The translational activation of several maternal mRNAs in Xenopus laevis is dependent on cytoplasmic poly(A) elongation. Messages harboring the UUUUUAU-type cytoplasmic polyadenylation element (CPE) in their 3 untranslated regions (UTRs) undergo polyadenylation and translation during oocyte maturation. This CPE is bound by the protein CPEB, which is essential for polyadenylation. mRNAs that have the poly(U) 12-27 embryonic-type CPE (eCPE) in their 3 UTRs undergo polyadenylation and translation during the early cleavage and blastula stages. A 36-kDa eCPE-binding protein in oocytes and embryos has been identified by UV cross-linking. We now report that this 36-kDa protein is ElrA, a member of the ELAV family of RNA-binding proteins. The proteins are identical in size, antibody directed against ElrA immunoprecipitates the 36-kDa protein, and the two proteins have the same RNA binding specificity in vitro. Cl2 and activin receptor mRNAs, both of which contain eCPEs, are detected in immunoprecipitated ElrA-mRNP complexes from eggs and embryos. In addition, this in vivo interaction requires the eCPE. Although a number of experiments failed to define a role for ElrA in cytoplasmic polyadenylation, the expression of a dominant negative ElrA protein in embryos results in an exogastrulation phenotype. The possible functions of ElrA in gastrulation are discussed.Early development in many animals is programmed by mRNAs inherited by the egg at the time of fertilization. While many of these mRNAs are translationally dormant in oocytes, they become activated in a sequence-specific manner at subsequent developmental periods. Several mechanisms are likely to control the translation of these messages, but one that has been studied extensively in Xenopus laevis is poly(A) elongation (31). Some mRNAs, which encode such proteins as c-Mos, cyclin B1, cyclin-dependent kinase 2 (Cdk2), and the earlydevelopment-specific histone B4, are polyadenylated and translated during oocyte maturation (27-29, 33, 39). Other mRNAs, however, undergo these processes sometime after fertilization; they encode such proteins as polypeptide chain release factor (Cl1), Cl2 (function unknown), and activin receptor (28,(34)(35)(36).Both groups of mRNAs contain two cis-acting elements located in their 3Ј untranslated regions (UTRs) that are required for cytoplasmic polyadenylation. One is the virtually ubiquitous hexanucleotide AAUAAA, which is also important for nuclear pre-mRNA cleavage and polyadenylation. The second, called the cytoplasmic polyadenylation element (CPE), is a U-rich sequence located upstream of the hexanucleotide. The exact sequence of CPE differs between the two groups of mRNAs mentioned. The maturation-type CPE is UUUUUAU (consensus), whereas the embryonic CPE (eCPE) is oligo (U) 12-27 (reviewed in reference 31). At least two mRNAs that are polyadenylated in embryos also contain a third cis-acting sequence in the 3Ј UTR: the masking element that prevents precocious polyadenylation during maturation (34, 35). Because deletion of the maskin...