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Molecular and Cellular Biology, December 2006, p. 9045-9059, Vol. 26, No. 23
0270-7306/06/$08.00+0     doi:10.1128/MCB.00248-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

The LRS and SIN Domains: Two Structurally Equivalent but Functionally Distinct Nucleosomal Surfaces Required for Transcriptional Silencing

Christopher J. Fry,^1, Anne Norris,^2, Michael Cosgrove,³ Jef D. Boeke,² and Craig L. Peterson^1*

Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts 01605,¹ Department of Molecular Biology and Genetics and High Throughput Biology Center, The Johns Hopkins University, Baltimore, Maryland 21205,² Department of Biology, Syracuse University, 406D Lyman Hall, 108 College Place, Syracuse, New York 13244³

Received 9 February 2006/ Returned for modification 3 March 2006/ Accepted 14 September 2006

Genetic experiments have identified two structurally similar nucleosomal domains, SIN and LRS, required for transcriptional repression at genes regulated by the SWI/SNF chromatin remodeling complex or for heterochromatic gene silencing, respectively. Each of these domains consists of histone H3 and H4 L1 and L2 loops that form a DNA-binding surface at either superhelical location (SHL) ±2.5 (LRS) or SHL ±0.5 (SIN). Here we show that alterations in the LRS domain do not result in Sin^– phenotypes, nor does disruption of the SIN domain lead to loss of ribosomal DNA heterochromatic gene silencing (Lrs^– phenotype). Furthermore, whereas disruption of the SIN domain eliminates intramolecular folding of nucleosomal arrays in vitro, alterations in the LRS domain have no effect on chromatin folding in vitro. In contrast to these dissimilarities, we find that the SIN and LRS domains are both required for recruitment of Sir2p and Sir4p to telomeric and silent mating type loci, suggesting that both surfaces can contribute to heterochromatin formation. Our study shows that structurally similar nucleosomal surfaces provide distinct functionalities in vivo and in vitro.

Published ahead of print on 2 October 2006.

These authors contributed equally to this work.

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