title: Characterization of the self-splicing products of two complex Naegleria LSU rDNA group I introns containing homing endonuclease genes.
abstract: The two group I introns Nae.L1926 and Nmo.L2563, found at two different sites in nuclear LSU rRNA genes of Naegleria amoebo-flagellates, have been characterized in vitro. Their structural organization is related to that of the mobile Physarum intron Ppo.L1925 (PpLSU3) with ORFs extending the L1-loop of a typical group IC1 ribozyme. Nae.L1926, Nmo.L2563 and Ppo.L1925 RNAs all self-splice in vitro, generating ligated exons and full-length intron circles as well as internal processed excised intron RNAs. Formation of full-length intron circles is found to be a general feature in RNA processing of ORF-containing nuclear group I introns. Both Naegleria LSU rDNA introns contain a conserved polyadenylation signal at exactly the same position in the 3' end of the ORFs close to the internal processing sites, indicating an RNA polymerase II-like expression pathway of intron proteins in vivo. The intron proteins I-NaeI and I-NmoI encoded by Nae.L1926 and Nmo.L2563, respectively, correspond to His-Cys homing endonucleases of 148 and 175 amino acids. I-NaeI contains an additional sequence motif homologous to the unusual DNA binding motif of three antiparallel beta sheets found in the I-PpoI endonuclease, the product of the Ppo.L1925 intron ORF.
authors.affiliation: RNA Research group, Department of Molecular Biotechnology, Institute of Medical Biology, University of Tromsø, Tromsø, Norway.
authors.forename: Peik
authors.initials: P
authors.lastname: Haugen
authors.forename: Johan F
authors.initials: JF
authors.lastname: De Jonckheere
authors.forename: Steinar
authors.initials: S
authors.lastname: Johansen
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D004247:DNA; D004275:DNA, Ribosomal / Q000737:chemistry / Q000235:genetics*; D015252:Deoxyribonucleases, Type II Site-Specific / Q000737:chemistry / Q000235:genetics*; D007438:Introns; D008969:Molecular Sequence Data; D009253:Naegleria / Q000201:enzymology / Q000235:genetics*; D009690:Nucleic Acid Conformation; D016366:Open Reading Frames; D012326:RNA Splicing; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid
chemical_list: D004275:DNA, Ribosomal; D029701:Saccharomyces cerevisiae Proteins; D004247:DNA; C066166:AI4 protein, S cerevisiae; D015252:Deoxyribonucleases, Type II Site-Specific