Early embryonic development in Xenopus laevis is programmed in part by maternally derived mRNAs, many of which are translated at the completion of meiosis (oocyte maturation). Polysomal recruitment of at least one of these mRNAs, G10, is regulated by cytoplasmic poly(A) elongation which, in turn, is dependent upon the cytoplasmic polyadenylation element (CPE) UUUUUUAUAAAG and the hexanucleotide AAUAAA (L. L. McGrew, E. Dworkin-Rastl, M. B. Dworkin, and J. D. Richter, Genes Dev. 3:803-815, 1989). We have investigated whether sequences similar to the G10 RNA CPE that are present in other RNAs could also be responsible for maturation-specific polyadenylation. B4 RNA, which encodes a histone Hl-like protein, requires a CPE of the sequence UUUUUAAU as well as the polyadenylation hexanucleotide. The 3' untranslated regions of Xenopus c-mos RNA and mouse HPRT RNA also contain U-rich CPEs since they confer maturation-specific polyadenylation when fused to Xenopus B-globin RNA. Polyadenylation of B4 RNA, which occurs very early during maturation, is limited to 150 residues, and it is this number that is required for polysomal recruitment. To investigate the possible diversity of factors and/or affinities that might control polyadenylation, egg extracts that faithfully adenylate exogenously added RNA were used in competition experiments. At least one factor is shared by B4 and GIO RNAs, although it has a much greater affinity for B4 RNA. Additional experiments demonstrate that an intact CPE and hexanucleotide are both required to compete for the polyadenylation apparatus. Gel mobility shift assays show that two polyadenylation complexes are formed on B4 RNA. Optimal complex formation requires an intact CPE and hexanucleotide but not ongoing adenylation. These data, plus additional RNA competition studies, suggest that stable complex formation is enhanced by an interaction of the trans-acting factors that bind the CPE and polyadenylation hexanucleotide.

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During Xenopus oocyte maturation, poly(A) elongation controls the translational recruitment of specific mRNAs that possess a CPE (cytoplasmic polyadenylation element). To investigate the activation of polyadenylation, we have employed oocyte extracts that are not normally competent for polyadenylation. Addition of cell lysates containing baculovirus-expressed cyclin to these extracts induces the polyadenylation of exogenous B4 RNA. The involvement of p34 cdc2 kinase in cyclin-mediated polyadenylation was demonstrated by pl3-Sepharose depletion; removal of the kinase from oocyte extracts with this affinity matrix abolishes polyadenylation activation. Reintroduction of cell lysates containing baculovirus-expressed p34 cd~2, however, completely restores this activity. To identify factors of the polyadenylation apparatus that might be responsible for the activation, we employed UV cross-linking and identified a 58-kD protein that binds the B4 CPE in oocyte extracts. In polyadenylation-proficient egg extracts, this protein has a slower electrophoretic mobility, which suggests a post-translational modification. A similar size shift of the protein is evident in oocyte extracts supplemented with lysates containing baculovirus-expressed cyclin and p34 ~dc2. This size shift, which is reversed by treatment with acid phosphatase, coincides temporally with cyclin-induced polyadenylation activation. We propose that p34 cd~2 kinase activity leads to the phosphorylation of the 58-kD CPE-binding protein and that this event is crucial for the cytoplasmic polyadenylation that occurs during oocyte maturation.

Fertilization ofXenopus laevis eggs triggers a period of rapid cell division comprising 12 nearly synchronous mitoses. Protein synthesis is required for these divisions, and new proteins appear after fertilization. Others proteins, however, which are synthesized in the unfertilized egg, are no longer made in the early embryo. To identify such proteins, a differential screen of an egg cDNA library gave nine clones corresponding to mRNAs that are deadenylylated soon after fertilization. The sequence ofone of these clones (Egi) revealed a high homology to p34Wdc2 the kinase subunit of maturationpromoting factor. Only 12 amino acids in the deduced amino acid sequence were unique to Egl when its sequence was compared to all other known examples of cdc2. Despite this strong similarity, however, Egl was unable to complement a yeast cdc2-mutant in Schizosaccharomyces pombe or a cdc28 mutant of Saccharomyces cerevisiae. Four Egl transcripts, two major and two minor, were found in Xenopus oocytes and early embryos. These RNAs appeared very early (stage I) in oogenesis and their level remained constant until the midlastfula transition, at which time they dedined. Egi RNA is found in the poly(A)+ fraction of oocytes only between the time of meiotic maturation and fertilization-that is to say, in the unfertilized egg. At fertilization the RNA loses its poly(A) tail and at the same time leaves the polyribosomes.