Symmetrical modification within a nucleosome is not required globally for histone lysine methylation.

Two copies of each core histone exist in every nucleosome; however, it is not known whether both histones within a nucleosome are required to be symmetrically methylated at the same lysine residues. We report that for most lysine methylation states, wild-type histones paired with mutant, unmethylatable histones in mononucleosomes have comparable methylation levels to bulk histones. Our results indicate that symmetrical histone methylation is not required on a global scale. However, wild-type H4 histones paired with unmethylatable H4K20R histones showed reduced levels of H4K20me2 and H4K20me3, suggesting that some fractions of these modifications might exist symmetrically, and enzymes mediating these modifications might, to some extent, favour nucleosome substrates with premethylated H4K20.