title: The mitochondrial LSU rRNA group II intron of Ustilago maydis encodes an active homing endonuclease likely involved in intron mobility.
abstract: BACKGROUND
  The a2 mating type locus gene lga2 is critical for uniparental mitochondrial DNA inheritance during sexual development of Ustilago maydis. Specifically, the absence of lga2 results in biparental inheritance, along with efficient transfer of intronic regions in the large subunit rRNA gene between parental molecules. However, the underlying role of the predicted LAGLIDADG homing endonuclease gene I-UmaI located within the group II intron LRII1 has remained unresolved.
  
  
  RESULTS
  We have investigated the enzymatic activity of I-UmaI in vitro based on expression of a tagged full-length and a naturally occurring mutant derivative, which harbors only the N-terminal LAGLIDADG domain. This confirmed Mg²⁺-dependent endonuclease activity and cleavage at the LRII1 insertion site to generate four base pair extensions with 3' overhangs. Specifically, I-UmaI recognizes an asymmetric DNA sequence with a minimum length of 14 base pairs (5'-GACGGGAAGACCCT-3') and tolerates subtle base pair substitutions within the homing site. Enzymatic analysis of the mutant variant indicated a correlation between the activity in vitro and intron homing. Bioinformatic analyses revealed that putatively functional or former functional I-UmaI homologs are confined to a few members within the Ustilaginales and Agaricales, including the phylogenetically distant species Lentinula edodes, and are linked to group II introns inserted into homologous positions in the LSU rDNA.
  
  
  CONCLUSIONS
  The present data provide strong evidence that intron homing efficiently operates under conditions of biparental inheritance in U. maydis. Conversely, uniparental inheritance may be critical to restrict the transmission of mobile introns. Bioinformatic analyses suggest that I-UmaI-associated introns have been acquired independently in distant taxa and are more widespread than anticipated from available genomic data.
authors.affiliation: Department of Genetics, Institute for Applied Biosciences of the Karlsruhe Institute of Technology-KIT, Karlsruhe, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Pfeifer
authors.forename: Bettina
authors.initials: B
authors.lastname: Martin
authors.forename: Jörg
authors.initials: J
authors.lastname: Kämper
authors.forename: Christoph W
authors.initials: CW
authors.lastname: Basse
mesh_terms: D004720:Endonucleases / Q000235:genetics* / Q000378:metabolism; D004789:Enzyme Activation; D015870:Gene Expression; D015966:Gene Expression Regulation, Fungal; D023061:Gene Order; D050259:Genes, Mitochondrial; D007438:Introns; D054730:Protein Interaction Domains and Motifs; D012313:RNA; D000077278:RNA, Mitochondrial; D012335:RNA, Ribosomal; D013379:Substrate Specificity; D014158:Transcription, Genetic; D014588:Ustilago / Q000235:genetics* / Q000378:metabolism
chemical_list: D000077278:RNA, Mitochondrial; D012335:RNA, Ribosomal; D012313:RNA; D004720:Endonucleases