Identification and Characterization of Histone H3 Lysine 23 Methylation in Germline Chromatin
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Eukaryotic DNA and histone proteins are packaged into chromatin that can occur in two cytologically distinct states: heterochromatin, which is highly compacted and transcriptionally repressive; or euchromatin, which is relatively open and transcriptionally permissive. Increasing experimental evidence suggest that the structure and function of chromatin is governed, in part, by histone post-translational modifications (PTMs), but their characterization is generally protracted because of the biological complexities associated with higher eukaryotes. Moreover, recent studies have shown that euchromatin and heterochromatin represent the extremes of a continuum of physiological chromatin states, which may be encoded for by distinct combinations of histone PTMs. These states also have transcription-independent roles during interphase, mitosis, and meiosis that contribute to, among other activities, maintaining genomic stability. To screen for novel heterochromatin associated histone PTMs, we took advantage of the nuclear dimorphism afforded by the single-celled ciliate Tetrahymena thermophila. This model organism has two physically separable nuclei with its cytoplasm: a heterochromatic micronucleus (germline genome), and a euchromatic macronucleus (somatic genome). In this dissertation, I present our work identifying and characterizing histone H3 lysine 23 methylation (H3K23me) in the micronucleus. We report that H3K23 trimethylation (H3K23me3) is a micronucleus-specific heterochromatin modification that becomes particularly enriched during meiosis. Loss of H3K23me3, through deletion of its putative methyltransferase Ezl3p, causes mislocalization of meiosis-induced DNA double-strand breaks to heterochromatin, and decreases progeny viability. We extend the significance of H3K23me3 to higher eukaryotes by showing its meiotic upregulation is conserved in germlines of C. elegans and mice. In summary, our results suggest that the evolutionary conserved H3K23me3 plays an important role in protecting meiotic, germline heterochromatin from programmed DSBs.