title: Detection and analysis of alternative splicing in Yarrowia lipolytica reveal structural constraints facilitating nonsense-mediated decay of intron-retaining transcripts.
abstract: BACKGROUND
  Hemiascomycetous yeasts have intron-poor genomes with very few cases of alternative splicing. Most of the reported examples result from intron retention in Saccharomyces cerevisiae and some have been shown to be functionally significant. Here we used transcriptome-wide approaches to evaluate the mechanisms underlying the generation of alternative transcripts in Yarrowia lipolytica, a yeast highly divergent from S. cerevisiae.
  
  
  RESULTS
  Experimental investigation of Y. lipolytica gene models identified several cases of alternative splicing, mostly generated by intron retention, principally affecting the first intron of the gene. The retention of introns almost invariably creates a premature termination codon, as a direct consequence of the structure of intron boundaries. An analysis of Y. lipolytica introns revealed that introns of multiples of three nucleotides in length, particularly those without stop codons, were underrepresented. In other organisms, premature termination codon-containing transcripts are targeted for degradation by the nonsense-mediated mRNA decay (NMD) machinery. In Y. lipolytica, homologs of S. cerevisiae UPF1 and UPF2 genes were identified, but not UPF3. The inactivation of Y. lipolytica UPF1 and UPF2 resulted in the accumulation of unspliced transcripts of a test set of genes.
  
  
  CONCLUSIONS
  Y. lipolytica is the hemiascomycete with the most intron-rich genome sequenced to date, and it has several unusual genes with large introns or alternative transcription start sites, or introns in the 5' UTR. Our results suggest Y. lipolytica intron structure is subject to significant constraints, leading to the under-representation of stop-free introns. Consequently, intron-containing transcripts are degraded by a functional NMD pathway.
authors.affiliation: INRA UMR1319 Micalis - AgroParisTech, Biologie intégrative du métabolisme lipidique microbien, Bât, CBAI, 78850 Thiverval-Grignon, France.
authors.forename: Meryem
authors.initials: M
authors.lastname: Mekouar
authors.forename: Isabelle
authors.initials: I
authors.lastname: Blanc-Lenfle
authors.forename: Christophe
authors.initials: C
authors.lastname: Ozanne
authors.forename: Corinne
authors.initials: C
authors.lastname: Da Silva
authors.forename: Corinne
authors.initials: C
authors.lastname: Cruaud
authors.forename: Patrick
authors.initials: P
authors.lastname: Wincker
authors.forename: Claude
authors.initials: C
authors.lastname: Gaillardin
authors.forename: Cécile
authors.initials: C
authors.lastname: Neuvéglise
mesh_terms: D017398:Alternative Splicing / Q000235:genetics*; D000595:Amino Acid Sequence; D001483:Base Sequence; D018389:Codon, Nonsense / Q000235:genetics*; D018076:DNA, Complementary / Q000235:genetics; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D015723:Gene Library; D005800:Genes, Fungal / Q000235:genetics; D007438:Introns / Q000235:genetics*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D020871:RNA Stability / Q000235:genetics*; D012331:RNA, Fungal / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D017422:Sequence Analysis, DNA; D025062:Yarrowia / Q000235:genetics*
chemical_list: D018389:Codon, Nonsense; D018076:DNA, Complementary; D005656:Fungal Proteins; D012331:RNA, Fungal; D012333:RNA, Messenger