Simon R, Tassan J P, Richter J D
Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545.
Genes Dev. 1992 Dec;6(12B):2580-91. doi: 10.1101/gad.6.12b.2580.
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.
非洲爪蟾卵母细胞成熟和胚胎发生的一个特征是通过细胞质多聚腺苷酸化延长来激活翻译抑制（封闭）的母体mRNA。在成熟过程中，多聚腺苷酸化延长由响应性mRNA的3'-非翻译区（UTR）中的两个顺式作用元件控制，即六核苷酸AAUAAA和细胞质多聚腺苷酸化元件（CPE），CPE由UUUUUAU或其他类似序列组成。为了研究胚胎发生过程中的多聚腺苷酸化延长和翻译激活，我们聚焦于Cl2 RNA，这是一个经历这些过程的代表性转录本。通过将放射性标记的Cl2 RNA注入受精卵并使其发育，我们发现该RNA在4000细胞囊胚期达到最大多聚腺苷酸化，并且这需要两个顺式作用元件，即六核苷酸AAUAAA和一个新的CPE，它是十二聚尿苷。有趣的是，两个元件之间距离的缩短将最大多聚腺苷酸化的时间改变到四细胞期。将氯霉素乙酰转移酶（CAT）-Cl2嵌合RNA注入受精卵不仅导致转录本的胚胎多聚腺苷酸化，而且与注入CAT RNA或因多聚腺苷酸化六核苷酸突变而无法进行多聚（A）延长的CAT-Cl2嵌合RNA的卵相比，CAT活性高5至15倍。然而，注入预先腺苷酸化但不能进一步进行多聚（A）延长的CAT-Cl2嵌合RNA的卵所含的CAT活性并不比注入没有多聚（A）尾的相同RNA的卵更多，这表明多聚（A）延长过程而非多聚（A）尾长度对翻译很重要。最后，我们表明卵母细胞成熟期间Cl2 RNA的过早多聚腺苷酸化延长被一个包括十二聚尿苷CPE的大“封闭”元件所阻止。讨论了CPE在多聚（A）延长的抑制和激活中的双重作用。
