히스톤 탈메틸화 효소에 의한 헤테로크로마틴 침묵 조절

Abstract

The post-translational modification of histones has been implicated in the regulation of gene expression and recently in that of cellular lifespan in yeast and nematodes. In the present study, we try to determine the role of histone modification in the regulation of heterochromatic silencing at telomeric proximal regions and ribosomal DNA loci. First, we show the antagonistic role of Set2 methyltransferase, which is specific for histone H3 at lysine 36, in regulating telomeric silencing and cellular lifespan. We observed that an intermediate state of chromatin, namely, unstable ON telomeres, exists when a gene is switched on near telomeres. This unstable state of chromatin is temporally maintained in a transcription-dependent manner and is preferentially restored to its original heterochromatic state, namely, OFF telomeres. We found that Set2 suppresses the restoration of unstable ON telomeres to the stable OFF state and promotes cellular aging. Second, we show that changes in histone H3 lysine methylation levels distinctly regulate rDNA silencing by recruiting different silencing proteins to rDNA, thereby contributing to rDNA silencing and nucleolar organization in yeast. Nucleolar ribosomal DNA is tightly associated with silent heterochromatin, which is important for rDNA stability, nucleolar integration and cellular senescence. We found that Lys4, Lys79 and Lys36 methylation within histone H3 acts as a bivalent marker for the regulation of rDNA recombination and RENT complex-mediated rDNA silencing, both of which are Sir2-dependent pathways. By contrast, we found that Jhd2, an evolutionarily conserved JARID1 family H3 Lys4 demethylase, affects all states of methylated H3K4 within the nontranscribed spacer (NTS) regions of rDNA and that its activity is required for the regulation of rDNA silencing in a Sir2-independent manner. In this context, Jhd2 regulates rDNA recombination through the Tof2/Csm1/Lrs4 pathway and prevents excessive recruitment of Tof2, Csm1/Lrs4 and condensin subunits to the replication fork barrier site within the NTS1 region. Our FISH analyses further demonstrate that the demethylase activity of Jhd2 regulates mitotic rDNA condensation and that JHD2-deficient cells contain the mostly hypercondensed rDNA mislocalized away from the nuclear periphery. Taken together, these results obtained from two heterochromatic regions show the accumulation of unstable ON telomeres maintained by Set2 is one of the features of aged cells in telomere-proximal regions and, in heterochromatic rDNA loci, yeast Jhd2, which demethylates histone H3 Lys4 near the rDNA locus, regulates rDNA repeat stability and rDNA silencing in a Sir2-independent manner by maintaining Csm1/Lrs4 and condensing association with rDNA regions during mitosis.