Gata.a, Tbx21, and Klf6/7 function cooperatively for zygotic genome activation in ascidian embryos

In ascidian embryos, gene expression from the zygotic genome begins between the 8- and 16-cell stages. While most zygotic genes are expressed in specific cell lineages at these stages, transcription factors that provide spatial cues for establishing specific expression patterns are insufficient to activate target genes at normal physiological levels. Gata.a is a transcription factor that provides spatial cues for targets expressed specifically in the animal hemisphere. Intriguingly, it is also required for physiological-level expression of many zygotic genes expressed in the vegetal hemisphere. In the present study, we found that Tbx21 and Klf6/7 augment the latter function of Gata.a. To determine the global extent of genes under control of these factors, we identified genes zygotically activated in early embryos using RNA-sequencing of BrU-labelled zygotic mRNAs. Our results revealed that approximately 80% of all zygotically activated genes were under control of these three factors. That is, together, Gata.a, Tbx21, and Klf6/7 are necessary to regulate target gene expression at physiological levels. This requirement for a specific set of broadly distributed factors resembles those of pioneer transcription factors that trigger zygotic genome activation (ZGA) in other animals, including flies and vertebrates. Regulatory factors involved in ZGA vary among animals, and our results indicate that ascidians use a distinct set of transcription factors for ZGA.