Nuclear Organization of RNA Polymerase III Transcription

描述（由申请人提供）：遗传信息通常以染色体上的线性阵列表示，但核DNA以高度紧凑的形式存在，其中对单个基因的访问必须被视为三维问题。在几种真核生物中有限数量的基因座的细胞学证据表明，至少一些基因的定位与基因的转录活性有关，并且这种定位可以响应代谢和发育信号而改变。利用出芽酵母（Saccharomyces cerevisiae），我们最近发现275个tRNA基因虽然广泛分布在线性基因组中，但在整个细胞周期中都聚集在核核附近。tRNA基因的这种聚类，虽然在RNA聚合酶III （pol III）转录调控和有序的tRNA前加工途径方面具有潜在的优势，但对酵母基因组的空间组织具有显著的意义。聚类是pol III转录依赖的，影响tRNA基因之间的重组和附近RNA聚合酶II的转录。在初步的研究中，我们现在已经表明核仁tRNA基因定位是一个两步的过程，tRNA基因的凝聚依赖的集群可与微管依赖的集群定位到核仁分开。该项目的近期目标是利用分子遗传学工具，表征酵母中单个tRNA基因的大规模组织机制。

DESCRIPTION (provided by applicant): Genetic information is usually represented as linear arrays on chromosomes, yet nuclear DNA exists in a highly compacted form in which access to individual genes must be considered as a three-dimensional problem. Cytological evidence for a limited number of loci in several eukaryotes has suggested that localization of at least some genes is related to the transcriptional activity of the genes, and that this positioning can change in response to metabolic and developmental signals. Using the budding yeast, Saccharomyces cerevisiae, we have recently shown that the 275 tRNA genes, although widely scattered in the linear genome, are gathered near the nucleolus throughout the cell cycle. This clustering of the tRNA genes, while potentially advantageous in terms of RNA polymerase III (pol III) transcriptional regulation and an ordered pre-Trna processing pathway, has striking implications for spatial organization of much of the yeast genome. The clustering is pol III transcription-dependent, and affects both recombinations between tRNA genes and nearby transcription by RNA polymerase II. In preliminary studies we have now shown that nucleolar tRNA gene localization is a two step process, with condensin-dependent clustering of the tRNA genes separable from microtubule-dependent localization of the clusters to the nucleolus. The near-term goal of this project is to characterize the mechanisms of this large-scale organization of the individual tRNA genes in yeast, to take advantage of the molecular genetic tools. In the longer term we will also investigate whether this phenomenon affects the behavior of RNA polymerase III transcription units in mammals. Genomes from humans and other vertebrates contain not only hundreds of tRNA genes, but hundreds of thousands of short interspersed DNA elements (SINEs) with tRNA-class promoters. Some of these short interspersed DNA elements (SINEs) are known to influence both nearby transcription by RNA polymerase II and recombination events that are associated with developmental abnormalities and cancers. We anticipate that understanding the spatial behavior of pol III transcription units could provide broad insights into constraints on genome organization and regulation. PUBLIC HEALTH RELEVANCE: DNA in living cells is extremely condensed and packaged in a highly organized fashion. This packaging facilitates regulated retrieval of genetic information and affects DNA recombination events that can lead to cancers and developmental abnormalities. We have identified a type of abundant DNA element, distributed throughout genomes of higher cells, that both encodes small RNAs and serves as a three-dimensional organization signal. We are investigating the molecular mechanisms of this organization and the nature of the resulting DNA packaging.