Activation of a two‐cell stage‐specific gene following transfer of heterologous nuclei into enucleated mouse embryos

Zygotic gene activation occurs at the two‐cell stage in the mouse embryo, resulting in the appearance of many new proteins, including a stage‐specific family of related proteins of M r 70,000. The mechanisms that regulate the stage‐specific expression of these proteins were examined by transplanting nuclei from oocytes, four‐cell‐stage blastomeres, inner cell mass cells, cultured embryonic stem cells, or differentiated endoderm‐like PYS2 cells to enucleated one‐cell embryos. Although none of these cell types synthesizes the 70 kDa complex, all were able to direct the synthesis of the 70 kDa complex following transplantation and overnight culture to the two‐cell stage. These results suggest that the embryonic cytoplasm can exert a dominant, positive regulatory influence on a variety of heterologous nuclei that results in the transcription of a stage‐specific gene. In addition, these results indicate that activation of the gene(s) coding for the 70 kDa complex is not dependent on prior programming during oogenesis and oocyte maturation, and that repression of the gene(s) coding for this complex after the two‐cell stage does not involve irreversible gene inactivation.