Further analysis of cytoplasmic polyadenylation in Xenopus embryos and identification of embryonic cytoplasmic polyadenylation element-binding proteins

Genome-wide analysis reveals a switch in the translational program upon oocyte meiotic resumption

During oocyte maturation, changes in gene expression depend exclusively on translation and degradation of maternal mRNAs rather than transcription. Execution of this translation program is essential for assembling the molecular machinery required for meiotic progression, fertilization, and embryo development. With the present study, we used a RiboTag/RNA-Seq approach to explore the timing of maternal mRNA translation in quiescent oocytes as well as in oocytes progressing through the first meiotic division. This genome-wide analysis reveals a global switch in maternal mRNA translation coinciding with oocyte re-entry into the meiotic cell cycle. Messenger RNAs whose translation is highly active in quiescent oocytes invariably become repressed during meiotic re-entry, whereas transcripts repressed in quiescent oocytes become activated. Experimentally, we have defined the exact timing of the switch and the repressive function of CPE elements, and identified a novel role for CPEB1 in maintaining constitutive translation of a large group of maternal mRNAs during maturation.

Structural transitions in poly(A), poly(C), poly(U), and poly(G) and their possible biological roles

The homopolynucleotide (homo-oligonucleotide) tracts function as regulatory elements at various stages of mRNAs life cycle. Numerous cellular proteins specifically bind to these tracts. Among them are the different poly(A)-binding proteins, poly(C)-binding proteins, multifunctional fragile X mental retardation protein which binds specifically both to poly(G) and poly(U) and others. Molecular mechanisms of regulation of gene expression mediated by homopolynucleotide tracts in RNAs are not fully understood and the structural diversity of these tracts can contribute substantially to this regulation. This review summarizes current knowledge on different forms of homoribopolynucleotides, in particular, neutral and acidic forms of poly(A) and poly(C), and also biological relevance of homoribopolynucleotide (homoribo-oligonucleotide) tracts is discussed. Under physiological conditions, the acidic forms of poly(A) and poly(C) can be induced by proton transfer from acidic amino acids of proteins to adenine and cytosine bases. Finally, we present potential mechanisms for the regulation of some biological processes through the formation of intramolecular poly(A) duplexes.

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

Determinants and implications of mRNA poly(A) tail size--does this protein make my tail look big?

While the phenomenon of polyadenylation has been well-studied, the dynamics of poly(A) tail size and its impact on transcript function and cell biology are less well-appreciated. The goal of this review is to encourage readers to view the poly(A) tail as a dynamic, changeable aspect of a transcript rather than a simple static entity that marks the 3' end of an mRNA. This could open up new angles of regulation in the post-transcriptional control of gene expression throughout development, differentiation and cancer.

Specificity factors in cytoplasmic polyadenylation

Poly(A) tail elongation after export of an messenger RNA (mRNA) to the cytoplasm is called cytoplasmic polyadenylation. It was first discovered in oocytes and embryos, where it has roles in meiosis and development. In recent years, however, has been implicated in many other processes, including synaptic plasticity and mitosis. This review aims to introduce cytoplasmic polyadenylation with an emphasis on the factors and elements mediating this process for different mRNAs and in different animal species. We will discuss the RNA sequence elements mediating cytoplasmic polyadenylation in the 3' untranslated regions of mRNAs, including the CPE, MBE, TCS, eCPE, and C-CPE. In addition to describing the role of general polyadenylation factors, we discuss the specific RNA binding protein families associated with cytoplasmic polyadenylation elements, including CPEB (CPEB1, CPEB2, CPEB3, and CPEB4), Pumilio (PUM2), Musashi (MSI1, MSI2), zygote arrest (ZAR2), ELAV like proteins (ELAVL1, HuR), poly(C) binding proteins (PCBP2, αCP2, hnRNP-E2), and Bicaudal C (BICC1). Some emerging themes in cytoplasmic polyadenylation will be highlighted. To facilitate understanding for those working in different organisms and fields, particularly those who are analyzing high throughput data, HUGO gene nomenclature for the human orthologs is used throughout. Where human orthologs have not been clearly identified, reference is made to protein families identified in man.

