Histone Modifications and Transcriptional Silencing in Chlamydomonas

真核生物的生长和发育是由它们的遗传组成和环境之间的相互作用决定的。在分子水平上，这转化为不同的基因表达，导致明显特化的细胞、组织和/或器官。基因表达或抑制的调节通常需要修改染色质，染色质是一种复杂的结构，由包裹在组蛋白八聚体(核小体)周围的DNA组成，作为基本的重复单位。翻译后的组蛋白修饰(如甲基化和磷酸化)在确定与基因表达或抑制相关的染色质状态中起着重要作用。这些共价修饰可以有丝分裂地传播，导致可遗传的表观遗传状态。然而，关于组蛋白甲基化或磷酸化在光合作用真核生物中常染色质基因调控中的作用，人们知之甚少。即使在后生动物和真菌系统中，组蛋白修饰在调节染色质中的功能已经被更广泛地研究，仍然有许多未解决的观察结果，有时甚至是令人困惑的观察结果。本项目将重点研究核心组蛋白甲基化和磷酸化在绿藻衣藻和高等植物拟南芥中常染色质转录沉默中的作用和分子机制。初步研究结果表明，单甲基组蛋白H3赖氨酸4(H3K4)似乎是一个常染色质沉默标志，而二甲基和三甲基H3K4与转录活性染色质相关。这项研究将研究通过H3K4单甲基化导致基因沉默的分子机制。该项目还将探讨一种新的丝氨酸/苏氨酸蛋白激酶在染色质结构和基因抑制中的作用，它使组蛋白H3和H2A磷酸化。这种蛋白的同源基因似乎仅限于植物谱系，它们在拟南芥发育中的作用也将得到测试。预计该项目将有助于我们了解决定常染色质状态的分子组成以及由这些机制控制的植物发育过程。该项目也有望对农业和医学具有实际意义。更好地了解表观遗传基因沉默机制可能会导致转基因技术的改进，以及对某些与表观遗传现象相关的疾病(如癌症)的治疗。作为这项研究的一部分而产生的突变菌株和载体也将是研究界的宝贵资源。此外，该项目将通过直接培训本科生、一名研究生和一名博士后研究员，对人力资源开发产生影响。学生将通过实践经验学习研究方法，目的是培养他们对科学发现的兴趣。博士后研究员和研究生还将为植物生物技术课程的开发和教学做出贡献。最后，染色质修饰基因的特征也将有助于对几乎完成的衣藻基因组序列中新基因的注释。

The growth and development of eukaryotic organisms is determined by the interactions between their genetic make-up and the environment. At the molecular level this translates into differential gene expression, giving rise to distinctly specialized cells, tissues, and/or organs. Regulation of gene expression or repression often entails modifying chromatin, a complex structure consisting of DNA wrapped around an octamer of histones (the nucleosome) as the basic repeating unit. Post-translational histone modifications (such as methylation and phosphorylation) play an important role in determining chromatin states associated with gene expression or repression. These covalent modifications can be propagated mitotically, resulting in heritable epigenetic states. However, relatively little is known about the role of histone methylation or phosphorylation in the regulation of euchromatic genes in photosynthetic eukaryotes. Even in metazoan and fungal systems, where the function of histone modifications in modulating chromatin has been examined much more extensively, there are still numerous unresolved and sometimes confusing observations. This project will focus on the molecular mechanisms responsible for, and the role of, methylation and phosphorylation of core histones in the transcriptional silencing of euchromatin in the green alga Chlamydomonas reinhardtii and the higher plant Arabidopsis thaliana. Preliminary findings suggest that monomethyl histone H3 lysine 4 (H3K4) appears to operate as a euchromatic silencing mark whereas di- and tri-methyl H3K4 correlate with transcriptionally active chromatin. The research will examine the molecular machinery(ies) responsible for gene silencing through H3K4 monomethylation. The project will also address the function of a novel serine/threonine protein kinase, which phosphorylates histones H3 and H2A, in the modulation of chromatin structure and gene repression. Orthologs of this protein appear to be limited to the plant lineage and their role in Arabidopsis development will also be tested. It is anticipated that the project will contribute to our understanding of both the molecular components involved in determining euchromatic states as well as the plant developmental processes controlled by these mechanisms.The project is also expected to have practical implications for both agriculture and medicine. A better understanding of epigenetic gene silencing mechanisms may lead to improvements in transgenic technology as well as in therapy against certain diseases associated with epigenetic phenomena such as cancer. The mutant strains and vectors generated as part of this research will also be a valuable resource for the research community. In addition, the project will have an impact on human resource development through the direct training of undergraduate students, one graduate student, and one postdoctoral fellow. Students will learn research methodology via hands on experience, with the goal of fostering their interest in scientific discovery. The postdoctoral fellow and the graduate student will also contribute to the development and teaching of a Plant Biotechnology course. Lastly, characterization of chromatin modifying genes will also facilitate the annotation of novel genes in the nearly completed Chlamydomonas genome sequence.