FURTHER ANALYSIS OF CYTOPLASMIC POLYADENYLATION IN XENOPUS EMBRYOS AND IDENTIFICATION OF EMBRYONIC CYTOPLASMIC POLYADENYLATION ELEMENT-BINDING PROTEINS

Early development in Xenopus laevis is programmed in part by maternall y inherited mRNAs that are synthesized and stored in the growing oocyt e. During oocyte maturation, several of these messages are translation ally activated by poly(A) elongation, which in turn is regulated by tw o cis elements in the 3' untranslated region, the hexanucleotide AAUAA A and a cytoplasmic polyadenylation element (CPE) consisting of UUUUUA U or similar sequence. In the early embryo, a different set of materna l mRNAs is translationally activated. We have shown previously that on e of these, C12, requires a CPE consisting of at least 12 uridine resi dues, in addition to the hexanucleotide, for its cytoplasmic polyadeny lation and subsequent translation (R. Simon, J.-P. Tassan, and J. D. R ichter, Genes Dev. 6:2580-2591, 1992). To assess whether this embryoni c CPE functions in other maternal mRNAs, we have chosen C11 RNA, which is known to be polyadenylated during early embryogenesis (J. Paris, B . Osborne, A. Couturier, R. LeGuellec, and M. Philippe, Gene 72:169-17 6, 1988). Wild-type as well as mutated versions of C11 RNA were inject ed into fertilized eggs and were analyzed for cytoplasmic polyadenylat ion at times hip to the gastrula stage. This RNA also required a poly( U) CPE for cytoplasmic polyadenylation in embryos, but in this case th e CPE consisted of 18 uridine residues. In addition, the timing and ex tent of cytoplasmic poly(A) elongation during early embryogenesis were dependent upon the distance between the CPE and the hexanucleotide. F urther, as was the case with C12 RNA, C11 RNA contains a large masking element that prevents premature cytoplasmic polyadenylation during oo cyte maturation. To examine the factors that may be involved in the cy toplasmic polyadenylation of both C12 and C11 RNAs, we performed UV cr oss-linking experiments in egg extracts. Two proteins with sizes of si milar to 36 and similar to 45 kDa interacted specifically with the CPE s of both RNAs, although they bound preferentially to the C12 CPE. The role that these proteins might play in cytoplasmic polyadenylation is discussed.