We previously suggested that the transcriptional machinery is rate-limiting for genome activation in the preimplantation mouse embryo (Aoki et al., 1997: Dev Biol 181:296-307), suggesting that genome activation requires the synthesis of some proteins following fertilization. To test this hypothesis, transcriptional activity was measured in 1-cell embryos in which protein synthesis was inhibited by cycloheximide from the time shortly after insemination. Transcription in both the male and female pronuclei, as assessed by BrUTP incorporation, was markedly inhibited when assayed 14 hr post-insemination. This result suggested that newly synthesized proteins derived from maternally recruited mRNA, and not maternally derived proteins that were post-translationally modified following fertilization/egg activation, were critical for the acquisition of transcriptional competence. To test this hypothesis, mobilization of maternally recruited mRNAs was inhibited by treating the embryos with 3'-deoxyadenosine (3'-dA), which prevents polyadenylation of mRNA. The results demonstrated that the transcriptional activity was markedly decreased in the embryos cultured with 2 mM 3'-dA, whereas 3'-deoxyguanosine had little inhibitory effect, and suggest that recruitment of maternal mRNAs is essential for acquisition of transcriptional competence.
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