Download Epigenetics: Histone Modification III

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)3-917 : Suppressor of Variegation, chromosome 3,
9th gene, 17th 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.25th)
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.