Summary

In mammals, enhancers are believed to stimulate transcription from RNA polymerase II promoters primarily by relieving their chromatin-mediated repression. Interactions responsible for enhancer function are developmentally acquired. Factors responsible for this repression are not present in the paternal pronuclei of 1-cell embryos, making them impervious to enhancer function. Although such chromatin-mediated repression is observed in oocytes, maternal pronuclei of 1-cell embryos, and the 2-cell embryos, the enhancer function first appears in 2-cell embryos, a stage in development that corresponds to the beginning of major zygotic gene expression (ZGE). The lack of enhancer function prior to 2-cell stage is also not due to the absence of specific activation proteins, but appears to be due to the absence of an essential coactivator activity. The coactivator activity first appears in 2-cell embryos, and is required for enhancers driven by different classes of transcription factors. The absence of the coactivator activity and the corresponding enhancer function appears to be unique to oocytes and 1-cell embryos, suggesting that it provides a safeguard against premature activation of genes prior to ZGE. I