Zelda's role in Drosophila zygotic genome activation. Yujia Sun¹, Sun Melody Foo¹, Chung-Yi Nien¹, Hsiao-Yun Liu¹, Kai Chen², Kevin O'Brien¹, Amruta Tamhane¹, Julia Zeitlinger², Christine Rushlow¹. 1) Department of Biology, New York University, New York, NY 10003; 2) Stowers Institute for Medical Research, 1000 East 50th Street, Kansas City, MO 64110.

   During embryogenesis, developmental control is transferred from maternal gene products to the zygotic genome in a process called the maternal-to-zygotic transition. Previously, Zelda (Zld) was identified by our lab to be a key activator of the early zygotic genome in Drosophila, without which the transcription of many genes is affected, and the embryo ceases development before gastrulation. However, the underlying mechanism of how Zld acts to ensure robust and timely activation of its target genes remains a mystery. The prevalence of Zld binding sites near transcription start sites, the great overlap between Zld-bound regions and "hotspots" where many other transcription factors co-occupy, and the early presence of Zld in nuclei prompted us to investigate the role of Zld in recruitment of transcription factors including RNA polymerase II. Here we present evidence that supports the hypothesis that Zld binding increases the accessibility of transcription factors to chromatin, and reveals the mechanistic role of Zld during zygotic genome activation. We propose a model of how Zld functions alone or together with transcription factors such as the Dorsal morphogen to activate target genes in a timely and robust manner during the maternal-to-zygotic transition.