Histone modifications play important roles in regulating DNA-based biological processes. Of the modified sites, histone H3 lysine 56 (H3K56) is unique in that it lies within the globular core domain near the entry-exit sites of the nucleosomal DNA superhelix and its acetylation state in yeast is a marker for newly synthesized histones in transcription, DNA repair, and DNA replication. We now report the presence of H3K56 monomethylation (H3K56me1) in mammalian cells and find that the histone lysine methytransferase G9a/KMT1C is required for H3K56me1 both in vivo and in vitro. We also find that disruption of G9a or H3K56 impairs DNA replication. Furthermore, H3K56me1 associates with the replication processivity factor PCNA primarily in G1 phase of the cell cycle and, directly, in vitro. These results find H3K56me1 in mammals and indicate a role for H3K56me1 as a chromatin docking site for PCNA prior to its function in DNA replication.
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