In all animals the initial events of embryogenesis are controlled by maternal gene products that are deposited into the developing oocyte. At some point after fertilization, control of embryogenesis is transferred to the zygotic genome in a process called the maternal to zygotic transition (MZT). During this time maternal RNAs are degraded and zygotic RNAs are transcribed. Although progress has been made on the mechanisms underlying RNA degradation (Giraldez et al., 2006), the activators of the zygotic genome have remained elusive. A hint came from the discovery that many Drosophila zygotic genes share a cis-regulatory motif related to CAGGTAG (ten Bosch et al.,2006;De Renzis et al., 2007). We recently isolated a zinc-finger protein, Zelda, which binds specifically to these sites. Mutant embryos lacking maternal zelda transcripts fail to activate the transcription of a sample of zygotic genes, and are defective in several aspects of the cellularization process, which corresponds to the point at which embryos can no longer survive on maternal products alone (Merrill et al., 1988). Our preliminary results suggest that Zelda activates batteries of genes during the MZT, each responsible for a key developmental process. Zelda may activate early-transcribed microRNAs that mediate maternal RNA degradation, thus providing a link between the two hallmark events of the MZT. Building on our preliminary data, we will further characterize the role of Zelda in the MZT. How many genes does Zelda activate and which early genes are independent of Zelda? We will use a genomics approach in Aim 1 to determine all Zelda targets in the early embryo. How many genes does Zelda activate, and are there groups of genes not regulated by Zelda? In Aim 2 we ask whether Zelda plays an instructive or permissive role, or both, in activating target genes, and how it interacts with other key regulators such as Dorsal and Dpp to activate their target genes.
In Aim 3 we investigate the structure and function of conserved Zelda protein domains, and test whether there is Zelda function in the long germ Nasonia wasp and if it plays a similar role in activating the zygotic genome. Finally, we address mechanistic questions regarding the timing of genome activation in Aim 4, and the possible role of Zelda in reversing the initial silencing of the genome. The experiments outlined in this proposal have the potential to advance greatly our understanding of the mechanisms underlying the MZT.
The study of the molecular mechanisms underlying basic developmental processes is relevant to understanding the cause and progression of human diseases such as cancer, and human disorders such as birth defects.
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