiate reverse splicing, the first step in retrotransposition. Moreover, the branchpoint and flanking helices must undergo a base-pairing switch after branch formation. A group II-based model of the active site of the nuclear splicing machinery (the spliceosome) is proposed. The crucial role of the lariat conformation in active-site assembly and catalysis explains its prevalence in modern splicing.
authors.affiliation: Group II introns as ribozymes and retrotransposons, Institute for Integrative Biology of the Cell (I2BC), UMR 9198 CNRS, Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), University Paris-Sud, University Paris-Saclay, 1 Avenue de la Terrasse, Bâtiment 26, 91198 Gif-sur-Yvette cedex, France. maria.costa@i2bc.paris-saclay.fr.
authors.forename: Maria
authors.initials: M
authors.lastname: Costa
authors.affiliation: Structure and Dynamics of RNA, I2BC, UMR 9198 CNRS, CEA, University Paris-Sud, University Paris-Saclay, 91198 Gif-sur-Yvette cedex, France.
authors.forename: Hélène
authors.initials: H
authors.lastname: Walbott
authors.affiliation: Group II introns as ribozymes and retrotransposons, Institute for Integrative Biology of the Cell (I2BC), UMR 9198 CNRS, Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), University Paris-Sud, University Paris-Saclay, 1 Avenue de la Terrasse, Bâtiment 26, 91198 Gif-sur-Yvette cedex, France.
authors.forename: Dario
authors.initials: D
authors.lastname: Monachello
authors.affiliation: Architecture and Reactivity of RNA, Institute of Molecular and Cellular Biology of the CNRS, University of Strasbourg, 15 rue René Descartes, 67084 Strasbourg, France.
authors.forename: Eric
authors.initials: E
authors.lastname: Westhof
authors.affiliation: Group II introns as ribozymes and retrotransposons, Institute for Integrative Biology of the Cell (I2BC), UMR 9198 CNRS, Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), University Paris-Sud, University Paris-Saclay, 1 Avenue de la Terrasse, Bâtiment 26, 91198 Gif-sur-Yvette cedex, France.
authors.forename: François
authors.initials: F
authors.lastname: Michel
mesh_terms: D055162:Biocatalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D005091:Exons; D007438:Introns; D009690:Nucleic Acid Conformation; D012322:RNA Precursors / Q000737:chemistry / Q000235:genetics; D012326:RNA Splicing; D016337:RNA, Catalytic / Q000737:chemistry* / Q000648:ultrastructure*; D017381:Spliceosomes / Q000737:chemistry / Q000235:genetics
chemical_list: D012322:RNA Precursors; D016337:RNA, Catalytic