Control of Histone H3 Lysine 27 Trimethylation in Neurospora crassa
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Trimethylation of histone H3 lysine 27 (H3K27me3) marks facultative heterochromatin, containing silent genes. My research investigated factors that influence the distribution of H3K27me3 in the filamentous fungus Neurospora crassa. The H3K27 methyltransferase complex, PRC2, is well conserved in eukaryotes and consists of four core members: E(Z), EED, SUZ12 and P55. I showed that three of the PRC2 subunits (SET-7, the homolog of E(Z), EED and SUZ12) are required for H3K27me3 in Neurospora, while NPF, the homolog of P55, is only required for a subset of H3K27me3 domains. H3K27me3 is organized into large, gene-rich domains in Neurospora and normally does not overlap with constitutive heterochromatin, which is marked by both H3K9me3 and DNA methylation and bound by heterochromatin protein 1 (HP1). I discovered that loss of HP1 binding results in a genome-wide relocalization of H3K27me3. Specifically, it is lost from many of its normal domains while it becomes associated with much of the genome that is constitutive heterochromatin. This contrasts plant and mouse studies in which the loss of DNA methylation relocalizes H3K27me3. The DCDC complex is the H3K9-specific methyltransferase consisting of DIM-5, DIM-7, DIM-9, CUL4 and DIM-8. Separate deletions of DCDC subunits, with the exception of dim-7, relocalized H3K27me3 to constitutive heterochromatin, presumably due to the loss of HP1 binding. The deletion of dim-7 resulted in the loss of all H3K27me3, suggesting a novel role for dim-7. To look for a recruitment signal for PRC2, I moved large fragments contained within an H3K27me3 domain to loci devoid of H3K27me3, his-3 and csr-1. None of the fragments induced H3K27me3, demonstrating that a recruitment signal is not present within every fragment of H3K27me3-marked DNA. Large chromosomal rearrangements had profound effects on H3K27me3 domains, resulting in the loss of some H3K27me3 domains and the formation of others. In Drosophila and mammals, a subset of PRC2 complexes contains the histone deacetylase, Rpd3. A close homolog of Rpd3 in Neurospora, HDA-3, did not appear to be a member of PRC2 in Neurospora. This dissertation includes both previously published and unpublished co-authored material.