Polyadenylation and deadenylation of maternal mRNAS during oocyte growth and maturation in the mouse

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Abstract
We previously showed that, during mouse oocyte maturation, specific maternal mRNAs (actins) are deadenylated, while others (hypoxanthine phosphoribosyltransferase:HPRT) are adenylated. As in other systems, these changes can be correlated with changes in transiational activities. Maturation‐specific polyadenylation in Xenopus depends on the presence of a U‐rich cytoplasmic polyadenylation element (CPE) close to the 3′ end of the RNA. RNAs that lack CPEs appear to be deadenylated by default when meiosis resumes. We show here that this default program also applies to maturing mouse oocytes. Microinjected β‐ and γ‐actin 3′ UTR (untranslated region) transcripts lacking CPEs but including polyA tails (100–200 N) behave as endogenous maternal actin mRNAs and are at deadenylated by maturing oocytes. “Nonsense” transcripts taht do not include CPEs, but that do contain polyA tails, are also deadenylated. β‐ and γ‐Actin 3′ UTRs with short polyA tails (50–80 N) are stable and exhibit no detectable change in adenylation when injected into growing, fullgrown, or maturing oocytes. In contrast, HPRT 3′ UTRs, which include the CPE UUUUAAAU and a short polyA tail (50 N), are polyadenylated during maturation. HPRT 3′ UTR transcripts with long polyA tails (100–200 N) are more extensively deadenylated by growing and full‐grown oocytes that retain germinal vesicles than by maturing oocytes. The presence of CPEs may be required for polyA tail shortening and translational inactivation of stable mRNAs during oocyte growth and subsequent selective readenylation and translation during meiotic maturation. We propose that the absence of CPEs on maternal mRNAs translationally active during oogenesis enables them to retain polyA tails of sufficient length for recruitment onto polysomes and later allows for their deadenylation by default and removal from polysomes after meiosis resumes.

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