Distribution of a limited Sir2 protein pool regulates the strength of yeast rDNA silencing and is modulated by Sir4p.
Distribution of a limited Sir2 protein pool regulates the strength of yeast rDNA silencing and is modulated by Sir4p.
复制标题
有限 Sir2 蛋白池的分布调节酵母 rDNA 沉默的强度，并受 Sir4p 调节。
DOI：
10.1093/genetics/149.3.1205
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发表时间：
1998
期刊：
影响因子：
3.3
通讯作者：
Boeke,JD
中科院分区：
文献类型：
--
作者：
Smith,JS;Brachmann,CB;Pillus,L;Boeke,JD
酿酒酵母的转录沉默发生在沉默的交配型基因座HML和HMR、端粒和核糖体DNA(RDNA)基因座RDN1。RDNA中的沉默是通过一种新的机制发生的，这种机制依赖于单个沉默信息调节因子(SIR)基因Sir2。SIR4是其他沉默基因所必需的，却矛盾地抑制了rDNA沉默。在这项研究中，我们基于rDNA沉默强度与细胞Sir2蛋白水平直接相关的发现，阐明了rDNA沉默的调节机制。内源性Sir2p水平对rDNA沉默有一定的限制作用。此外，Sir2p水平的微小变化改变了rDNA沉默的强度。在rDNA沉默表型中，sir2突变对sir4突变具有上位性，表明sir4抑制rDNA沉默是通过sir2介导的。此外，rDNA沉默对SIR3的过度表达不敏感，但通过全长Sir4p或与Sir2p相互作用的Sir4p片段的过度表达而严重减少。Sir4过表达的这种负面影响被Sir2的共表达所克服，这表明Sir4直接抑制了Sir2的rDNA沉默功能。最后，先前被证明可以延长寿命的SIR4的遗传操作也导致了rDNA沉默的增强。我们提出了一个简单的模型，在这个模型中，端粒通过竞争有限数量的Sir2蛋白来调节rDNA沉默。
Transcriptional silencing in Saccharomyces cerevisiae occurs at the silent mating-type loci HML and HMR, at telomeres, and at the ribosomal DNA (rDNA) locus RDN1. Silencing in the rDNA occurs by a novel mechanism that depends on a single Silent Information Regulator (SIR) gene, SIR2. SIR4, essential for other silenced loci, paradoxically inhibits rDNA silencing. In this study, we elucidate a regulatory mechanism for rDNA silencing based on the finding that rDNA silencing strength directly correlates with cellular Sir2 protein levels. The endogenous level of Sir2p was shown to be limiting for rDNA silencing. Furthermore, small changes in Sir2p levels altered rDNA silencing strength. In rDNA silencing phenotypes, sir2 mutations were shown to be epistatic to sir4 mutations, indicating that SIR4 inhibition of rDNA silencing is mediated through SIR2. Furthermore, rDNA silencing is insensitive to SIR3 overexpression, but is severely reduced by overexpression of full-length Sir4p or a fragment of Sir4p that interacts with Sir2p. This negative effect of SIR4 overexpression was overridden by co-overexpression of SIR2, suggesting that SIR4 directly inhibits the rDNA silencing function of SIR2. Finally, genetic manipulations of SIR4 previously shown to promote extended life span also resulted in enhanced rDNA silencing. We propose a simple model in which telomeres act as regulators of rDNA silencing by competing for limiting amounts of Sir2 protein.