Reporter gene expression in G2 of the 1-cell mouse embryo.

Similar amounts of expression of a luciferase reporter gene that is driven by the SV40 early promoter are detected in G2 of 1-cell mouse embryo following injection of the male pronucleus in either early or mid/late S. No luciferase activity is observed when the male pronucleus is injected in early S and the embryos analyzed for expression in late S. Last, no luciferase activity is observed when the female pronucleus is injected in mid/late S and the embryos analyzed for luciferase activity in G2 of the first cell cycle. These results suggest that zygotic gene activation initiates in the 1-cell embryo rather than in the 2-cell embryo, as previously thought, and that differences between the transcriptional activity of male and female pronuclei exist.