Figure 2. Model for epigenetic regulation of muscle promoters in undifferentiated and differentiated muscle cells. Transcriptional repression of muscle gene expression in undifferentiated cells is achieved through recruitment of HDACs (HDAC1, HDAC4/5, and SirT1) and HMTs (Suv39h1, G9a, Ezh2), that interact with MyoD, MEF2, and YY1 as indicated. HMTs mediate two signature repressive chromatin marks H3K9me2/me3 and H3K27me3 that restrict MyoD and MEF2 activity. In differentiated cells, recruitment of HATs (p300, P/CAF, SRC1, GRIP1), HMTs (Set7/9, PRMT4/5, CARM1, Ezh1; Ash2L), demethylases (JMJD2A, LSD1); and SWI/SNF Brg1/Brm chromatin remodeling complexes permit extensive reprograming of muscle promoters resulting H3K9ac and H3K14ac, and H3K4, H3R8, and H3R17 methylation that allow for an open chromatin configuration and activation of MyoD- and MEF2-dependent gene expression.