Practice question

Histone lysine methylation status is read by specialized domains that dictate transcriptional outcome depending on residue position and methylation state. Trimethylation of H3 lysine 9 creates binding platform for chromodomain of heterochromatin protein 1, stimulating Suv39h1 recruitment, spreading of repressive domains and interaction with DNA methyltransferases. This enforces chromatin compaction, transcriptional silencing and maintenance of constitutive heterochromatin at pericentromeres and telomeres. In contrast, H3K4me3 and H3K27ac at promoters recruit activating complexes, illustrating residue-specific decoding of methylation into activation versus repression programs.