Enhancer-associated H3K4 mono-methylation by Trithorax-related. Hans-Martin Herz¹, Man Mohan¹, Alexander S. Garruss¹, Kaiwei Liang¹, Yoh-hei Takahashi¹, Kristen Mickey¹, Olaf Voets², C. Peter Verrijzer², Ali Shilatifard¹. 1) Shilatifard lab, Stowers Institute for Medical Research, Kansas City, MO; 2) Department of Biochemistry and Center for Biomedical Genetics, Erasmus University Medical Center, Rotterdam, The Netherlands.

   Mono-methylation of histone H3 on lysine 4 (H3K4me1) and acetylation of histone H3 on lysine 27 (H3K27ac) are histone modifications that are highly enriched over the body of actively transcribed genes and on enhancers. Although in yeast all H3K4 methylation patterns including H3K4me1 are implemented by Set1/COMPASS (complex of proteins associated with Set1), there are three classes of COMPASS-like complexes in Drosophila that could carry out H3K4me1 on enhancers: dSet1, Trithorax, and Trithorax-related (Trr). Here, we report that Trr, the Drosophila homolog of the mammalian Mll3/Mll4 COMPASS-like complexes, can function as a major H3K4 mono-methyltransferase on enhancers in vivo. Loss of Trr results in a global decrease of H3K4me1 and H3K27ac levels in various tissues. Assays with the cut wing margin enhancer imply a functional role for Trr in enhancer-mediated processes. A genome-wide analysis demonstrates that Trr is required to maintain the H3K4me1 and H3K27ac chromatin signature that resembles the histone modification patterns described for enhancers. Since the Trr complex is distinguished by bearing a unique subunit, the H3K27 demethylase, dUTX, we propose a model in which the H3K4 mono-methyltransferase, Trr, and the H3K27 demethylase, dUTX, cooperate to regulate the transition from inactive/poised to active enhancers.