title: Stalled spliceosomes are a signal for RNAi-mediated genome defense.
abstract: Using the yeast Cryptococcus neoformans, we describe a mechanism by which transposons are initially targeted for RNAi-mediated genome defense. We show that intron-containing mRNA precursors template siRNA synthesis. We identify a Spliceosome-Coupled And Nuclear RNAi (SCANR) complex required for siRNA synthesis and demonstrate that it physically associates with the spliceosome. We find that RNAi target transcripts are distinguished by suboptimal introns and abnormally high occupancy on spliceosomes. Functional investigations demonstrate that the stalling of mRNA precursors on spliceosomes is required for siRNA accumulation. Lariat debranching enzyme is also necessary for siRNA production, suggesting a requirement for processing of stalled splicing intermediates. We propose that recognition of mRNA precursors by the SCANR complex is in kinetic competition with splicing, thereby promoting siRNA production from transposon transcripts stalled on spliceosomes. Disparity in the strength of expression signals encoded by transposons versus host genes offers an avenue for the evolution of genome defense.
authors: Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA.
authors: Phillip A
authors: PA
authors: Dumesic
authors: Prashanthi
authors: P
authors: Natarajan
authors: Changbin
authors: C
authors: Chen
authors: Ines A
authors: IA
authors: Drinnenberg
authors: Benjamin J
authors: BJ
authors: Schiller
authors: James
authors: J
authors: Thompson
authors: James J
authors: JJ
authors: Moresco
authors: John R
authors: JR
authors: Yates
authors: David P
authors: DP
authors: Bartel
authors: Hiten D
authors: HD
authors: Madhani
mesh_terms: D003455:Cryptococcus neoformans / Q000235:genetics*; D004251:DNA Transposable Elements; D016681:Genome, Fungal; D007438:Introns; D007700:Kinetics; D034622:RNA Interference; D012333:RNA, Messenger / Q000378:metabolism; D015138:RNA, Nuclear / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism; D012324:RNA-Dependent RNA Polymerase / Q000378:metabolism; D017381:Spliceosomes / Q000378:metabolism*
chemical_list: D004251:DNA Transposable Elements; D012333:RNA, Messenger; D015138:RNA, Nuclear; D034741:RNA, Small Interfering; D012324:RNA-Dependent RNA Polymerase