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Fig. 5. Myod targets chromatin-remodeling complexes to the Myog promoter. (A) In
the undifferentiated myoblast, a nucleosome (gray) is likely to be positioned
over the E-box and the binding sites for Mef2 and Six factors, based on the
limited access that restriction endonucleases have to this region
(Gerber et al., 1997);
however, Pbx/Meis is bound even in the presence of the nucleosome
(Berkes et al., 2004).
Chromatin immunoprecipitation (ChIP) analysis indicates that Myod (MD)
recruits an Hdac to the Myog promoter in myoblasts
(Mal and Harter, 2003),
possibly by interacting with the Pbx/Meis complex. Id proteins are expressed
in the myoblast and dimerize with E-proteins, preventing the formation of
Myod/E-protein heterodimers (Jen et al.,
1992). Mef2 isoforms are present but are probably not bound to the
Myog promoter in the myoblast (de
La Serna et al., 2005), and the same is likely to be true for the
Six proteins. (B) Early on during differentiation, Id levels decrease, leading
to the formation of Myod/E-protein heterodimers that interact with the
Pbx/Meis complex at the Myog promoter
(Berkes et al., 2004;
de La Serna et al., 2005).
Myod recruits HATs and the Swi/Snf complex
(de La Serna et al., 2005;
Simone et al., 2004), which
acetylate the histones and remodel the nucleosome, respectively. (C) The
remodeling of the nucleosome permits Mef2 and Six protein isoforms to access
their cognate sites and the stable binding of Myod to its E-box. In this
model, therefore, Myod-directed chromatin remodeling must occur before Myod
and other factors can access their cognate sites in the promoter. E,
E-protein; Ac, acetylation of histone tail; E-box, Myod-binding site; HATs,
histone acetyltransferases; Hdac, histone deacetylase.
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