Translational control by poly(A) elongation during Xenopus development: differential repression and enhancement by a novel cytoplasmic polyadenylation element.

One characteristic of oocyte maturation and embryogenesis in Xenopus laevis is the activation of translationally repressed (masked) maternal mRNAs by cytoplasmic poly(A) elongation. At maturation, poly(A) elongation is controlled by two cis-acting elements in the 3'-untranslated regions (UTRs) of responsive mRNAs, the hexanucleotide AAUAAA and the cytoplasmic polyadenylation element (CPE), consisting of UUUUUAU or other similar sequences. To investigate poly(A) elongation and translational activation during embryogenesis, we have focused on Cl2 RNA, a representative transcript that undergoes these processes. By injecting radiolabeled Cl2 RNA into fertilized eggs and allowing development to proceed, we found that maximal polyadenylation of this RNA is reached by the 4000-cell blastula stage and that it requires two cis-acting elements, the hexanucleotide AAUAAA and a novel CPE, which is dodecauridine. Interestingly, a shortening of the distance between the two elements changes the timing of maximal polyadenylation to the four-cell stage. The injection of a chloramphenicol acetyl transferase (CAT)-Cl2 chimeric RNA into fertilized eggs not only results in embryonic polyadenylation of the transcript but also 5- to 15-fold more CAT activity compared with eggs injected with CAT RNA or CAT-Cl2 chimeric RNA that is prevented from undergoing poly(A) elongation by a mutation in the polyadenylation hexanucleotide. However, eggs injected with a CAT-Cl2 chimeric RNA that is preadenylated but that cannot undergo further poly(A) elongation contain no more CAT activity than eggs injected with the same RNA without a poly(A) tail, suggesting that the process of poly(A) elongation, and not poly(A) tail length, is important for translation. Finally, we show that precocious poly(A) elongation of Cl2 RNA during oocyte maturation is prevented by a large "masking" element that includes the dodecauridine CPE. The dual role of the CPE in both repression and activation of poly(A) elongation is discussed.