Expanded Roles of the Origin Recognition Complex in the Architecture and Function of Silenced Chromatin in Saccharomyces cerevisiae

Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3220, USA.
Molecular and Cellular Biology (Impact Factor: 4.78). 11/2009; 30(3):626-39. DOI: 10.1128/MCB.00614-09
Source: PubMed


The silenced chromatin at the cryptic mating-type loci (HML and HMR) of Saccharomyces cerevisiae requires a cell cycle event between early S phase and G(2)/M phase to achieve repression. Although DNA replication per se is not essential for silencing, mutations in many of the proteins involved in DNA replication affect silencing. Each of the four silencers, which flank the silenced loci, includes an origin recognition complex (ORC) binding site (ACS). ORC directly interacted with Sir1 and recruits Sir1 to the silencers. This study describes additional roles for ORC in the architecture of silenced chromatin. Using chromatin immunoprecipitation (ChIP) analysis, we found that ORC physically interacts throughout the internal regions of HMR as well as with silencers. This interaction depended on the presence of Sir proteins and, in part, on the HMR-I silencer. ORC remained associated with the internal regions of HMR even when these regions were recombinationally separated from the silencers. Moreover, ORC could be recruited to the silencers lacking an ACS through its Sir1 interaction.

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Available from: Bilge Ozaydin, Oct 31, 2014
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    • "Thus, Sir1 and ORC serve to nucleate the remaining Sir proteins which spread across the locus, resulting in hypoacetylation and structural changes in the chromatin leading to the loss of transcription. Recent high-resolution ChIP studies from the Rine laboratory suggest that ORC may have a more extensive role in silencing and perhaps contribute to the local chromatin structure and organization of silenced sequences (Özaydin and Rine, 2010). "
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    • "Alternatively, in light of a recent study showing that ORC binding spread throughout the HMR silent domain rather than being restricted to the HMR silencers (52), it is also possible that ORC similarly binds HML beyond the silencer, and that this binding, and thus the contribution of ORC to silencing, is not abrogated by orc2-1 and orc5-1. "
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    • "Transformants were selected by auxotrophic complementation and the functionality of the recombinant strains analyzed for testing their ability to produce CO2 in high-sucrose LD. However, only overexpression of CAF16 and ORC2, two of the six transcriptional factor-encoding genes [14-16], identified as specifically induced in high-sucrose LD (additional file 2), had significant positive effects on leavening activity of baker's yeast cells (Table 3 and Figure 3, control, 0 days). Because of this, only strains overexpressing these two genes were further characterized. "
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