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Fonctions et organisations de l’hétérochromatine au cours du développement sexué chez le champignon filamenteux Podospora anserina

Abstract : In pezizomycotina, transposable elements are targeted by a genome defense system named Repeat Induced Point Mutation (RIP). First described in Neurospora crassa, RIP occurs before karyogamy in each parental haploid nucleus of the dikaryotic cells and results, within the repeats, in de novo methylation of cytosine (5mC) and mutations, mainly C to T transitions. This initial step triggers local assembly of constitutive heterochromatin, which allows transcriptional gene silencing. RID (RIP Defective) is a putative cytosine methyltransferase essential for RIP. Despite the absence of 5mC in its genome, PaRid inactivation in Podospora anserina results in sexual reproduction arrest right after fertilization. In this context, we asked whether PaRid is required to silence expression of some of sexual development-specific genes by nucleation of constitutive heterochromatin. To this end, we identified PaKmt1 and PaHp1 genes encoding respectively the histone methyltransferase PaKmt1 (SU(VAR)39 homologue protein) and the heterochromatin protein 1 (PaHP1). To assemble constitutive heterochromatin, PaKmt1 catalyses tri-methylation of H3K9 (H3K9me3), latter on bound by PaHP1. By contrast, the E(Z) histone methyltransferase homologue PaKmt6, as part of the PRC2 complex, catalyses tri-methylation of H3K27 (H3K27me3) to form facultative heterochromatin. Our results showed that loss of either PaKmt1 or PaHP1 does not cause major defects. Conversely, PaKmt6 gene inactivation results in severe defects: altered mycelium and vegetative growth rate, overproduction of male gamete, development of crippled fructifications, reduced production ascospores, part of which does not germinate. Furthermore, epistatic study showed that PaRid and PaKmt6 likely act in two different developmental pathways, with respect to sexual reproduction. In addition, using chromatin immuno-precipitation we characterized H3K9me3, H3K27me3 and H3K4me3 genome-wide distribution patterns. We observed an uncommon overlapping distribution between H3K9me3 and H3K27me3 on transcriptionally repressed genes and RIP target repeats. As expected, H3K4me3 localizes in 5’ of the transcribed genes and is excluded from the H3K9me3 and H3K27me3 domains. As expected, PaKmt6 is essential for H3K27me3 modification, but surprisingly, could also be responsible for some of the H3K9me3 setting up or maintenance.
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  • HAL Id : tel-02426209, version 1

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Florian Carlier. Fonctions et organisations de l’hétérochromatine au cours du développement sexué chez le champignon filamenteux Podospora anserina. Biologie moléculaire. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS457⟩. ⟨tel-02426209⟩

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