Practice question

Methyllysine-binding modules convey repressive or active marks depending context. Chromodomain, from Greek chromatin organization modifier, first found in Drosophila Polycomb and HP1 proteins, folds into three beta sheets plus alpha helix creating aromatic cage capturing trimethylated lysine side chains, especially H3K9me3 and H3K27me3 driving heterochromatin spreading silencing. Some chromodomains also bind H3K4me3 in active transcription via distinct mechanism. Unlike bromodomains recognizing acetylation, chromodomains discriminate mono di tri methyl states. Phosphothreonine recognized by FHA or BRCT domains, unmodified tails lack high affinity reader interactions, ensuring modification-specific recruitment necessary for epigenetic memory.