Zygotic control of maternal cyclin A1 translation and mRNA stability

Abstract

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 3UTR 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 3UTR 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.

Deadenylation of maternal mRNAs mediated by miR-427 in Xenopus laevis embryos

“…1B) raised the possibility that this miRNA promotes the inactivation and destruction of these maternal cyclin mRNAs. Previous deletion mapping of the cis-acting regions needed for this deadenylation revealed no extended sequences shared between the two mRNAs (Audic et al 2001(Audic et al , 2002. However, a re-examination showed that the required region in each mRNA contained a single short miRNA recognition element (referred to as MRE 427 ) comprised of sequences complementary to the seed sequence of miR-427 plus short sequences that could pair with additional regions of the miRNA (Figs.…”

“…Polyadenylation of these reporter RNAs was assured even in the absence of native CPE and HEX elements by appending an RNA region containing CPE plus HEX sequences derived from histone B4 mRNA ( Fig. 4D; Paris and Philippe 1990;Audic et al 2002). Like the full-length cyclin B2 39 UTR, a transcript lacking the 39 terminal 60 nt underwent MRE 427 -dependent deadenylation, as did a transcript with a substitution of nucleotides 61-88 (Fig.…”

“…Hartley and co-workers (Audic et al 2002) previously showed that the first 99 nt of the cyclin A1 39 UTR are sufficient for deadenylation of Gb d A1 39 UTR reporter mRNAs. That region contains the miR-427-seed match defined above.…”

“…The chimeric reporter constructs pGbA1, pGbA1D384, and pGbB2 have been described previously (Audic et al 2001(Audic et al , 2002. pT7A1, containing the cyclin A1 39 UTR, was constructed from Table 2).…”

“…]), maternal cyclin A1 and B2 mRNAs undergo rapid deadenylation and subsequent decay, in a process that requires zygotic transcription (Audic et al 2001(Audic et al , 2002. Hartley and coworkers identified regions in the 39 UTRs of these two cyclin mRNAs that were both necessary and sufficient for 3 the deadenylation (Audic et al 2001(Audic et al , 2002R Hartley, unpubl.…”