The Ezh2 protein endows the Polycomb PRC2 and PRC3 complexes with histone lysine 
methyltransferase (HKMT) activity that is associated with transcriptional 
repression. We report that Ezh2 expression was developmentally regulated in the 
myotome compartment of mouse somites and that its down-regulation coincided with 
activation of muscle gene expression and differentiation of 
satellite-cell-derived myoblasts. Increased Ezh2 expression inhibited muscle 
differentiation, and this property was conferred by its SET domain, required for 
the HKMT activity. In undifferentiated myoblasts, endogenous Ezh2 was associated 
with the transcriptional regulator YY1. Both Ezh2 and YY1 were detected, with 
the deacetylase HDAC1, at genomic regions of silent muscle-specific genes. Their 
presence correlated with methylation of K27 of histone H3. YY1 was required for 
Ezh2 binding because RNA interference of YY1 abrogated chromatin recruitment of 
Ezh2 and prevented H3-K27 methylation. Upon gene activation, Ezh2, HDAC1, and 
YY1 dissociated from muscle loci, H3-K27 became hypomethylated and MyoD and SRF 
were recruited to the chromatin. These findings suggest the existence of a 
two-step activation mechanism whereby removal of H3-K27 methylation, conferred 
by an active Ezh2-containing protein complex, followed by recruitment of 
positive transcriptional regulators at discrete genomic loci are required to 
promote muscle gene expression and cell differentiation.