Onset of paternal and maternal Gpi-1 expression in preimplantation mouse embryos

The initial activation of the glucose phosphate isomerase gene, Gpi-1, was studied in mouse embryos produced by transplanting pronuclei between two strains of mice differing in alleles for this enzyme. Protein isozymes encoded by the embryonic cell nuclei were first detected on Day 4 of embryogenesis, and the maternal and paternal genes are seen to be activated simultaneously. Comparison of isozymes produced by these nuclear-transfer embryos and by F1 embryos from these two strains suggests the absence of oocyte mRNA for GPI-1 at the time when these genes are first activated. Thus, the GPI-1 present is derived from newly transcribed mRNA contributed by both maternal and paternal genes. The relative proportion of maternal cytoplasmic GPI-1 enzyme declines from Day 3 to Day 6, such that on Day 6, almost no oocyte GPI-1 is detected.