Enhancers act to regulate cell type specific gene expression by facilitating the transcription of target genes. In mammalian cells active or primed enhancers are commonly marked by monomethylation of Histone H3 at lysine 4 (H3K4me1) in a cell-type specific manner. Whether and how this histone modification regulates enhancer-dependent transcription programs in mammals is unclear. In this study, we conducted SILAC Mass-spec experiments with mono-nucleosomes and identified multiple H3K4me1 associated proteins, including many involved in chromatin remodeling. We demonstrate that H3K4me1 augments the association of the chromatin remodeling complex BAF to enhancers in vivo and that in vitro, H3K4me1 nucleosomes are more efficiently remodeled by the BAF complex. Crystal structures of BAF component BAF45c reveal that monomethylation, but not trimethylation, is accommodated by BAF45c’s H3K4 binding site. Our results suggest that H3K4me1 plays an active role at enhancers by facilitating the binding of the BAF complex and possibly other chromatin regulators.

We apologize for an error that we just found in the paper published online on January 9th, 2018. The 2nd column/6th row of Figure 5B (Hoxd13-DKO) was inadvertently duplicated from 2nd column/5th row of Figure 5B (Hoxd13-WT). A corrected version of Figure 5B is provided below. No conclusion was affected by this error, but we apologize for not detecting it before publication.We also regret for the failure to acknowledge the technical support by Ms Rong Hu towards the generation of ChIP-seq of Rad21 in wild type and dCD MLL3/4 mutant mouse ES cells. We would like to add the following statement in the section of Acknowledgments: "We also thank Ms Rong Hu for her technical assistance in ChIPseq experiments."