Department of Genetics, Harvard Medical School, 77 Avenue Louis Pasteur, Boston, MA 02115, USA.
Mol Cell Biol. 2011 Oct;31(20):4193-204. doi: 10.1128/MCB.05568-11. Epub 2011 Aug 15.
Spt6 is a conserved factor, critically required for several transcription- and chromatin-related processes. We now show that Spt6 and its binding partner, Iws1, are required for heterochromatic silencing in Schizosaccharomyces pombe. Our studies demonstrate that Spt6 is required for silencing of all heterochromatic loci and that an spt6 mutant has an unusual combination of heterochromatic phenotypes compared to previously studied silencing mutants. Unexpectedly, we find normal nucleosome positioning over heterochromatin and normal levels of histone H3K9 dimethylation at the endogenous pericentric repeats. However, we also find greatly reduced levels of H3K9 trimethylation, elevated levels of H3K14 acetylation, reduced recruitment of several silencing factors, and defects in heterochromatin spreading. Our evidence suggests that Spt6 plays a role at both the transcriptional and posttranscriptional levels; in an spt6 mutant, RNA polymerase II (RNAPII) occupancy at the pericentric regions is only modestly increased, while production of small interfering RNAs (siRNAs) is lost. Taken together, our results suggest that Spt6 is required for multiple steps in heterochromatic silencing by controlling chromatin, transcriptional, and posttranscriptional processes.
Spt6 是一种保守的因子，对于几个转录和染色质相关的过程至关重要。我们现在表明，Spt6 和它的结合伙伴 Iws1，在裂殖酵母中对于异染色质沉默是必需的。我们的研究表明，Spt6 对于所有异染色质位点的沉默都是必需的，而且与之前研究的沉默突变体相比，spt6 突变体具有异常的异染色质表型组合。出乎意料的是，我们发现异染色质上的核小体定位正常，组蛋白 H3K9 二甲基化水平正常，但 H3K9 三甲基化水平大大降低，H3K14 乙酰化水平升高，几个沉默因子的募集减少，异染色质扩展缺陷。我们的证据表明，Spt6 在转录和转录后水平都发挥作用；在 spt6 突变体中，着丝粒区域的 RNA 聚合酶 II（RNAPII）占有率仅略有增加，而小干扰 RNA（siRNA）的产生则丢失。总之，我们的结果表明，Spt6 通过控制染色质、转录和转录后过程，在异染色质沉默的多个步骤中是必需的。
