Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3220, USA.
Mol Cell Biol. 2010 Feb;30(3):626-39. doi: 10.1128/MCB.00614-09. Epub 2009 Nov 30.
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.
酿酒酵母隐秘交配型位点（HML 和 HMR）的沉默染色质需要在早期 S 期和 G2/M 期之间的细胞周期事件来实现抑制。尽管 DNA 复制本身对于沉默不是必需的，但许多参与 DNA 复制的蛋白质的突变会影响沉默。每个侧翼沉默位点的四个沉默子都包括一个原点识别复合物（ORC）结合位点（ACS）。ORC 直接与 Sir1 相互作用，并将 Sir1 招募到沉默子上。本研究描述了 ORC 在沉默染色质结构中的其他作用。通过染色质免疫沉淀（ChIP）分析，我们发现 ORC 在 HMR 的内部区域以及与沉默子之间存在物理相互作用。这种相互作用依赖于 Sir 蛋白的存在，部分依赖于 HMR-I 沉默子。即使这些区域与沉默子发生重组分离，ORC 仍与 HMR 的内部区域保持关联。此外，ORC 可以通过与 Sir1 的相互作用被招募到缺乏 ACS 的沉默子上。
