2 DOMAINS OF MYOD MEDIATE TRANSCRIPTIONAL ACTIVATION OF GENES IN REPRESSIVE CHROMATIN - A MECHANISM FOR LINEAGE DETERMINATION IN MYOGENESIS

Genetic studies have demonstrated that MyoD and Myf5 establish the ske letal muscle lineage, whereas myogenin mediates terminal differentiati on, yet the molecular basis for this distinction is not understood. We show that MyoD can remodel chromatin at binding sites in muscle gene enhancers and activate transcription at previously silent loci. TGF-be ta, basic-FGF, and sodium butyrate blocked MyoD-mediated chromatin reo rganization and the initiation of transcription. In contrast, TGF-beta and sodium butyrate did not block transcription when added after chro matin remodeling had occurred. MyoD and Myf-5 were 10-fold more effici ent than myogenin at activating genes in regions of transcriptionally silent chromatin. Deletion mutagenesis of the MyoD protein demonstrate d that the ability to activate endogenous genes depended on two region s: a region rich in cysteine and histidine residues between the acidic activation domain and the bHLH domain, and a second region in the car boxyl terminus of the protein. Neither region has been shown previousl y to regulate gene transcription and both have domains that are conser ved in the Myf5 protein. Our results establish a mechanism for chromat in modeling in the skeletal muscle lineage and define domains of MyoD, independent of the activation domain, that participate in chromatin r eorganization.