1. Epigenetics: Histone Modification II

2. trithorax-Group (trxG)
Maintain the “on” state of developmental regulatory genes
trx: founding member (HMT on H3K4)

3. Transcriptional memory by trxG and PcG
trxG – on state memory
PcG – off state memory

4. Regulation of Hox by trxG and PcG
segmentation genes establish
14 segments
Hox genes maintain the identity
of these segments
trxG and PcG control Hox
defects in trxG or PcG cause
hometic mutatios

5. Other examples of trxG phenotypes
a vs b : wild type and trx mut
c : balancer into small wing
d : fifth seg mimicking anterior
seg

6. Identification Strategy for trxG genes reversal for PcG mutants
trxG mutants as suppressor
of PcG mutants
reversal of Polycomb phenotype
in double mutants
opposite outcome on
same pathways
activator vs repressor on
Hox gene

7. Two major groups of trxG
Group 1: ATP-dependent chromatin remodeling complexes
(SWI2/SNF2, Brahma, BRG1)
Group 2: HMTs on H3K4 (Set1, MLL1, MLL2)

8. ATP-dep chromatin remodeling complexes
Bromo-domain: recognize Acetylated
Histone
Many complexes are equipped with
Both readers (bromo-domain)
and writers (Histone Acetyl Transferase)
-> maintaining active “ON” state

9. Modes of chromatin remodeling

10. Genome-wide distributions of trxG
A) BRM complex: more global roles
In general transcription control
B) TRX: less limited number of loci

11. Inter-dependence of trxG and PcG
PcG-dep repression is a default state
trxG-dep activation an anti-repression
to PcG
share targeting proteins between trxG
and PcG: Zeste and GAGA factor
mechanistically feasible:
H3K4me3 -> HAT -> Acetylation on
H3K9 or H3K27 -> block methylation
on H3K9 or H3K27