Abundant non-A residues in the poly(A) tail orchestrate the mouse oocyte-to-embryo transition

Non-A (U, G, and C) residues can be added to the 5’-end, internal, and 3’-end positions of poly(A) tails of RNA transcripts[1][1]–[3][2], and some of these have been shown to regulate mRNA stability[4][3], [5][4]. The mammalian oocyte-to-embryo transition (OET) relies on post-transcriptional regulation of maternal RNA, because transcription is silent during this process until the point of zygotic genome activation (ZGA)[6][5]–[9][6]. Although the regulation of mRNA translation by poly(A) tail length plays an important role in the mammalian OET, the dynamics and functions of non-A residues in poly(A) tails are completely unknown. In this study, we profiled the genome-wide presence, abundance, and roles of non-A residues during the OET in mice using PAIso-seq1 and PAIso-seq2[2][7], [10][8], two complementary methods of poly(A) tail analysis. We found that non-A residues are highly dynamic in maternal mRNA, following a general pattern of beginning to increase at the MII stage, becoming highly abundant after fertilization with U residues in about half of poly(A) tails in 1-cell embryos, and declining in 2-cell embryos. We revealed that Btg4-mediated global maternal mRNA deadenylation created the substrates for U residue addition by Tut4/7 at their 3’-ends and further re-polyadenylation. In addition, G residues can be added by Tent4a/b. Finally, we demonstrate that G residues stabilize the modified mRNA, while the U residues mark maternal RNA for faster degradation in 2-cell mouse embryos. Taken together, these findings demonstrate that non-A residues are abundant and re-sculpt the maternal transcriptome to initiate zygotic development, which reveals the functional importance of the post-transcriptional regulation mediated by non-A residues in mRNA poly(A) tails. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-3 [3]: #ref-4 [4]: #ref-5 [5]: #ref-6 [6]: #ref-9 [7]: #ref-2 [8]: #ref-10