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REGULATION OF EMBRYONIC GENE ACTIVATION BY POLYADENYLATION OF MATERNAL MRNA IN MOUSE EMBRYOS.
Hara, Kentaro1, Aoki, Fugaku1,2, Schultz, Richard2, 1 2
ABSTRACT- The mechanism for the initiation of embryonic gene activation has not been elucidated. We previously suggested that transcriptional machinery is rate-limiting for transcriptional activity in 1-cell mouse embryos. In this case, embryonic gene activation would be dependent on the synthesis of some proteins involved in transcriptional activation after fertilization. To address this hypothesis, the transcriptional activity was measured in the 1-celll embryos in which protein synthesis was inhibited by the treatment with cycloheximide. When the embryos were cultured with cycloheximide from 1h after insemination, the transcriptional activity, which was measured by in vitro transcription assay 14 h after insemination, was decreased to the marginal level (less than 5% of the control level) in both male and female pronuclei. Such a requirement of a newly synthesized protein(s) for transcriptional activation could be interpreted as followings. Either proteins required for transcriptional activation are absent in the oocytes before fertilization but begin to be synthesized after fertilization; or proteins required for transcriptional activation are present in the oocytes before fertilization but are labile and need to be continuously synthesized. Recent works demonstrated that some proteins begin to be synthesized after fertilization and that this beginning of protein synthesis is associated with poly(A) tail elongation of maternal mRNA. To assess the first possibility, poly(A) tail elongation was inhibited by 3'-deoxyadenosine (3'-dA) to prevent the beginning of protein synthesis after fertilization. 3'-dA is expected to be metabolized to 3'-ATP in the embryos and prevent polyadenylation of mRNA. As expected, the transcriptional activity was prominently decreased in the embryos cultured with 2 mM 3'-dA. This result suggested that the synthesis of a protein(s) required for transcriptional activation is induced by polyadenylation of its (their) mRNA.
KEY WORDS: mouse, preimplantation embryos, polyadenylation, gene expression
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