Project description:During the first stages of development, the fertilized germ cells rapidly transition to totipotency. Maternally deposited mRNAs encode the proteins necessary for reprogramming the transcriptionally quiescent zygotic genome during this maternal-to-zygotic transition (MZT). The transcription factor Zelda is essential for this reprogramming in the Drosophila embryo. Zelda is necessary for transcriptional activation of the zygotic genome, and the absence of Zelda leads to embryonic lethality during the MZT. Excess Zelda activity is also lethal to the embryo, demonstrating that Zelda levels must be precisely controlled during early development. Because Zelda is encoded by a maternally deposited mRNA, Zelda levels in the embryo are controlled at the level of translation. To understand how levels of this essential reprogramming factor were regulated to allow for embryonic development and zygotic genome activation, we investigated the factors that regulated translation of zelda. Brain Tumor (BRAT) is a translational regulator that was previously shown to bind to zelda mRNA in the embryo. We showed that BRAT functions to repress zelda translation, as embryos deficient for maternal BRAT activity prematurely express Zelda. We further showed that in the larval brain, BRAT similarly regulates Zelda levels and identified specific BRAT-binding sites that mediate these effects. Thus, BRAT regulates Zelda in multiple tissues. Because both too little and too much Zelda are lethal to the embryo, we hypothesized that precocious expression of this transcriptional activator might be capable of driving precocious activation of the zygotic genome, leading to embryonic lethality. To test this hypothesis, we performed single embryo RNA-seq at distinct nuclear cycles throughout zygotic genome activation (NC10, NC12, NC13, and NC14) in control and brat-mutant embryos. Our results conclusively demonstrated that in embryos lacking functional BRAT, Zelda target genes were not prematurely activated. Rather, these genes were expressed normally, but become significantly upregulated at nuclear cycle 14, when the division cycle slows. Our data support a model in which zygotic genome activation requires precise coordination between expression of reprogramming factors, such as Zelda, and the slowing of the cell cycle.

Project description:The earliest stages of development in most metazoans are driven by maternally deposited proteins and mRNAs, with widespread transcriptional activation of the zygotic genome occurring hours after fertilization, at a period known as the maternal-to-zygotic transition (MZT). In Drosophila, the MZT is preceded by the transcription of a small number of genes that initiate sex determination, patterning and other essential developmental processes. The zinc-finger transcription factor Zelda (ZLD) plays a key role in the transcriptional activation of these earliest-expressed genes. To better understand the mechanisms of ZLD activation and the range of its targets, we used chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-Seq) to map regions bound by ZLD prior to (mitotic cycles 8 and 9), during (mitotic cycles 13 and early 14) and after (late mitotic cycle 14) the MZT. Although only a handful of genes are transcribed prior to mitotic cycle 10, we identified thousands of regions bound by ZLD in cycle 8-9 embryos, most of which remain bound through mitotic cycle 14. As expected, these ZLD-bound regions include the promoters and enhancers of the small subset of genes transcribed at this early stage. However we also observed ZLD bound at cycle 8-9 to the promoters of a large fraction of the several thousand genes whose first transcription does not occur until roughly an hour and four mitotic cycles later. These early ZLD-bound regions include virtually all of the thousands of known and presumed enhancers bound at cycle 14 by the transcription factors that regulate patterned gene activation during the MZT. The association between early ZLD binding and MZT activity is so strong that ZLD binding alone can be used to identify active promoters and regulatory sequences with high specificity and selectivity. This strong early association of ZLD with regions not active until the MZT suggests that ZLD is not only required for the earliest wave of transcription, but also plays a major role in activating the genome at the MZT. Genome-wide mapping of Zelda in wild-type Drosophila melanogaster embryos prior (mitotic cycles 8-9), during (cycles 13-14), and after (late cycle 14) maternal-to-zygotic transition

Project description:In nearly all metazoans, the earliest stages of development are controlled by maternally deposited mRNAs and proteins. The zygotic genome only becomes transcriptionally active hours later. Transcriptional activation is tightly coordinated with the degradation of maternally provided mRNAs during this maternal-to-zygotic transition (MZT). In Drosophila melanogaster, the transcription factor Zelda plays an essential role in widespread activation of the zygotic genome. While Zelda expression is required both maternally and zygotically, the mechanisms by which it functions both during and after the MZT remain unclear. Zelda is a large protein with six C2H2 zinc fingers, but no additional identified domains or predicted enzymatic activities. Using Cas9-mediated genome editing to generate targeted mutations in Zelda, we determined the functional relevance of protein domains conserved amongst Zelda orthologs. Generating these mutations in vivo revealed that neither a conserved N-terminal zinc finger nor an acidic patch were required for activity. Similarly, using a variety of Cas9-enabled approaches we showed that a previously identified splice isoform of zelda is dispensable for viability, and the predicted protein product is not expressed at detectable levels. By contrast, we identified a highly conserved zinc-finger domain that is essential for the maternal, but not zygotic functions of Zelda. A mutation in this zinc-finger domain does not interfere with the capacity of Zelda to activate transcription, but rather leads to a hyperactive form of the protein and enhanced Zelda-dependent gene expression. Embryos inheriting this allele from their mothers die late in embryogenesis. These data define a protein domain critical for controlling Zelda activity and, for the first time, identify a separation between the maternal and zygotic requirements for Zelda. This demonstrates that highly regulated levels of Zelda activity are essential for establishing the developmental program during the MZT and that failure to precisely execute this program leads to embryonic lethality.

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Drosophila melanogaster

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