title: Molecular characterization of a new member of the lariat capping twin-ribozyme introns.
abstract: BACKGROUND
  Twin-ribozyme introns represent a complex class of mobile group I introns that harbour a lariat capping (LC) ribozyme and a homing endonuclease gene embedded in a conventional self-splicing group I ribozyme (GIR2). Twin-ribozyme introns have so far been confined to nucleolar DNA in Naegleria amoeboflagellates and the myxomycete Didymium iridis.
  
  
  RESULTS
  We characterize structural organization, catalytic properties and molecular evolution of a new twin-ribozyme intron in Allovahlkampfia (Heterolobosea). The intron contains two ribozyme domains with different functions in ribosomal RNA splicing and homing endonuclease mRNA maturation. We found Allovahlkampfia GIR2 to be a typical group IC1 splicing ribozyme responsible for addition of the exogenous guanosine cofactor (exoG), exon ligation and circularization of intron RNA. The Allovahlkampfia LC ribozyme, by contrast, represents an efficient self-cleaving ribozyme that generates a small 2',5' lariat cap at the 5' end of the homing endonuclease mRNA, and thus contributes to intron mobility.
  
  
  CONCLUSIONS
  The discovery of a twin-ribozyme intron in a member of Heterolobosea expands the distribution pattern of LC ribozymes. We identify a putative regulatory RNA element (AP2.1) in the Allovahlkampfia LC ribozyme that involves homing endonuclease mRNA coding sequences as an important structural component.
authors.affiliation: RNA and Molecular Pathology group, Department of Medical Biology, Faculty of Health Sciences, UiT - The Arctic University of Norway, MH-building Breivika, N-9037 Tromsø, Norway ; Department of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Yunjia
authors.initials: Y
authors.lastname: Tang
authors.affiliation: Department of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Henrik
authors.initials: H
authors.lastname: Nielsen
authors.affiliation: Génétique Moléculaire, Génomique, Microbiologie, IPCB, Université de Strasbourg, CNRS, Strasbourg, France.
authors.forename: Benoît
authors.initials: B
authors.lastname: Masquida
authors.affiliation: School of Biological Sciences, University of Canterbury, Christchurch, New Zealand.
authors.forename: Paul P
authors.initials: PP
authors.lastname: Gardner
authors.affiliation: RNA and Molecular Pathology group, Department of Medical Biology, Faculty of Health Sciences, UiT - The Arctic University of Norway, MH-building Breivika, N-9037 Tromsø, Norway.
authors.forename: Steinar D
authors.initials: SD
authors.lastname: Johansen
keywords: Allovahlkampfia; Heterolobosea; LC ribozyme; Naegleria; RNA structure; circular RNA; intron splicing