Histone methylation

Histone methylation is the modification of certain amino acids in a histone protein by the addition of one, two, or three methyl groups.

Function
This modification alters the properties of the nucleosome and affects its interactions with other proteins.


 * Histone methylation is generally associated with transcriptional repression.
 * However, methylation of some lysine and arginine residues of histones results in transcriptional activation. Examples include methylation of lysine 4 of histone 3 (H3K4), and arginine (R) residues on H3 and H4.

Demethylation
For many years histone methylation was thought to be a permanent modification. Very recently two families of histone demethylating enzymes were discovered.
 * The first was Lysine Specific Demethylase 1 (LSD1) which is an flavin-dependent monoamine oxidase which can demethylate mono- and di-methylated lysines, specifically histone 3, lysines 4 and 9 (H3K4 and H3K9). This enzyme cannot demethylate tri-methylated lysines and for a short while it was thought that tri-methylated lysines may indeed be permanent modifications.
 * In late 2005 the Jumonji domain-containing (JmjC) histone demethylases were discovered which are able to demethylate mono-, di-, or tri-methylated lysines thereby disproving the theory that histone methylation is permanent once and for all.