After fertilization, maternal factors direct development and trigger zygotic 
genome activation (ZGA) at the maternal-to-zygotic transition (MZT). In 
zebrafish, ZGA is required for gastrulation and clearance of maternal messenger 
RNAs, which is in part regulated by the conserved microRNA miR-430. However, the 
factors that activate the zygotic program in vertebrates are unknown. Here we 
show that Nanog, Pou5f1 (also called Oct4) and SoxB1 regulate zygotic gene 
activation in zebrafish. We identified several hundred genes directly activated 
by maternal factors, constituting the first wave of zygotic transcription. 
Ribosome profiling revealed that nanog, sox19b and pou5f1 are the most highly 
translated transcription factors pre-MZT. Combined loss of these factors 
resulted in developmental arrest before gastrulation and a failure to activate 
>75% of zygotic genes, including miR-430. Our results demonstrate that maternal 
Nanog, Pou5f1 and SoxB1 are required to initiate the zygotic developmental 
program and induce clearance of the maternal program by activating miR-430 
expression.