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Molecular and Cellular Biology, December 2001, p. 8082-8094, Vol. 21, No. 23
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.23.8082-8094.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Multiple Interactions in Sir Protein Recruitment by
Rap1p at Silencers and Telomeres in Yeast
Paolo
Moretti1,
and
David
Shore1,2,*
Department of Microbiology, College of
Physicians & Surgeons of Columbia University, New York, New York
10032,1 and Department of Molecular
Biology, Sciences II, University of Geneva, 1211 Geneva 4, Switzerland2
Received 14 June 2001/Returned for modification 13 July
2001/Accepted 28 August 2001
Initiation of transcriptional silencing at mating type loci
and telomeres in Saccharomyces cerevisiae
requires the recruitment of a Sir2/3/4 (silent information regulator)
protein complex to the chromosome, which occurs at least in part
through its association with the silencer- and telomere-binding protein
Rap1p. Sir3p and Sir4p are structural components of silent chromatin
that can self-associate, interact with each other, and bind to the
amino-terminal tails of histones H3 and H4. We have identified a
small region of Sir3p between amino acids 455 and 481 that is
necessary and sufficient for association with the carboxyl terminus of
Rap1p but not required for Sir complex formation or histone binding.
SIR3 mutations that delete this region cause a silencing
defect at HMR and telomeres. However, this
impairment of repression is considerably less than that displayed by
Rap1p carboxy-terminal truncations that are defective in Sir3p binding.
This difference may be explained by the ability of the Rap1p
carboxyl terminus to interact independently with Sir4p, which we
demonstrate by in vitro binding and two-hybrid assays.
Significantly, the Rap1p-Sir4p two-hybrid interaction does not require
Sir3p and is abolished by mutation of the carboxyl terminus of Rap1p.
We propose that both Sir3p and Sir4p can directly and independently
bind to Rap1p at mating type silencers and telomeres and suggest that
Rap1p-mediated recruitment of Sir proteins operates through
multiple cooperative interactions, at least some of which are
redundant. The physical separation of the Rap1p interaction region of
Sir3p from parts of the protein required for Sir complex formation and
histone binding raises the possibility that Rap1p can participate
directly in the maintenance of silent chromatin through the
stabilization of Sir complex-nucleosome interactions.
*
Corresponding author. Mailing address: Department of
Molecular Biology, Sciences II, University of Geneva, 30, Quai
Ernest-Ansermet, 1211 Geneva 4, Switzerland. Phone: 41 22 702 6183. E-mail: David.Shore{at}molbio.unige.ch.

Present address: Department of Molecular and Human Genetics, Baylor
College of Medicine, Houston, TX
77030.
Molecular and Cellular Biology, December 2001, p. 8082-8094, Vol. 21, No. 23
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.23.8082-8094.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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