Histone methyltransferases (HMTases) selectively methylate evolutionarily conserved arginine or lysine residues, primarily in the N-terminal tails of histones H3 and H4. Signal transduction pathways affecting the N-terminal tails of histones lead to a number of post-translational modifications including acetylation, phosphorylation, poly(ADP-ribosylation), ubiquitination and methylation. These modifications play critical roles in regulating chromatin structure and gene expression. Set7/9 is a histone specific HMTase that methylates histone H3 lysine 4. Set7/9 transfers methyl groups to lysine 4 of histone H3 in complex with S-adenosyl-L-methionine. In yeast, H4-K20 methylation does not have any apparent role in the regulation of gene expression or heterochromatin function; rather it appears to play a role in DNA damage response. Loss of Set9 activity or mutation of H4-K20 markedly impairs yeast cell survival after genotoxic challenge and compromises the ability of cells to maintain checkpoint mediated cell cycle arrest. Genetic experiments link Set9 to Crb2, a homolog of the mammalian checkpoint protein 53BP1, and the enzyme is required for Crb2 localization to sites of DNA damage.

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