Control of mating-type switching by Sir2 and condensin

项目摘要。 自从大约40年前首次描述芽殖酵母中的交配型转换以来， 这一过程的表征导致了许多关键的进展，了解机制， 基因沉默（异染色质）、细胞命运决定（交配型）和同源重组。 例如，高度保守的NAD+依赖性蛋白质脱乙酰酶Sir 2和其他“沉默的”酶。 信息调节因子（SIR）蛋白，最初是由于它们在沉默细胞中的作用而被鉴定出来的。 异染色质HMLα和HMR α基因座，它们作为活性MATα的沉默拷贝被维持， MATa基因座。当程序性dsDNA断裂发生时， MAT基因座被称为HO的核酸内切酶所取代。然后通过同源的修复， 使用HMLα或HMR α作为模板进行重组，强烈倾向于切换到 相反的交配类型。这种转换中的“供体偏好”是由邻近的顺式作用序列指导的。 称为重组增强子（RE）。许多染色质因子与 交配型转换，但涉及染色体结构动力学的机制仍然存在 难以捉摸。在这个提议中，我们调查了一个新发现的协调“感知”的过程， HO诱导的dsDNA在MAT位点断裂，伴有必需的染色体III结构变化。内 我们鉴定了Sir 2、凝聚素和cohibin（Lrs 4/Csm 1）的一个强结合位点。 长链非编码RNA（lncRNA）基因RDT 1。这个位点维持着三号染色体的转换- 结构构象Sir 2通常抑制非转换细胞中RDT 1的转录，但 被重新分配到MAT上HO诱导的断裂位点，激活RDT 1转录并触发 配对式切换我们提出了3个具体的目标，不仅将确定RDT 1 lncRNA如何 诱导开关，但也研究了凝聚素和cohibin在程序化结构中的作用。 染色体重组III。这提供了一个独特的和明确的系统， 有丝分裂染色体压缩之外的凝聚素功能，目前该领域缺少的东西。

Project Summary. Since the first descriptions of mating-type switching in budding yeast approximately 40 years ago, characterization of this process has led to numerous key advances in understanding the mechanisms of gene silencing (heterochromatin), cell-fate determination (mating-type), and homologous recombination. For example, the highly conserved NAD+-dependent protein deacetylase, Sir2, and other “silent information regulator (SIR) proteins, were initially identified due to their roles in silencing the heterochromatic HMLα and HMRa loci, which are maintained as silenced copies of the active MATα and MATa loci, respectively. Mating-type switching occurs when a programmed dsDNA break is generated at the MAT locus by an endonuclease called HO. The break is then repaired through homologous recombination using either HMLα or HMRa as a template, with a strong preference for switching to the opposite mating type. Such “donor preference” in switching is directed by a cis-acting sequence adjacent to HMLα called the recombination enhancer (RE). Numerous chromatin factors have been implicated in mating-type switching, but mechanisms involving chromosome structural dynamics have remained elusive. In this proposal we investigate a newly discovered process of coordinating the “sensing” of an HO-induced dsDNA break at the MAT locus with requisite chromosome III structural changes. Within the RE we identified a strong binding site for Sir2, condensin, and cohibin (Lrs4/Csm1) at the promoter of a long non-coding RNA (lncRNA) gene called RDT1. This site maintains chromosome III in a switching- competent structural conformation. Sir2 normally represses RDT1 transcription in non-switching cells, but is redistributed to the HO-induced break site at MAT, which activates RDT1 transcription and triggers mating-type switching. We propose 3 specific aims that will determine not only how RDT1 lncRNA induces switching, but also investigate the roles of condensin and cohibin in the programmed structural reorganization of chromosome III. This provides a unique and well-defined system for elucidating condensin function outside of mitotic chromosome compaction, something currently missing in the field.