Translational control is a key level in regulating gene expression in oocytes and eggs because many mRNAs are synthesized and stored during oogenesis for latter use at various stages of oocyte maturation and embryonic development. Understanding the molecular mechanisms that underlie this translational control is therefore crucial. Another important issue is the evolutionary conservation of these mechanisms-in other words the determination of their universal and specific aspects. We report here a comparative analysis of a translational repression mechanism that depends on the EDEN (embryo deadenylation element) element. This small cis-acting element, localized in the 3 untranslated region of c-mos and Eg mRNAs, was shown to be involved in a deadenylation process. We demonstrate here that in Xenopus embryos, mRNAs that contain an EDEN are translationally repressed. Next, transgenic flies were used to study the effect of the EDEN motif on translation in Drosophila oocytes. We show that this element also causes the translational repression of a reporter gene in Drosophila demonstrating that the EDEN-dependent translational repression is functionally conserved between Xenopus and Drosophila.yemanuclein-alpha ͉ oogenesis

In this paper we show that large changes in ornithine decarboxylase (ODC) activity occured during early Xenopus development. Following fertilization, this enzyme activity rises with a quantitatively correlated accumulation of putrescine and spermidine. This increase in ODC activity was associated with an increased translation of the maternal ODC mRNA, which was stable in the embryo and whose polyadenylation increased slightly between fertilization and the mid-blastula transition (MBT). ODC activity was stable in cycloheximide-treated embryos, indicating that before the MBT this enzyme was not degraded. After the MBT, ODC activity fell, but no decrease in this mRNA was observed. In gastrulae, ODC mRNA was both increased in amount and polyadenylated. The reduced ODC activity at this stage of development was not associated with a fall in ribosome loading of the mRNA. Treatment of post-MBT embryos with cycloheximide lead to an accentuation of the normally observed decrease in ODC activity. Expression of Xenopus ODC in mutant ODC-deficient Chinese hamster ovary cells (C 55.7 cells) showed that the Xenopus enzyme was rapidly degraded and can be regulated post-translationally by polyamines, indicating that the post-MBT fall in ODC activity could be caused by a change in protein turnover or by polyamine-mediated regulation.In many species, the first hours of development are devoid of transcriptional activity in the zygotic nucleus [l]. This means that post-transcriptional, translational and post-translational modes of regulation are responsible for the changes in gene expression which occur during the initial stages of early development. In Xenopus embryos, transcription of the zygotic genome is clearly detected only after the eleventh cleavage, when the embryos contain more than 4000 cells. Also, at this time, the cell cycle slows with the appearance of G I and G2 phases [for review, see 21. This change in the celldivision cycle is called the mid-blastula transition (MBT) [3]. The ten cell divisions which follow the first cleavage, 1.5 h after fertilization, are rapid (every 20-30 min) and quasisynchronous.A gene product whose expression is intimately associated with cell proliferation is ornithine decarboxlase (ODC). This enzyme catalyses the first and one of the rate limiting steps in polyamine biosynthesis [for review, see 41. The addition of aminopropyl groups to the putrescine formed by the decarboxylation of ornithine, successively produces spermidine and spermine. In mammalian cell cultures, stimulation of growth is associated with an increase in polyamine biosynthesis [reviewed in 51. Furthermore, a decrease in intracellular polyamine concentrations below a critical level, caused by pharmacologically [6 -81 or genetically [9] inhibiting ODC activity, leads to a slowing or arrest of cell growth. The expression of ODC in mammals can be regulated at multiple levels (transcription, translation and protein turnover). For instance, serum stimulation of quiescent 3T3 cells leads to an increase in ODC and S-adenosylmet...