Mechanism of heterochromatin assembly

描述(由申请人提供)： 项目摘要组蛋白的共价修饰，如乙酰化、甲基化、磷酸化和泛素化，是染色质结构和功能的重要调节因素。这些修饰的调节缺陷在许多发育障碍和疾病中起到了因果作用。然而，将组蛋白修饰酶定位于特定的基因组位置并调节其酶活性的机制尚不清楚。我们的长期目标是了解不同的组蛋白修饰活动如何协调以启动和维持不同的表观遗传状态。异染色质优先组装在重复的DNA元件上。它对于在发育过程中建立基因表达模式和通过使重复结构重组惰性来维持基因组完整性至关重要。它对着丝粒和端粒等重要染色体结构的功能组织也是至关重要的，确保有丝分裂和减数分裂过程中遗传物质的准确分离。异染色质的形成需要多种组蛋白修饰酶的协同作用以及RNA干扰(RNAi)机制。矛盾的是，尽管异染色质通常抑制转录，但适当的异染色质组装也需要潜在重复DNA元件的转录。这些转录本不仅作为RNAi产生小干扰RNAs(SiRNAs)的底物，也作为通过含有siRNA的效应复合体招募组蛋白修饰酶的平台。组蛋白的修饰反过来又稳定了RNAi机制与异染色质的结合。因此，RNAi对转录本的处理和组蛋白修饰活性的招募是紧密耦合的，使得识别针对异染色质组装的重复区域的初始信号变得困难。我们最近发现，一些因子的丢失允许细胞绕过RNAi机制对异染色质组装的要求。我们对其中一类因子Mst2组蛋白乙酰转移酶复合体的研究表明，在DNA复制过程中减少RNA聚合酶向异染色质的募集对于异染色质状态的世代遗传至关重要。这项建议的目标是进一步了解多样化活性的丧失如何绕过异染色质组装的RNAi要求。这些分析将提供如何调节异染色质促进活性以控制异染色质结构域的启动、扩散和维持的分子机制。

DESCRIPTION (provided by applicant): Project Summary Covalent modifications of histones, such as acetylation, methylation, phosphorylation, and ubiquitylation, are essential regulators of chromatin structure and function. Defects in the regulation of these modifications have causal roles in numerous developmental disorders and diseases. However, the mechanisms that target histone-modifying enzymes to specific genomic locations and regulate their enzymatic activities are not well understood. Our long-term goal is to understand how diverse histone modification activities are coordinated to initiate and maintain different epigenetic states. Heterochromatin preferentially assembles at repetitive DNA elements. It is critical for setting up gene expression patterns during development and maintaining genome integrity by rendering repetitive structures recombinationally inert. It is also crucial for functional organization of vital chromosomal structures such as centromeres and telomeres, ensuring the accurate segregation of genetic material during mitosis and meiosis. Heterochromatin formation requires the concerted actions of diverse histone modifying enzymes as well as the RNA interference (RNAi) machinery. Paradoxically, transcription of the underlying repetitive DNA elements is also required for proper heterochromatin assembly, although heterochromatin generally represses transcription. These transcripts not only serve as substrates for RNAi to produce small interfering RNAs (siRNAs), but also as a platform for the recruitment of histone-modifying enzymes through siRNA-containing effector complexes. Histone modifications in turn stabilize the binding of the RNAi machinery to heterochromatin. As a result, processing of transcripts by RNAi and recruitment of histone modifying activities are tightly coupled, making it difficult to identify the initial signas that target repetitive regions for heterochromatin assembly. We have recently discovered that loss of a number of factors allows cells to bypass the requirement of the RNAi machinery for heterochromatin assembly. Our studies of the one class of these factors, the Mst2 histone acetyltransferase complex, revealed that reducing RNA polymerase recruitment to heterochromatin during DNA replication is essential for the inheritance of the heterochromatic state through generations. The goal of this proposal is to further understand how loss of diverse activities could bypass the RNAi requirement for heterochromatin assembly. These analyses will provide molecular mechanisms of how heterochromatin-promoting activities are regulated to control the initiation, spreading, and maintenance of heterochromatin domains.