Normal development of animals, plants and fungi rely heavily on chromatin-based signals. The proposed research focuses on a particularly central example, trimethylation of histone H3 Lysine 27 (H3K27me3). Work in flies, mammals and plants have implicated this mark in long-term repression of genes in development, as well as in X-inactivation, genomic imprinting and cancer. Details of how H3K27me3 functions and how it is controlled remain unknown, however. The recent discovery of regions of H3K27me3 in the genome of the simple eukaryote, Neurospora crassa, and identification of elements of the underlying methylation machinery in this organism provide an opportunity to apply the exceptional power of Neurospora molecular and genetic methods to explore the function and control of this histone mark. H3K27me3 has not been found in other simpler systems (e.g. yeasts).
Specific aims of the project are: 1. To test for heritability of H3K27me3 and to determine the kinetics of de novo K27 methylation. We will test if methylation of H3K27 induced at an ectopic site is maintained after the inducing construct is removed. We will also use genetic and molecular methods to determine the kinetics of de novo H3K27 methylation. 2. To identify the components of the H3K27 methylation machinery. We will characterize the K27 methyltransferase complex and investigate what reads the H3K27me3 mark. Candidate binders (chromo domain proteins and the SET-7 complex) will be tested and proteomic and genetic (mutant hunt) methods will be used to identify unsuspected elements of the machinery. 3. To identify cis-acting sequences that control H3K27me3. To address the possibility that H3K27me3 in Neurospora is directed by sequences comparable to PREs of Drosophila, we will: a. use ChIP-Seq. to map binding sites of Neurospora PRC2 components;b. test deletions of native H3K27me3 regions for loss of H3K27me;c. test candidate control regions for the ability to direct H3K27 methylation at an ectopic site. 4. To define the role and mechanism of H3K27 methylation in gene repression. We will explore conditions that may control K27me3 genes and determine whether repression by K27 methylation results from a block in transcription initiation or elongation. 5. To test whether the SET-7 complex reads histone marks. We will explore the possibility that the SET-7 complex is sensitive to modifications in the N-terminus of H3 in vivo using our collection of mutants, and if the complex is found to bind H3K27me in vitro, we will follow up by testing it's binding to modified peptides.
Trimethylation of the lysine 27 residue of histone H3 plays a critical role in the epigenetic repression of genes during development, X-inactivation, genomic imprinting, and the aberrant inactivation of genes in cancer. Understanding how this epigenetic mark is regulated will lead to better insight of its role in tumorigenesis and ultimately to the development of therapeutic interventions. Neurospora crassa is the simplest model organism known to have the H3K27me3 mark and provides a system in which to make great strides in our understanding of the H3K27me3 mark.
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