Following fertilization, activation of the embryonic genome is delayed in many organisms until a major transition known as zygotic gene activation (ZGA) in mammals and the midblastula transition (MET) in model organisms such as Xenopus, zebrafish, and Drosophila. It has been widely accepted that zygotic transcription does not occur before this critical transition, and most research on gene expression in early embryos has focused on the mechanisms of transcription repression. This laboratory has identified genes that are robustly transcribed in Xenopus before the MET. Furthermore, maternal transcription factors, including the Wnt-regulated S-catenin/Tcf complex, are required for preMBT transcription;interference with their preMBT function markedly disrupts embryonic development, including formation of the neural tube, notochord, and muscle. We have identified promoter sequences that are active in preMBT embryos, which have not previously been reported for any Xenopus promoter (or for conventional viral promoters such as CMV or SV40 promoters). We have identified specific chromatin modifications associated with preMBT gene expression and have demonstrated direct binding of B-catenin to Wnt regulated promoters in preMBT embryos. We will examine the mechanisms by which selected genes are activated in early embryos, in the setting of global transcription silencing, and investigate how specific transcription factors, upstream regulatory sequences, and overall chromatin architecture regulate preMBT transcription. Specific sequences that confer preMBT transcription will be identified and loss of function approaches will be used to test the role of maternal transcription factors in regulating preMBT transcription. We will examine the access of maternal transcription factors to the promoters of genes active before vs. after the MBT. DNA methylation, histone modifications, and nucleosome remodeling play key roles in gene silencing and activation mechanisms in early embryos, and interference with these mechanisms causes multi-system developmental defects, including Rett, Beckwith-Wiedeman, and Rubenstein-Taybi syndromes. Our data suggest that selective gene activation before MBT, as well as global repression, is critical for normal development of the embryo. The research proposed here should provide insights into how errors in embryonic gene expression could cause profound developmental defects in humans.
