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Etude des voies de silencing transciptionnel indépendantes de la méthylation ADN chez Arabidopsis thaliana

Abstract : Transcriptional gene silencing hinders deleterious transcription of some genes and transposable elements. Silencing is maintained by numerous chromatin modifications such as DNA methylation and repressive histone marks. To better understand the molecular mechanisms of silencing, we conducted a forward genetic screen using a transgene reporter system targeted by transcriptional gene silencing in the model plant Arabidopsis thaliana. We isolated a first type of mutants with diminished maintenance of silencing and a second category that displayed deficient release of transgene silencing upon heat stress. We then combined molecular, cytological and genomic methods to characterize the defects associated with these mutations.First, we show that the Mediator subunit MED14 and the TFIIH complex subunit UVH6 are required for heat-stress-induced release of silencing. We further show that MED14, but not UVH6, promotes transcriptional activation of transposable elements in mutant contexts where silencing is defective. Importantly, MED14 is only required when DNA methylation is not affected, suggesting that MED14 has a specialized function to promote transcription of heterochromatin. Furthermore, we show that MED14 promote DNA methylation at targets regulated by RNA-directed DNA methylation.Characterizing mutants from the first category, we unveil the contribution of the MAIN and MAIL1 proteins into transcriptional gene silencing, and show that they likely act through a pathway independent of known silencing factors. Interestingly, MAIN and MAIL1 bear a protein domain that is shared with transposable elements, and that has been captured by transposable elements and genes throughout the evolutionary history of flower plants.Additionally, we confirm the involvement of the DNA polymerase epsilon in transcriptional gene silencing by isolating a new mutation of the POL2A gene among mutants of the first category. We characterize the effects of the pol2a mutation on several heterochromatin properties, and show that the pol2a mutant retains high levels of heterochromatin marks despite having highly disorganized heterochromatin. We actually detect a strong elevation of DNA methylation in the pol2a mutant and explore different hypothesis to explain this unusual phenotype. We show that increased expression of the CMT3 chromomethylase is a likely cause, but that additional molecular mechanisms are probably involved. Further exploration suggests that constitutive replicative stress occurring in pol2a mutants could be an additional cause of DNA hypermethylation.To summarize, this work provide putative causes for DNA hypermethylation and silencing defects in a situation of replicative deficiency. Further investigation will be required to identify the molecular components involved in the mechanism. Our data further suggest that MED14 has a function dedicated to heterochromatin transcription that could promote DNA methylation maintenance.
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Submitted on : Thursday, October 3, 2019 - 3:47:13 PM
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  • HAL Id : tel-02304926, version 1



Pierre Bourguet. Etude des voies de silencing transciptionnel indépendantes de la méthylation ADN chez Arabidopsis thaliana. Génétique des plantes. Université Clermont Auvergne, 2018. Français. ⟨NNT : 2018CLFAC091⟩. ⟨tel-02304926⟩



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