Sujin Lee a and Scott W. Stevens b,c,1
aGraduate Program in Cellular and Molecular Biology, The University of Texas at Austin, Austin, TX 78712;
bDepartment of Molecular Biosciences, The University of Texas at Austin, Austin, TX 78712;
cInstitute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, TX 78712
Edited by Jef D. Boeke, New York University School of Medicine, New York, NY, and approved April 20, 2016 (received for review March 30, 2016)
Mere illustration/Mera ilustração
Eukaryotic transcripts contain spliceosomal introns that need to be removed by pre-mRNA splicing. Although several models have been proposed to identify the mechanism of intron gain over the evolution of eukaryotes, they remain models due to a lack of experimental validation. We developed a reporter system to detect selected intron gain and loss events and captured two intron gain events in which the intron derived from the reporter was transposed into the chromosomal loci of RPL8B and ADH2. This is, to our knowledge, the first demonstration of intron gain via intron transposition in any organism, and we suggest that these events are likely to have occurred by a reversal of the pre-mRNA splicing reaction followed by homologous recombination.
The presence of intervening sequences, termed introns, is a defining characteristic of eukaryotic nuclear genomes. Once transcribed into pre-mRNA, these introns must be removed within the spliceosome before export of the processed mRNA to the cytoplasm, where it is translated into protein. Although intron loss has been demonstrated experimentally, several mysteries remain regarding the origin and propagation of introns. Indeed, documented evidence of gain of an intron has only been suggested by phylogenetic analyses. We report the use of a strategy that detects selected intron gain and loss events. We have experimentally verified, to our knowledge, the first demonstrations of intron transposition in any organism. From our screen, we detected two separate intron gain events characterized by the perfect transposition of a reporter intron into the yeast genes RPL8B and ADH2, respectively. We show that the newly acquired introns are able to be removed from their respective pre-mRNAs by the spliceosome. Additionally, the novel allele, RPL8Bint, is functional when overexpressed within the genome in a strain lacking the Rpl8 paralogue RPL8A, demonstrating that the gene targeted for intronogenesis is functional.
spliceosome intron evolution
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Author contributions: S.W.S. designed research; S.L. performed research; S.L. and S.W.S. analyzed data; and S.L. and S.W.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1605113113/-/DCSupplemental.
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