Regulation of zygotic genome activation by Zelda

描述（由申请人提供）：有很多关于早期发展的不了解。在受精后的某个时刻，合子基因组被激活，对即将到来的发育事件所必需的基因被转录。许多早期基因已经被描述并被放置到基因网络中，但是它们是如何在适当的时间，以适当的顺序，以一种强有力的方式被激活的还不清楚。在上一个资助期，发现果蝇胚胎使用一种简单的策略来集体和选择性地激活基因-转录因子Zelda （Zld）识别并结合特定的序列基序（CAGGTA），随后将基因网络整合到一个连贯和不连贯的前馈循环系统中。在此过程中，Zld驱动发生在母体到合子转变（MZT）期间的主要重编程事件，在MZT中，胚胎从母亲那里接管了对发育的控制。这是首次在任何生物体中证明这种调节因子，因此，Zld为早期发育中基因网络的全球协调提供了范例。Zld类似于一类被称为先锋因子的特殊转录因子，它是第一个参与目标基因的，当发育信号出现时，为未来的激活提供能力。本研究的目的是揭示Zld调控合子基因组激活（ZGA）的潜在机制，并为关键发育调控因子的活性提供强有力的时间控制。Zld蛋白最初处于低水平，浓度上升一小时，与ZGA一致，然后降低

DESCRIPTION (provided by applicant): There is much about early development that is not understood. At some point after fertilization the zygotic genome is activated, and genes that are necessary for upcoming developmental events are transcribed. Many early genes have been characterized and placed into gene networks, but how they come to be activated at the proper time, in the proper order, and in a robust manner is unclear. In the last grant period, it was revealed that the Drosophila embryo uses a simple strategy to collectively and selectively activate genes - the transcription factor Zelda (Zld) recognizes and binds a specific sequence motif (CAGGTA), and subsequently integrates gene networks into a system of coherent and incoherent feed-forward loops. In doing so, Zld drives the major reprogramming event that occurs during the maternal-to-zygotic transition (MZT), where the embryo takes over control of development from the mother. This is the first demonstration of such a regulator in any organism, thus, Zld provides a paradigm for the global coordination of gene networks in early development. Zld resembles a special class of transcription factors called pioneer factors, which are the first to engage target genes, providing competence for future activation when developmental signals come along. The goal of this proposal is to reveal the underlying mechanisms by which Zld functions to regulate zygotic genome activation (ZGA) and to provide robust temporal control to the activities of the key developmental regulators. Zld protein is initially at low levels, rises in concentration by one hour, coincident with ZGA, then decreases in the third hour. The regulation of Zld protein dynamics and how Zld times the on-off state of early genes is unclear. The hypothesis that a threshold level of Zld triggers genome activation will be tested by altering Zld concentrations and monitoring target gene activation. Whether Zld itself is regulated will be assessed, with focus on a potential role of the zld 3' UTR. The idea that Zld binding leads to local remodeling of the chromatin landscape, thus increasing accessibility of other factors, leading to increased "expressivity" of target genes will be tested. Global changes in chromatin states at enhancers and transcription start sites before ZGA when the genome is inactive, after ZGA when the early set of zygotic genes are transcribed, and later when many early genes are turned off and others are activated will be examined. Importantly, comparing chromatin states in wild-type and zld mutants will reveal if Zld mediates epigenetic changes during the MZT, and if certain chromatin marks prepare genes for future transcriptional activation/repression. Genetic, genomic, and biochemical approaches will be used to test these hypotheses. Key reagents include zld mutants, zld rescue transgenes to alter Zld levels, and transcriptional reporter transgenes to assay Zld target- gene responses to changes in Zld binding sites. The experimental approaches will lead to a greater understanding of zygotic genome reprogramming from a quiescent state to a robustly active state in early development.