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We report here unusual features of c-Myc specific to early embryonic development in Xenopus laevis, a period characterized by generalized transcriptional quiescence and rapid biphasic cell cycles. Two c-Myc protein forms, p61 and p64, are present in large amounts in the oocyte as well as during early development. In contrast, only p64 c-Myc is present in Xenopus somatic cells. p61 c-Myc is the direct translation product from both endogenous c-myc mRNAs and c-myc recombinant DNA. It is converted to the p64 c-Myc form after introduction into an egg extract, in the presence of phosphatase inhibitors. p61 and p64 belong to two distinct complexes localized in the cytoplasm of the oocyte. A 15S complex contains p64 c-Myc, and a 17.4S complex contains p61 c-Myc. Fertilization triggers the selective and total entry of only p64 c-Myc into the nucleus. This translocation occurs in a nonprogressive manner and is completed during the first cell cycles. This phenomenon results in an exceptionally high level of c-Myc in the nucleus, which returns to a somatic cell-like level only at the end of the blastulation period. During early development, when the entire embryonic genome is transcriptionally inactive, c-Myc does not exhibit a DNA binding activity with Max. Moreover, embryonic nuclei not only prevent the formation of c-Myc/Max complexes but also dissociate such preformed complexes. These peculiar aspects of c-Myc behavior suggest a function that could be linked to the rapid DNA replication cycles occurring during the early cell cycles rather than a function involving transcriptional activity.
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