Histone methylation refers to the addition of a methyl group to the side chains of arginine and lysine residues found on core histone subunits. Histones act as protein spools around which DNA is wound in the nuclei of eukaryotic cells.
Until recently, biologists regarded histones as little more than structural scaffolds for packaging nuclear DNA. The first histone modification discovered was acetylation. Acetate has a negative charge, which neutralizes the positive charge on arginine and lysine side chains. Acetylated histones bind loosely to DNA, allowing transcription enzymes access to the genes in that region. Conversely, deacetylated histones bind DNA tightly and render that region of chromatin transcriptionally inactive.
Histone methylation has more varied effects on gene transcription than acetylation. Although methylation does not alter the charge on arginine or lysine side chains, it does recruit methyl binding proteins to the histone complex. These methyl binding proteins may promote or suppress gene transcription depending on the presence of other transcription factors as well as various other histone modifications, including the phosphorylation of serines and threonines or the addition of a protein called ubiquitin to lysine side chains.