Mechanism of heterochromatin assembly

描述（由申请人提供）： 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子。这些修饰的调节缺陷在许多发育障碍和疾病中具有因果作用。然而，靶向组蛋白修饰酶的特定基因组位置和调节其酶活性的机制还没有很好地理解。我们的长期目标是了解不同的组蛋白修饰活动是如何协调启动和维持不同的表观遗传状态的。异染色质优先在重复DNA元件处组装。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。它对于重要染色体结构（如着丝粒和端粒）的功能组织也至关重要，确保有丝分裂和减数分裂期间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。有趣的是，异染色质组装也需要潜在的重复DNA元件的转录，尽管异染色质通常抑制转录。这些转录物不仅作为RNAi的底物以产生小干扰RNA（siRNA），而且作为通过含有siRNA的效应复合物募集组蛋白修饰酶的平台。组蛋白修饰反过来稳定RNAi机制与异染色质的结合。因此，通过RNAi对转录物的加工和组蛋白修饰活性的募集是紧密耦合的，使得难以鉴定靶向异染色质组装的重复区域的初始信号。我们最近发现，许多因子的丢失允许细胞绕过RNAi机制对异染色质组装的要求。我们的一类这些因素，Mst 2组蛋白乙酰转移酶复合物的研究表明，减少RNA聚合酶招聘异染色质在DNA复制是必不可少的异染色质状态通过几代人的遗传。该提案的目标是进一步了解多样性活动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动的分子机制，以控制异染色质结构域的起始，扩散和维持。

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.