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Epigenetics: Histone Modification III

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Position-effect variegation (PEV) - Large segments of eukaryotic genomes are made of repetitive sequences that are constitutively heterochromatin - Juxtaposition of a gene to the heterochromatic regions derives PEV. - Spreading heterochromatic features to a nearby gene in a clonal fashion. - The drosophila white gene is the first known example, which has been an important tool for identifying all the machineries involved in the heterochromatin formation.

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Screening of PEV modifiers - The white gene in heterochromatic region was used as a reporter. - A large-scale mutagenesis experiments to find either suppressing or enhancing the PEV. - su(var) -> loss of silencing -> components for repression and heterochromatin - e(var) -> enhancing of silencing -> components for activation

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P-element based screening - Transposon P element = inverted element + transposase - co-injection of the P-based reporter with active transposase into embryos - 1% of the recovered line with PEV - Visualizing the heterochromatin structure by MNase assays.

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Su(var) and E(var) - Nomenclature Su(var)3-9 17 : Suppressor of Variegation, chromosome 3, 9 th gene, 17 th allele ---- H3K9 methylase 5 known K3K9 methylase in mammals, including Suv39h1, Suv39h2. - Su(var)2-5 : Heterochromatic protein 1 (HP1) recognizes H3K9me2 and spreads through interacting Su(var)3-9 - About 150 genes are involved in PEV; chromosomal proteins + histone modifiers - PcG and TrxG are also identified as Su(var) or E(var)

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Immuno-staining: demonstration of heterochromatin components -HP1 = su(var)2-5 -HKMT for H3K9 = su(var)3-9

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Histone Modification on H3K9 - su(var)3-9 dimethylation on H3K9 - not known for mono and tri-methylases - HP1 and SU(VAR)3-9 are inter-dependent for the formation of heterochromatin

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Many pre-steps for the transition between hetero and euchromatins

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Targeting HP1 and SU(VAR)3-9 repetitive sequences location within pericentromeric, centromeric, and telomeric regions well conserved through eukaryotes (S. pombe to mammals however, the actual histone modifications are interpreted in a different manner between Individual species!!

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Paper to discuss Thursday (Sept.25 th ) Ooi, S.K., Qiu, C., Bernstein, E., Li, K., Jia, D., Yang, Z., Erdjument-Bromage, H., Tempst, P., Lin, S.P., Allis, C.D., Cheng, X., and Bestor, T.H. (2007). DNMT3L connects unmethylated lysine 4 of histone H3 to de novo methylation of DNA. Nature 448, 714-717. Ciccone, D.N., Su, H., Hevi, S., Gay, F., Lei, H., Bakjo, J., Xu, G., Li, E., and Chen, T. (2009). KDM1B is a histone H3K4 demethylase required to establish maternal genomic imprints. Nature 461, 415-418.