Deadenylation of maternal mRNAs during Xenopus oocyte maturation does not require specific cis-sequences: a default mechanism for translational control.

The meiotic maturation of Xenopus oocytes initiates significant changes in the translation of a number of maternal mRNAs that coincide with alterations in their polyadenylation states. A considerable number of maternal mRNAs are deadenylated in mature oocytes, thereby reducing their translational efficiencies. In this report we demonstrate that deadenylation does not require specific cis-acting sequences. Polyadenylated RNAs derived from either ribosomal protein or beta-globin mRNAs, or that contain non-mRNA-derived sequences, are deadenylated in mature oocytes. Translation of a substrate RNA is not required for its deadenylation. G10 mRNA is representative of a class of mRNAs that is translationally activated at maturation and contains the cytoplasmic polyadenylation element (U)6AU. A deletion mutant G10 transcript that lacks the (U)6AU element is not polyadenylated in mature oocytes but is deadenylated instead. Insertion of the (U)6AU element into the 3'-untranslated region of the ribosomal protein L1 mRNA is sufficient to prevent both its deadenylation and polysomal release in mature oocytes. These results indicate that the deadenylation and translational inactivation of maternal mRNAs during Xenopus oocyte maturation occur by a default pathway in which transcripts lacking a cytoplasmic polyadenylation element undergo poly(A) removal.