Investigating the mechanism of the Exon Junction Complex in pre-mRNA splicing and its implication in the piRNA pathway
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Abstract
The exon junction complex (EJC) is a highly conserved ribonucleoprotein complex
which binds RNAs at a late stage of the splicing reaction and remains associated with them following export to the cytoplasm. This complex is involved in several cellular post-transcriptional processes including mRNA localization, translation and degradation. The EJC plays an additional role in the splicing of a subset of genes in Drosophila and in human cells but the underlying mechanism remains to be elucidated.
The goal of my PhD project was to decipher the mechanism of the EJC in the splicing process. To this end, we have searched for new developmentally regulated EJC targets. We have found a novel function for the EJC and its splicing subunit RnpS1 in preventing transposon accumulation in both Drosophila germline and surrounding follicular cells. We have shown that this function appears to be mediated through the control of the splicing of the piwi transcript. Piwi is involved in the piRNA pathway, a mechanism leading to transposon repression. In absence of RnpS1, one of the piwi intron is retained. This intron contains a weak 5’ splice site as well as degenerate transposon fragments, reminiscent of heterochromatic introns. Furthermore, we identified a small A/T rich region, which alters its polypyrimidine tract and confers dependency to RnpS1. We demonstrated that the removal of this intron by RnpS1 requires the initial splicing of the flanking introns, suggesting a model in which the EJC facilitates the splicing of weak introns following its initial deposition to adjacent exon junctions.
This work provides an interesting mechanism underlying the cooperation between
introns in facilitating their splicing. In addition, it opens new avenues in the
investigation of the regulation of the piRNA pathway as well as in the understanding of the polarity and kinetics of the splicing reaction.