1. Epigenetics: Histone Modification I

2. Nucleosome A packaging unit for DNA (=H3/H4 tetramer + two sets of H2A/H2B dimer) DNA (- charge) and histones (+ charge) histones = tails and globular domains

3. Higher-Order Packaging of Chromatins An efficient packaging strategy (10,000-fold compaction) A barrier for gene transcription

4. Three strategies to change nucleosome arrangement used for regulating gene transcription cis-effects: acetylation- driven activation trans-effects: reader- driven transcriptional control Nucleosome repositioning and displacement – ATP- driven reaction

5. Various histone modifications H3 and H4 are most modified Lys and Arg are most frequent targets Acetylation on Lys: activation Methylation on Lys: activation or repression Methylation on Arg: repression Ubiquitylation: repression Phosphorylation on Ser Thr: cell cycle signal

6. Major enzymes for histone modifications - Modifications are reversible (HAT vs HDAC, Kinase vs PPase) - Methylation on Lys has different states (mono, di, tri) - Methylations are very stable

7. Consequences of histone modifications - Modification on each amino acid has a different meaning - Two different modifications on one amino acid are exclusive and have opposite signals (K27me3 vs K27ace) - Readers (Acetylation- Bromo proteins, Methylation- Chromo proteins) - Main epigenetic regulators Polycomb (repression memory)  H3K27me and H2AK119ub Trithorax (activation memory)  H3K4me Position Effect Variegation(repression)  H3K9me

8. Polycomb mutant Sex com in the front legs of male flies Pc mutants have sex comb in other legs -> homeotic transformation mis-expression of Hox genes Polycomb-Group (PcG) genes control Hox genes and other developmental genes Main repression mechanism -> provides “cellular memory or transcriptional memory”

9. Cellular memory - Mammals 200 cell types vs plants 30 cell type - Different cell types have their own identity in terms of transcriptional programs - Epigenetic modifications serve as a molecular basis for cellular memory

10. Cellular memory - Early genetic studies identified two types of fly mutants - Polycomb mutants – defects with repression memory (H3K27me3 and H2AK119ub) - Trithorax mutants – defects with activation memory (H3K4me3)

11. PcG mutants - a & b – imaginal disc - c & d – suz12 mutant - e & f – Ring1A mutant

12. Core PcG genes PRC2 well conserved PRC1 only in metazoans Many duplications of PRC2 members in plants

13. Core PcG genes of PRC2 - three main components Ezh1/2, Suz12, Eed Ezh1/2- HMT enzyme Eed – glue protein Suz12 – no function yet - several other proteins RbAp48/49, Jarid2, Aebp2 RbAp48/49 – histone binding Jarid2 – DNA binding Aebp2 – DNA binding

14. PcG-dependent repression PRC2 -> H3K27 H3K27 -> PRC1 PRC1 -> H2AK119ub Targeting of PRC2 & PRC1 is mediated either by ncRNA or DNA-bindig protein Maintenance of H3K27me3 during DNA replication is mediated through Eed

15. Genome-wide PRE sites - more than Hox cluster - many genes with tissue-specific genes are controlled through PRC-mediated mechanisms - PRC2 serves as temporary repression mechanism (poised state) in early stage embryo - modify CpG islands