The poly(rC)-binding protein alphaCP2 is a noncanonical factor in X. laevis cytoplasmic polyadenylation

Post-transcriptional control of mRNA stability and translation is central to multiple developmental pathways. This control can be linked to cytoplasmic polyadenylation in certain settings. In maturing Xenopus oocytes, specific mRNAs are targeted for polyadenylation via recruitment of the Cytoplasmic Polyadenylation Element (CPE) binding protein (CPEB) to CPE(s) within the 3' UTR. Cytoplasmic polyadenylation is also critical to early embryonic events, although corresponding determinants are less defined. Here, we demonstrate that the Xenopus ortholog of the poly(rC) binding protein αCP2 can recruit cytoplasmic poly(A) polymerase activity to mRNAs in Xenopus post-fertilization embryos, and that this recruitment relies on cis sequences recognized by αCP2. We find that the hα-globin 3' UTR, a validated mammalian αCP2 target, constitutes an effective target for cytoplasmic polyadenylation in Xenopus embryos, but not during Xenopus oocyte maturation. We further demonstrate that the cytoplasmic polyadenylation activity is dependent on the action of the C-rich αCP-binding site in conjunction with the adjacent AAUAAA. Consistent with its ability to target mRNA for poly(A) addition, we find that XαCP2 associates with core components of the Xenopus cytoplasmic polyadenylation complex, including the cytoplasmic poly(A) polymerase XGLD2. Furthermore, we observe that the C-rich αCP-binding site can robustly enhance the activity of a weak canonical oocyte maturation CPE in early embryos, possibly via a direct interaction between XαCP2 and CPEB1. These studies establish XαCP2 as a novel cytoplasmic polyadenylation trans factor, indicate that C-rich sequences can function as noncanonical cytoplasmic polyadenylation elements, and expand our understanding of the complexities underlying cytoplasmic polyadenylation in specific developmental settings.

Positive and negative cis-regulatory elements directing postfertilization maternal mRNA translational control in mouse embryos

Mechanisms providing for temporally complex patterns of maternal mRNA translation after fertilization are poorly understood. We employed bioinformatics analysis to compare populations of mRNAs enriched specifically on polysomes at the metaphase II (MII) stage oocyte and late one-cell stages and a detailed deletion/truncation series to identify elements that regulate translation. We used the Bag4 3' untranslated region (UTR) as a model. Bioinformatics analysis revealed one conserved motif, subsequently confirmed by functional studies to be a key translation repressor element. The deletion/truncation studies revealed additional regulatory motifs, most notably a strong translation activator element of <30 nt. Analysis of mRNA secondary structure suggests that secondary structure plays a key role in translation repression. Additional bioinformatics analysis of the regulated mRNA population revealed a diverse collection of regulatory motifs found in small numbers of mRNAs, highlighting a high degree of sequence diversity and combinatorial complexity in the overall control of the maternal mRNA population. We conclude that translational control after fertilization is driven primarily by negative regulatory mechanisms opposing strong translational activators, with stage-specific release of the inhibitory influences to permit recruitment. The combination of bioinformatics analysis and deletion/truncation studies provides the necessary approach for dissecting postfertilization translation regulatory mechanisms.

Recruitment of Orc6l, a dormant maternal mRNA in mouse oocytes, is essential for DNA replication in 1-cell embryos

Mouse oocytes acquire the ability to replicate DNA during meiotic maturation, presumably to ensure that DNA replication does not occur precociously between MI and MII and only after fertilization. Acquisition of DNA replication competence requires protein synthesis, but the identity of the proteins required for DNA replication is poorly described. In Xenopus, the only component missing for DNA replication competence is CDC6, which is synthesized from a dormant maternal mRNA recruited during oocyte maturation, and a similar situation also occurs during mouse oocyte maturation. We report that ORC6L is another component required for acquisition of DNA replication competence that is absent in mouse oocytes. The dormant maternal Orc6l mRNA is recruited during maturation via a CPE present in its 3' UTR. RNAi-mediated ablation of maternal Orc6l mRNA prevents the maturation-associated increase in ORC6L protein and inhibits DNA replication in 1-cell embryos. These results suggest that mammalian oocytes have more complex mechanisms to establish DNA replication competence when compared to their Xenopus counterparts.

Cytoplasmic polyadenylation and cytoplasmic polyadenylation element-dependent mRNA regulation are involved in Xenopus retinal axon development

BACKGROUND

Translation in axons is required for growth cone chemotropic responses to many guidance cues. Although locally synthesized proteins are beginning to be identified, how specific mRNAs are selected for translation remains unclear. Control of poly(A) tail length by cytoplasmic polyadenylation element (CPE) binding protein 1 (CPEB1) is a conserved mechanism for mRNA-specific translational regulation that could be involved in regulating translation in axons.

RESULTS

We show that cytoplasmic polyadenylation is required in Xenopus retinal ganglion cell (RGC) growth cones for translation-dependent, but not translation-independent, chemotropic responses in vitro, and that inhibition of CPE binding through dominant-negative interference severely reduces axon outgrowth in vivo. CPEB1 mRNA transcripts are present at low levels in RGCs but, surprisingly, CPEB1 protein was not detected in eye or brain tissue, and CPEB1 loss-of-function does not affect chemotropic responses or pathfinding in vivo. UV cross-linking experiments suggest that CPE-binding proteins other than CPEB1 in the retina regulate retinal axon development.

