Zhang Ke, Mosch Kerstin, Fischle Wolfgang, Grewal Shiv I S
Laboratory of Biochemistry and Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Nat Struct Mol Biol. 2008 Apr;15(4):381-8. doi: 10.1038/nsmb.1406. Epub 2008 Mar 16.
Heterochromatin assembly, involving methylation of histone H3 lysine 9 (H3K9me), regulates various chromosomal processes. In fission yeast, heterochromatin targeted to specific repeat loci in an RNAi-dependent manner spreads across extended domains to exert regional epigenetic control. The Clr4 methyltransferase complex (ClrC) is responsible for nucleation and spreading of heterochromatin; however, its recruitment to heterochromatic repeats is poorly understood. Here we demonstrate that ClrC components are distributed throughout heterochromatic domains. To nucleate heterochromatin, Rik1, a WD domain-containing subunit of ClrC, is loaded onto the transcribed repeats via RNAi machinery including the RNA-induced transcriptional silencing (RITS) complex. Furthermore, we show that the chromodomain of Clr4 binds specifically to H3K9me that is essential for the spreading of heterochromatin. Our analyses delineate sequential steps for the assembly of heterochromatic domains and suggest that the ability of Clr4 to both 'write' and 'read' H3K9me facilitates heterochromatin maintenance through successive cell divisions.
异染色质组装涉及组蛋白H3赖氨酸9（H3K9me）的甲基化，调控各种染色体过程。在裂殖酵母中，以RNA干扰依赖的方式靶向特定重复序列位点的异染色质会在扩展区域扩散，以施加区域表观遗传控制。Clr4甲基转移酶复合物（ClrC）负责异染色质的成核和扩散；然而，其被招募到异染色质重复序列的机制尚不清楚。在这里，我们证明ClrC组分分布于整个异染色质区域。为了使异染色质成核，ClrC中一个含WD结构域的亚基Rik1通过包括RNA诱导转录沉默（RITS）复合物在内的RNA干扰机制被加载到转录重复序列上。此外，我们表明Clr4的染色体结构域特异性结合H3K9me，这对异染色质的扩散至关重要。我们的分析描绘了异染色质区域组装的连续步骤，并表明Clr4“写入”和“读取”H3K9me的能力有助于异染色质在连续细胞分裂中得以维持。
