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Molecular and Cellular Biology, October 2006, p. 7616-7631, Vol. 26, No. 20
0270-7306/06/$08.00+0     doi:10.1128/MCB.01082-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Sir3 C-Terminal Domain Involvement in the Initiation and Spreading of Heterochromatin{triangledown}

Hungjiun Liaw and Arthur J. Lustig*

Department of Biochemistry, Tulane University Health Sciences Center, New Orleans, Louisiana

Received 15 June 2006/ Returned for modification 20 July 2006/ Accepted 31 July 2006

Heterochromatin is nucleated at a specific site and subsequently spreads into distal sequences through multiple interactions between modified histones and nonhistone proteins. In the yeast Saccharomyces cerevisiae, these nonhistone proteins include Sir2, Sir3, and Sir4. We have previously shown that loss of the C-terminal Rap1 domain containing Sir3 and Sir4 association sites can be overcome by tethering a 144-amino-acid C-terminal domain (CTD) of Sir3 adjacent to the telomere. Here, we explore the substructure and functions of the CTD. We demonstrate that the CTD is the minimum domain for Sir3 homodimerization, a function that is conserved in related yeasts. However, CTD heterodimers associate at only low efficiencies and correspondingly have low levels of tethered silencing, consistent with an essential role for dimerization in tethered silencing. Six missense alleles were generated, with ctd-Y964A producing the most extreme phenotypes when tethered to the LexA binding sites. Although ctd-Y964A is capable of dimerization, telomere silencing is abrogated, indicating that the CTD serves a second essential function in silencing. Chromatin immunoprecipitation analyses of wild-type and ctd-Y964A mutant cells indicate an association of the CTD with the deacetylated histone tails of H3 and H4 that is necessary for the recruitment of Sir3. The efficiency of spreading depends upon the apparent stoichiometry and stability during the initiation event. The predicted Cdc6 domain III winged-helix structure may well be responsible for dimerization.

* Corresponding author. Mailing address: Tulane University Health Sciences Center, 1430 Tulane Avenue, New Orleans, LA 70112. Phone: (504) 988-3688. Fax: (504) 988-3687. E-mail: alustig{at}tulane.edu.

{triangledown} Published ahead of print on 14 August 2006.

Molecular and Cellular Biology, October 2006, p. 7616-7631, Vol. 26, No. 20
0270-7306/06/$08.00+0     doi:10.1128/MCB.01082-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Buchberger, J. R., Onishi, M., Li, G., Seebacher, J., Rudner, A. D., Gygi, S. P., Moazed, D. (2008). Sir3-Nucleosome Interactions in Spreading of Silent Chromatin in Saccharomyces cerevisiae. Mol. Cell. Biol. 28: 6903-6918 [Abstract] [Full Text]  


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