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J. Biol. Chem., Vol. 282, Issue 2, 1018-1028, January 12, 2007

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Inverted Factor Access and Slow Reversion Characterize SWI/SNF-altered Nucleosome Dimers^*

From the Department of Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 02111

Human SWI/SNF (hSWI/SNF) is an ATP-dependent chromatin remodeling complex with important functions in activation and repression of cellular genes. Previously, we showed that hSWI/SNF creates structurally altered dimers from mononucleosome cores. More recently we found that hSWI/SNF also generates abundant structurally altered dinucleosomes, called altosomes, on polynucleosomal templates. Here, we find that dimers revert to normal nucleosomes at a similar rate as altosomes and can also be cleaved to yield nucleosomal particles with mobilities similar to mononucleosomes. Using these and other shared properties we propose a single model for both types of hSWI/SNF product. In addition, we further characterize the accessibility of altered dimers to transcription factors, and find that the DNA in dimers is most accessible in the middle and least accessible at the ends, directly opposite the profile of normal mononucleosomes. We also find that transcription factor binding can influence the ratio of normal nucleosomes and dimers as hSWI/SNF products. Implications for the interplay between hSWI/SNF products and transcription factors are discussed.

Received for publication, October 6, 2006 , and in revised form, November 14, 2006.

^* This work was supported by grants from The Medical Foundation, National Cancer Institute, and the American Cancer Society (to G. R. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement"in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry, Tufts University School of Medicine, Boston, MA 02111. Tel.: 617-636-2441; Fax: 617-636-2409; E-mail: gavin.schnitzler{at}tufts.edu.

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