Different demethylases act on different residues and methylation states, so removal of H3K4 methylation and removal of H3K27 trimethylation are distinct activities. Regulated removal makes a chromatin mark reversible without requiring replacement of the underlying DNA sequence. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapters 3, 6 and 15, protein function, translation and cell signalling
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