Epigenetics: Histone Modification III

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.

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

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.

Su(var) and E(var) - Nomenclature Su(var) : 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)

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

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

Many pre-steps for the transition between hetero and euchromatins

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!!

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, 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,