Figure 7.

Two-step activation model of muscle gene expression. Regulatory regions of certain muscle-specific genes are occupied by a protein complex containing the DNA-binding protein YY1, the methyltransferase Ezh2, and the deacetylase HDAC1. Deacetylation of lysine residues by HDAC1 and di-/trimethylation of H3-K27 by Ezh2 actively prevent transcription (repressed state). At the onset of transcriptional activation, YY1 is displaced from the chromatin—with consequent loss of Ezh2 and HDAC1—and replaced by SRF. H3-K27 becomes hypomethylated, and loading of the MyoD family of transcription factors allows engagement of histone acetyltransferases (HATs) and permits initiation of transcription (activated state).