Repression of gene expression at the beginning of mouse development.

The transition from maternal to zygotic gene expression in the mouse occurs in the 2-cell embryo. Previous studies in which DNA was injected into 2-cell embryos revealed that transcription promoters and origins of DNA replication are strongly repressed in cleavage stage embryos unless linked to an embryo-responsive enhancer. Repression also occurs when DNA is injected into the paternal pronucleus of a 1-cell embryo and the injected embryo subsequently undergoes mitosis, except that repression is no longer relieved by enhancers. Here we extend this observation to maternal pronuclei in 1-cell embryos and show that this repression could not be relieved either by linking the promoter to an embryo-responsive enhancer or by inducing hyperacetylation of chromatin inorder to increase its accessibility to transcription factors. However, repression could be relieved by transplanting the injected pronucleus to a 2-cell embryo, even when the recipient cell subsequently underwent mitosis. Both the extent of promoter repression and the ability of enhancers to stimulate promoter activity increased as development proceeded from the early 2-cell stage to the 4-cell stage. Once repression was established in late 2-cell embryos, transplanting an injected 2-cell embryo nucleus back to an early 1-cell embryo failed to restore activity to the injected promoter, even when it was linked to an enhancer. These and other data demonstrate that cytoplasmic factors appear during formation of a 2-cell embryo that can repress promoter activity and activate enhancer activity. These factors are absent from the paternal pronucleus and cytoplasm of early (S-phase arrested) 1-cell embryos. Moreover, the cytoplasm of early 1-cell embryos appears to lack the ability to reprogram expression of genes once they have progressed to the late 2-cell stage in mouse development.