Re-polyadenylation occurs predominantly on maternal mRNA degradation intermediates during mammalian oocyte-to-embryo transition

The nascent mRNA transcribed in the nucleus is cleaved and polyadenylated before it is transported to the cytoplasm for translation1. Polyadenylation can also occur in the cytoplasm for post-transcriptional regulation, especially in neurons, oocytes and early embryos1,2. Recently, we revealed transcriptome-wide maternal mRNA cytoplasmic re-polyadenylation during the mammalian oocyte-to-embryo transition (OET)3-6. However, the mechanism of re-polyadenylation during mammalian OET, including the sites to be re-polyadenylated and the enzymes involved, is still poorly understood. Here, by analyzing the PAIso-seq1 and PAIso-seq2 poly(A) inclusive transcriptome data during OET in mice, rats, pigs, and humans, we reveal conserved re-polyadenylation of mRNA degradation intermediates. These re-polyadenylated mRNA degradation intermediates account for over half of the polyadenylated mRNA during OET in all four species. We find that mRNA degradation intermediates for re-polyadenylation are generated through Btg4-mediated deadenylation in both mouse and human. Interestingly, the poly(A) tails on the re-polyadenylated mRNA degradation intermediates are of different lengths and contain different levels of non-A residues compared to regular polyadenylation sites, suggesting specific regulation and function of these poly(A) tails in mammalian OET. Together, our findings reveal the maternal mRNA degradation intermediates as substrates for conserved cytoplasmic dominant re-polyadenylation during mammalian OET, and uncover the mechanism of production of these mRNA degradation intermediates. These findings provide new insights into mRNA post-transcriptional regulation, and a new direction for the study of mammalian OET.