CONCLUSION

These results indicate that cytoplasmic polyadenylation and CPE-mediated translational regulation are involved in retinal axon development, but that CPEB1 may not be the key regulator of polyadenylation in the developing retina.

Analysis of the 3' untranslated regions of alpha-tubulin and S-crystallin mRNA and the identification of CPEB in dark- and light-adapted octopus retinas

PURPOSE

We previously reported the differential expression and translation of mRNA and protein in dark- and light-adapted octopus retinas, which may result from cytoplasmic polyadenylation element (CPE)-dependent mRNA masking and unmasking. Here we investigate the presence of CPEs in alpha-tubulin and S-crystallin mRNA and report the identification of cytoplasmic polyadenylation element binding protein (CPEB) in light- and dark-adapted octopus retinas.

METHODS

3'-RACE and sequencing were used to isolate and analyze the 3'-UTRs of alpha-tubulin and S-crystallin mRNA. Total retinal protein isolated from light- and dark-adapted octopus retinas was subjected to western blot analysis followed by CPEB antibody detection, PEP-171 inhibition of CPEB, and dephosphorylation of CPEB.

RESULTS

The following CPE-like sequence was detected in the 3'-UTR of isolated long S-crystallin mRNA variants: UUUAACA. No CPE or CPE-like sequences were detected in the 3'-UTRs of alpha-tubulin mRNA or of the short S-crystallin mRNA variants. Western blot analysis detected CPEB as two putative bands migrating between 60-80 kDa, while a third band migrated below 30 kDa in dark- and light-adapted retinas.

CONCLUSIONS

The detection of CPEB and the identification of the putative CPE-like sequences in the S-crystallin 3'-UTR suggest that CPEB may be involved in the activation of masked S-crystallin mRNA, but not in the regulation of alpha-tubulin mRNA, resulting in increased S-crystallin protein synthesis in dark-adapted octopus retinas.

A deadenylation negative feedback mechanism governs meiotic metaphase arrest

In vertebrate oocytes, meiotic progression is driven by the sequential translational activation of maternal messenger RNAs stored in the cytoplasm. This activation is mainly induced by the cytoplasmic elongation of their poly(A) tails, which is mediated by the cytoplasmic polyadenylation element (CPE) present in their 3' untranslated regions. In Xenopus oocytes, sequential phase-specific translation of CPE-regulated mRNAs is required to activate the maturation-promoting factor, which in turn mediates entry into the two consecutive meiotic metaphases (MI and MII). Here we report a genome-wide functional screening to identify previously unknown mRNAs cytoplasmically polyadenylated at meiotic phase transitions. A significant fraction of transcripts containing, in addition to CPEs, (A + U)-rich element (ARE) sequences (characteristic of mRNAs regulated by deadenylation) were identified. Among these is the mRNA encoding C3H-4, an ARE-binding protein that we find to accumulate in MI and the ablation of which induces meiotic arrest. Our results suggest that C3H-4 recruits the CCR4 deadenylase complex to ARE-containing mRNAs and this, in turn, causes shortening of poly(A) tails. We also show that the opposing activities of the CPEs and the AREs define the precise activation times of the mRNAs encoding the anaphase-promoting complex inhibitors Emi1 and Emi2 during distinct phases of the meiotic cycle. Taken together, our results show that an 'early' wave of cytoplasmic polyadenylation activates a negative feedback loop by activating the synthesis of C3H-4, which in turn would recruit the deadenylase complex to mRNAs containing both CPEs and AREs. This negative feedback loop is required to exit from metaphase into interkinesis and for meiotic progression.

Translational control by cytoplasmic polyadenylation in Xenopus oocytes

Elongation of the poly(A) tails of specific mRNAs in the cytoplasm is a crucial regulatory step in oogenesis and early development of many animal species. The best studied example is the regulation of translation by cytoplasmic polyadenylation elements (CPEs) in the 3′ untranslated region of mRNAs involved in Xenopus oocyte maturation. In this review we discuss the mechanism of translational control by the CPE binding protein (CPEB) in Xenopus oocytes as follows:1.

The cytoplasmic polyadenylation machinery such as CPEB, the subunits of cleavage and polyadenylation specificity factor (CPSF), symplekin, Gld-2 and poly(A) polymerase (PAP).

2.

The signal transduction that leads to the activation of CPE-mediated polyadenylation during oocyte maturation, including the potential roles of kinases such as MAPK, Aurora A, CamKII, cdk1/Ringo and cdk1/cyclin B.

3.

The role of deadenylation and translational repression, including the potential involvement of PARN, CCR4/NOT, maskin, pumilio, Xp54 (Ddx6, Rck), other P-body components and isoforms of the cap binding initiation factor eIF4E.

Finally we discuss some of the remaining questions regarding the mechanisms of translational regulation by cytoplasmic polyadenylation and give our view on where our knowledge is likely to be expanded in the near future.

