Zygotic Control of Maternal Cyclin A1 Translation and MRNA Stability

Cyclin mRNAs are unstable in the adult cell cycle yet are stable during the first 12 cell divisions in Xenopus laevis. We recently reported that cyclin A1 and B2 maternal mRNAs are deadenylated upon completion of the 12th division (Audic et al. [2001] Mol. Cell Biol. 21:1662-1671). Deadenylation is mediated by the 3' untranslated region (UTR) of the mRNA and precedes the terminal disappearance of the cyclin proteins, with both processes requiring zygotic transcription. The purpose of the current study was (1) to ask whether deadenylation leads to translational repression and/or destabilization of endogenous cyclin A1 and B2 mRNAs, and (2) to further characterize the regulatory sequences required. We show that zygote-driven deadenylation leads to translational repression and mRNA destabilization. A 99-nucleotide region of the 3'UTR of the cyclin A1 mRNA mediates both deadenylation and destabilization. Surprisingly, two AU-rich consensus elements within this region are dispensable for this activity. These results suggest that zygote-dependent deadenylation, translational repression, and mRNA destabilization by means of novel 3'UTR elements contribute to the disappearance of maternal cyclins. They also suggest that translational control of cyclins may play a role in the transition to the adult cell cycle. These data concur with previous studies in Drosophila showing that zygote-mediated degradation of maternal cdc25 mRNA may be a general mechanism whereby transition to the adult cell cycle proceeds.