Sherman J M, Stone E M, Freeman-Cook L L, Brachmann C B, Boeke J D, Pillus L
Department of Molecular, Cellular, and Developmental Biology, Porter Biosciences, University of Colorado, Boulder, Colorado 80309-0347, USA.
Mol Biol Cell. 1999 Sep;10(9):3045-59. doi: 10.1091/mbc.10.9.3045.
Silencing is a universal form of transcriptional regulation in which regions of the genome are reversibly inactivated by changes in chromatin structure. Sir2 (Silent Information Regulator) protein is unique among the silencing factors in Saccharomyces cerevisiae because it silences the rDNA as well as the silent mating-type loci and telomeres. Discovery of a gene family of Homologues of Sir Two (HSTs) in organisms from bacteria to humans suggests that SIR2's silencing mechanism might be conserved. The Sir2 and Hst proteins share a core domain, which includes two diagnostic sequence motifs of unknown function as well as four cysteines of a putative zinc finger. We demonstrate by mutational analyses that the conserved core and each of its motifs are essential for Sir2p silencing. Chimeras between Sir2p and a human Sir2 homologue (hSir2Ap) indicate that this human protein's core can substitute for that of Sir2p, implicating the core as a silencing domain. Immunofluorescence studies reveal partially disrupted localization, accounting for the yeast-human chimeras' ability to function at only a subset of Sir2p's target loci. Together, these results support a model for the involvement of distinct Sir2p-containing complexes in HM/telomeric and rDNA silencing and that HST family members, including the widely expressed hSir2A, may perform evolutionarily conserved functions.
基因沉默是一种普遍的转录调控形式，基因组区域通过染色质结构的变化被可逆性失活。沉默信息调节因子2（Sir2）蛋白在酿酒酵母的沉默因子中独具特色，因为它能使核糖体DNA（rDNA）以及沉默交配型基因座和端粒沉默。在从细菌到人类的生物体中发现了Sir2同源物基因家族（HSTs），这表明Sir2的沉默机制可能是保守的。Sir2和Hst蛋白共享一个核心结构域，该结构域包括两个功能未知的诊断性序列基序以及一个假定锌指结构的四个半胱氨酸。我们通过突变分析证明，保守的核心及其每个基序对于Sir2p的沉默作用至关重要。Sir2p与人类Sir2同源物（hSir2Ap）之间的嵌合体表明，这种人类蛋白的核心可以替代Sir2p的核心，这意味着该核心是一个沉默结构域。免疫荧光研究揭示了定位的部分破坏，这解释了酵母 - 人类嵌合体仅在Sir2p靶基因座的一个子集中发挥作用的能力。总之，这些结果支持了一个模型，即不同的含Sir2p复合物参与了HM/端粒和rDNA沉默，并且包括广泛表达的hSir2A在内的HST家族成员可能执行进化上保守的功能。
