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The chromatin-remodeling factor FACT contributes to centromeric heterochromatin independently of RNAi.

ABSTRACT: Centromeres exert vital cellular functions in mitosis and meiosis. A specialized histone and other chromatin-bound factors nucleate a dynamic protein assembly that is required for the proper segregation of sister chromatids. In several organisms, including the fission yeast, Schizosaccharomyces pombe, the RNAi pathway contributes to the formation of silent chromatin in pericentromeric regions. Little is known about how chromatin-remodeling factors contribute to heterochromatic integrity and centromere function. Here we show that the histone chaperone and remodeling complex FACT is required for centromeric-heterochromatin integrity and accurate chromosome segregation. We show that Spt16 and Pob3 are two subunits of the S. pombe FACT complex. Surprisingly, yeast strains deleted for pob3+ are viable and alleviate gene silencing at centromeric repeats and at the silent mating-type locus. Importantly, like heterochromatin and RNAi pathway mutants, Pob3 null strains exhibit lagging chromosomes on anaphase spindles. Whereas the processing of centromeric RNA transcripts into siRNAs is maintained in Pob3 mutants, Swi6-association with the centromere is reduced. Our studies provide the first experimental evidence for a role of the RNA polymerase II cofactor FACT in heterochromatin integrity and in centromere function.

SUBMITTER: Lejeune E

PROVIDER: S-EPMC7612134 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Chromatin modifications play a fundamental role in controlling transcription and genome stability and yet despite their importance, are poorly understood in early-diverging fungi. We present a comprehensive study of histone lysine and DNA methyltransferases across the Mucoromycota, emphasizing heterochromatin formation pathways that rely on the Clr4 complex involved in H3K9-methylation, the Polycomb-repressive complex 2 driving H3K27-methylation, or DNMT1-like methyltransferases that catalyze 5mC DNA methylation. Our analysis uncovered H3K9-methylated heterochromatin as the major chromatin modification repressing transcription in these fungi, which lack both Polycomb silencing and cytosine methylation. Although small RNAs generated by RNA interference (RNAi) pathways facilitate the formation of heterochromatin in many eukaryotic organisms, we show that RNAi is not required to maintain either genomic or centromeric heterochromatin in Mucor. H3K9-methylation and RNAi act independently to control centromeric regions, suggesting a functional subspecialization. Whereas the H3K9 methyltransferase Clr4 and heterochromatin formation are essential for cell viability, RNAi is dispensable for viability yet acts as the main epigenetic, regulatory force repressing transposition of centromeric GremLINE1 elements. Mutations inactivating canonical RNAi lead to rampant transposition and insertional inactivation of targets resulting in antimicrobial drug resistance. This fine-tuned, Rdrp2-dependent RNAi activity is critical for genome stability, restricting GremLINE1 retroelements to the centromeres where they occupy long heterochromatic islands. Taken together, our results suggest that RNAi and heterochromatin formation are independent genome defense and regulatory mechanisms in the Mucorales, contributing to a paradigm shift from the cotranscriptional gene silencing observed in fission yeasts to models in which heterochromatin and RNAi operate independently in early-diverging fungi.

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The chromatin-remodeling factor FACT contributes to centromeric heterochromatin independently of RNAi.

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