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Molecular and Cellular Biology, June 2007, p. 4198-4205, Vol. 27, No. 11
0270-7306/07/$08.00+0 doi:10.1128/MCB.00238-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Institute of Molecular Biology and Biotechnology, Foundation of Research and Technology,1 Department of Biology, University of Crete, P.O. Box 1527, Vassilika Vouton, 711 10 Heraklion, Crete, Greece2
Received 9 February 2007/ Returned for modification 13 March 2007/ Accepted 15 March 2007
The SWR1 complex (SWR1-C)-dependent deposition of the histone variant Htz1 on promoter nucleosomes is typical of Saccharomyces cerevisiae genes whose expression is frequently reprogrammed. Although this epigenetic marking is of significant physiological importance, the determinants of Htz1 deposition, the conditions that set off SWR1-C occupancy, and the implications of Htz1 in transcriptional initiation are issues that remain unresolved. In this report, we addressed these questions by investigating the GAL1 promoter. We show that Htz1 is required for efficient Mediator recruitment and transcription only when the GAL1 promoter is under the influence of the Tup1 corepressor. In fact, we show that it is Tup1 that specifies Htz1 deposition for the promoter nucleosome covering the transcription start site. This deposition occurs rapidly following transcriptional repression, and it correlates with a Tup1-independent transient recruitment of the SWR1 complex. We propose that Tup1 cooperates with SWR1-C and specifies Htz1 deposition at GAL1, thereby marking the promoter for rapid neutralization from its repressive effects.
Published ahead of print on 26 March 2007.
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