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Molecular and Cellular Biology, December 2001, p. 8504-8511, Vol. 21, No. 24
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.24.8504-8511.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Direct Imaging of Human SWI/SNF-Remodeled Mono- and Polynucleosomes by Atomic Force Microscopy Employing Carbon Nanotube Tips

Gavin R. Schnitzler,1,2,* Chin Li Cheung,3 Jason H. Hafner,3,dagger Andrew J. Saurin,2 Robert E. Kingston,2 and Charles M. Lieber3,*

Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 021111; Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts 021142; and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 021383

Received 6 July 2001/Returned for modification 8 August 2001/Accepted 19 September 2001

Chromatin-remodeling complexes alter chromatin structure to facilitate, or in some cases repress, gene expression. Recent studies have suggested two potential pathways by which such regulation might occur. In the first, the remodeling complex repositions nucleosomes along DNA to open or occlude regulatory sites. In the second, the remodeling complex creates an altered dimeric form of the nucleosome that has altered accessibility to transcription factors. The extent of translational repositioning, the structure of the remodeled dimer, and the presence of dimers on remodeled polynucleosomes have been difficult to gauge by biochemical assays. To address these questions, ultrahigh-resolution carbon nanotube tip atomic force microscopy was used to examine the products of remodeling reactions carried out by the human SWI/SNF (hSWI/SNF) complex. We found that mononucleosome remodeling by hSWI/SNF resulted in a dimer of mononucleosomes in which ~60 bp of DNA is more weakly bound than in control nucleosomes. Arrays of evenly spaced nucleosomes that were positioned by 5S rRNA gene sequences were disorganized by hSWI/SNF, and this resulted in long stretches of bare DNA, as well as clusters of nucleosomes. The formation of structurally altered nucleosomes on the array is suggested by a significant increase in the fraction of closely abutting nucleosome pairs and by a general destabilization of nucleosomes on the array. These results suggest that both the repositioning and structural alteration of nucleosomes are important aspects of hSWI/SNF action on polynucleosomes.

* Corresponding author. Mailing address for G. R. Schnitzler: Department of Biochemistry, Tufts University School of Medicine, 136 Harrison Ave., Boston, MA 02111. Phone: (617) 636-2441. Fax: (617) 636-2409. E-mail: gavin.schnitzler{at}tufts.edu. Mailing address for C. M. Lieber: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, MA 02138. Phone: (617) 496-3169. Fax: (617) 496-5442. E-mail: cml{at}cmliris.harvard.edu.

dagger Present address: Department of Physics and Astronomy, Rice University, Houston, TX 77005.

Molecular and Cellular Biology, December 2001, p. 8504-8511, Vol. 21, No. 24
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.24.8504-8511.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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