Johnson Aaron, Li Geng, Sikorski Timothy W, Buratowski Stephen, Woodcock Christopher L, Moazed Danesh
Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
Mol Cell. 2009 Sep 24;35(6):769-81. doi: 10.1016/j.molcel.2009.07.030.
Heterochromatin assembly in budding yeast requires the SIR complex, which contains the NAD-dependent deacetylase Sir2 and the Sir3 and Sir4 proteins. Sir3 binds to nucleosomes containing deacetylated histone H4 lysine 16 (H4K16) and, with Sir4, promotes spreading of Sir2 and deacetylation along the chromatin fiber. Combined action of histone modifying and binding activities is a conserved hallmark of heterochromatin, but the relative contribution of each activity to silencing has remained unclear. Here, we reconstitute SIR-chromatin complexes using purified components and show that the SIR complex efficiently deacetylates chromatin templates and promotes the assembly of altered structures that silence Gal4-VP16-activated transcription. Silencing requires all three Sir proteins, even with fully deacetylated chromatin, and involves the specific association of Sir3 with deacetylated H4K16. These results define a minimal set of components that mediate heterochromatic gene silencing and demonstrate distinct contributions for histone deacetylation and nucleosome binding in the silencing mechanism.
芽殖酵母中的异染色质组装需要SIR复合物，该复合物包含NAD依赖的去乙酰化酶Sir2以及Sir3和Sir4蛋白。Sir3与含有去乙酰化组蛋白H4赖氨酸16（H4K16）的核小体结合，并与Sir4一起促进Sir2沿着染色质纤维的扩散以及去乙酰化作用。组蛋白修饰和结合活性的联合作用是异染色质的一个保守特征，但每种活性对沉默作用的相对贡献仍不清楚。在这里，我们使用纯化的组分重建了SIR-染色质复合物，并表明SIR复合物能有效地使染色质模板去乙酰化，并促进改变结构的组装，从而沉默Gal4-VP16激活的转录。即使染色质完全去乙酰化，沉默也需要所有三种Sir蛋白，并且涉及Sir3与去乙酰化的H4K16的特异性结合。这些结果定义了介导异染色质基因沉默的一组最小组分，并证明了组蛋白去乙酰化和核小体结合在沉默机制中的不同贡献。
