Developmental acquisition of enhancer function requires a unique coactivator activity

Enhancers are believed to stimulate promoters by relieving chromatin-mediated repression. However, injection of plasmid-encoded genes into mouse oocytes and embryos revealed that enhancers failed to stimulate promoters prior to formation of a two-cell embryo, even though the promoter was repressed in the maternal nucleus of both oocytes and one-cell embryos. The absence of enhancer function was not due to the absence of a required sequence-specific enhancer activation protein, because enhancer function was not elicited even when these proteins either were provided by an expression vector (GAL4:VP16) or were present as an endogenous transcription factor (TEF-1) and shown to be active in stimulating promoters. Instead, enhancer function in vivo required a unique coactivator activity in addition to enhancer-specific DNA binding proteins and promoter repression. This coactivator activity first appeared during mouse development in two- to four-cell embryos, concurrent with the major onset of zygotic gene expression. Competition between various enhancers was observed in these embryos, but not competition between enhancers and promoters, and competition between enhancers was absent in one-cell embryos. Moreover, enhancer function in oocytes could be partially restored by pre-injecting mRNA from cells in which enhancers were active, the same mRNA did not affect enhancer function in two- to four-cell embryos.