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Molecular and Cellular Biology, January 2009, p. 43-56, Vol. 29, No. 1
0270-7306/09/$08.00+0     doi:10.1128/MCB.00983-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

A Silencer Promotes the Assembly of Silenced Chromatin Independently of Recruitment {triangledown}

Patrick J. Lynch and Laura N. Rusche*

Institute for Genome Sciences and Policy and Department of Biochemistry, Duke University, Durham, North Carolina

Received 23 June 2008/ Returned for modification 17 July 2008/ Accepted 17 October 2008

In Saccharomyces cerevisiae, silenced chromatin occurs at telomeres and the silent mating-type loci HMR and HML. At these sites, the Sir proteins are recruited to a silencer and then associate with adjacent chromatin. We used chromatin immunoprecipitation to compare the rates of Sir protein assembly at different genomic locations and discovered that establishment of silenced chromatin was much more rapid at HMR than at the telomere VI-R. Silenced chromatin also assembled more quickly on one side of HMR-E than on the other. Despite differences in spreading, the Sir proteins were recruited to HMR-E and telomeric silencers at equivalent rates. Additionally, insertion of HMR-E adjacent to the telomere VI-R increased the rate of Sir2p association with the telomere. These data suggest that HMR-E functions to both recruit Sir proteins and promote their assembly across several kilobases. Observations that association of Sir2p occurs simultaneously throughout HMR and that silencing at HMR is insensitive to coexpression of catalytically inactive Sir2p suggest that HMR-E acts by enabling assembly to occur in a nonlinear fashion. The ability of silencers to promote assembly of silenced chromatin over several kilobases is likely an important mechanism for maintaining what would otherwise be unstable chromatin at the correct genomic locations.

* Corresponding author. Mailing address: Institute for Genome Sciences and Policy, 101 Science Drive, Box 3382, Durham, NC 27708. Phone: (919) 684-0354. Fax: (919) 668-0795. E-mail: lrusche{at}biochem.duke.edu

{triangledown} Published ahead of print on 27 October 2008.

Molecular and Cellular Biology, January 2009, p. 43-56, Vol. 29, No. 1
0270-7306/09/$08.00+0     doi:10.1128/MCB.00983-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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