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Molecular and Cellular Biology, May 2005, p. 3997-4009, Vol. 25, No. 10
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.10.3997-4009.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

MyoD Targets Chromatin Remodeling Complexes to the Myogenin Locus Prior to Forming a Stable DNA-Bound Complex{dagger}

Ivana L. de la Serna,1,{ddagger} Yasuyuki Ohkawa,1,{ddagger} Charlotte A. Berkes,2 Donald A. Bergstrom,2,3 Caroline S. Dacwag,1 Stephen J. Tapscott,2,3 and Anthony N. Imbalzano1*

Department of Cell Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01655,1 Division of Human Biology, Fred Hutchinson Cancer Research Center,2 Department of Pathology, University of Washington School of Medicine, Seattle, Washington 981093

Received 12 January 2005/ Returned for modification 13 February 2005/ Accepted 19 February 2005

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.

* Corresponding author. Mailing address: University of Massachusetts Medical School, Department of Cell Biology, 55 Lake Avenue North, Worcester, MA 01655. Phone: (508) 856-1029. Fax: (508) 856-5612. E-mail: anthony.imbalzano{at}umassmed.edu.

{dagger} Supplemental material for this article may be found at http://mcb.asm.org/.

{ddagger} Equal contributors.

Molecular and Cellular Biology, May 2005, p. 3997-4009, Vol. 25, No. 10
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.10.3997-4009.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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