No abstract

Skeletal muscle differentiation is initiated by the transcription factor MyoD, which binds directly to the regulatory regions of genes expressed during skeletal muscle differentiation and initiates chromatin remodeling at specific promoters. It is not known, however, how MyoD initially recognizes its binding site in a chromatin context. Here we show that the H/C and helix III domains, two domains of MyoD that are necessary for the initiation of chromatin remodeling at the myogenin locus, together regulate a restricted subset of genes, including myogenin. These domains are necessary for the stable binding of MyoD to the myogenin promoter through an interaction with an adjacent protein complex containing the homeodomain protein Pbx, which appears to be constitutively bound at this site. This demonstrates a specific mechanism of targeting MyoD to loci in inactive chromatin and reveals a critical role of homeodomain proteins in marking specific genes for activation in the muscle lineage.

The activation of muscle-specific gene expression requires the coordinated action of muscle regulatory proteins and chromatin-remodeling enzymes. Microarray analysis performed in the presence or absence of a dominant-negative BRG1 ATPase demonstrated that approximately one-third of MyoD-induced genes were highly dependent on SWI/SNF enzymes. To understand the mechanism of activation, we performed chromatin immunoprecipitations analyzing the myogenin promoter. We found that H4 hyperacetylation preceded Brg1 binding in a MyoD-dependent manner but that MyoD binding occurred subsequent to H4 modification and Brg1 interaction. In the absence of functional SWI/SNF enzymes, muscle regulatory proteins did not bind to the myogenin promoter, thereby providing evidence for SWI/SNF-dependent activator binding. We observed that the homeodomain factor Pbx1, which cooperates with MyoD to stimulate myogenin expression, is constitutively bound to the myogenin promoter in a SWI/SNF-independent manner, suggesting a two-step mechanism in which MyoD initially interacts indirectly with the myogenin promoter and attracts chromatin-remodeling enzymes, which then facilitate direct binding by MyoD and other regulatory proteins.In eukaryotes, activation of gene expression involves the ordered assembly of transcriptional regulators, chromatinmodifying enzymes, RNA polymerase II, and associated general transcription factors onto cis-acting elements that are embedded in chromatin. Chromatin-remodeling enzymes play an integral role in gene activation by perturbing chromatin structure and making specific loci permissive for transcription. Molecular analysis of multiple gene activation events suggests that the temporal recruitment of transcription factors and chromatin-remodeling enzymes is gene specific and dictated by the interplay between specific activators and local chromatin structure (1,52,55).Two classes of enzymes have been shown to remodel chromatin structure either by catalyzing covalent modifications of histones or by hydrolyzing ATP to mobilize nucleosomes. Among the latter class of enzymes are the SWI/SNF chromatin-remodeling complexes. A distinguishing feature of this family is the presence of a bromodomain in the ATPase subunit, which promotes interaction with acetylated histones and links the activities of the two classes of chromatin remodelers in the regulation of gene expression (22). SWI/SNF enzymes physically interact with histone acetyltransferases (HATs), histone deacetylases (HDACs), and methyltransferases, showing the potential for coordination of chromatin-remodeling activities (reviewed in reference 53).Mammalian SWI/SNF chromatin-remodeling enzymes are multisubunit complexes that contain either the Brg1 or Brm ATPase subunits and can activate or repress expression of a subset of genes (39, 53). They function in cell cycle control, and some of the subunits are tumor suppressors (49). Diverse SWI/ SNF complexes exist that are distinguished by the particular ATPase, the presence of unique subunits, and tissue-specific ...