ElrA binding to the 3'UTR of cyclin E1 mRNA requires polyadenylation elements

The early cell divisions of Xenopus laevis and other metazoan embryos occur in the presence of constitutively high levels of the cell cycle regulator cyclin E1. Upon completion of the 12th cell division, a time at which many maternal proteins are downregulated by deadenylation and destabilization of their encoding mRNAs, maternal cyclin E1 protein is downregulated while its mRNA is polyadenylated and stable. We report here that stable polyadenylation of cyclin E1 mRNA requires three cis-acting elements in the 3' untranslated region; the nuclear polyadenylation sequence, a contiguous cytoplasmic polyadenylation element and an upstream AU-rich element. ElrA, the Xenopus homolog of HuR and a member of the ELAV gene family binds the cyclin E1 3'UTR with high affinity. Deletion of these elements dramatically reduces the affinity of ElrA for the cyclin E1 3'UTR, abolishes polyadenylation and destabilizes the mRNA. Together, these findings provide compelling evidence that ElrA functions in polyadenylation and stabilization of cyclin E1 mRNA via binding these elements.

A novel maternally transcribed homeobox gene,Eso-1, is preferentially expressed in oocytes and regulated by cytoplasmic polyadenylation

The homeobox gene families play important roles in the transcriptional regulation of gene expression prior to and during embryo development. To identify novel homeobox genes expressed in early embryonic development, we conducted a degenerated oligonucleotide polymerase chain reaction (PCR) to screen a mouse embryonic stem (ES) cell cDNA library. A novel homeobox-containing gene, Eso-1, which is preferentially expressed in ES cells and ovaries, was identified. The full-length Eso-1 cDNA was found to be 1,710 bp with a predicted homeodomain that has no significant homology to previously reported homeodomain proteins. Eso-1 was mapped to chromosome 14A3. Reverse transcription-polymerase chain reaction (RT-PCR) analyses showed that Eso-1 was expressed through oogenesis and continuing to be expressed through to the blastocyst stage. De novo expression of Eso-1 started at 13.5 days postcoitum in the ovaries, which coincides with the initiation of oogenesis. Northern blot analyses demonstrated that Eso-1 is preferentially expressed in both ovaries and ES cells as a 1.7-kb transcript. Results from whole mount in situ hybridization revealed that Eso-1 in oocytes showed increased expression from primordial to antral follicles. The 3'-untranslated region of Eso-1 transcripts contained cytoplasmic polyadenylation sequences while the length of poly (A) tails changed during oocyte maturation, indicating that Eso-1 expression is controlled by time-dependent translational activation. We suggest that the novel homeodomain protein, Eso-1, plays a role during oocyte maturation and early embryonic development.

RNA-binding proteins in early development

RNA-binding proteins play a major part in the control of gene expression during early development. At this stage, the majority of regulation occurs at the levels of translation and RNA localization. These processes are, in general, mediated by RNA-binding proteins interacting with specific sequence motifs in the 3'-untranslated regions of their target RNAs. Although initial work concentrated on the analysis of these sequences and their trans-acting factors, we are now beginning to gain an understanding of the mechanisms by which some of these proteins function. In this review, we will describe a number of different families of RNA-binding proteins, grouping them together on the basis of common regulatory strategies, and emphasizing the recurrent themes that occur, both across different species and as a response to different biological problems.

A novel method for constructing murine cDNA library enriched with maternal mRNAs exhibiting de novo independent post-fertilization polyadenylation

Recently, mouse maternal mRNAs such as SSEC-D, Spin, beta-catenin, Ptp4a1, and Maid have been found to exhibit de novo independent polyadenylation after fertilization. To obtain an overall picture of post-fertilization polyadenylation events, we developed a novel method for constructing murine fertilized egg cDNA library enriched with cDNAs exhibiting de novo independent polyadenylation. As a pilot study, we isolated at least four new maternal mRNAs exhibiting extension of poly(A) tail in fertilized 1-cell eggs. Moreover, various types of polyadenylation of maternal RNAs were observed at this stage, suggesting the presence of novel mechanisms for regulating the length of poly(A) tails of maternal mRNA. This is the first report of successful construction of a cDNA library enriched with newly polyadenylated maternal mRNAs derived from post-fertilized mouse eggs. This cDNA library will be useful for molecular analysis of the mechanisms underlying post-fertilization polyadenylation of mammalian maternal RNAs.

Analysis of cDNAs coding for immunologically dominant antigens from an oncosphere-specific cDNA library of Echinococcus multilocularis

A cDNA library based on mRNA from oncospheres of Echinococcus multilocularis was constructed and screened with an oncosphere-specific rabbit serum. cDNA sequences of three clones that were isolated out of this library are discussed: one codes for a serpin-like proteinase inhibitor, the first isolated from cestodes. Two other clones code for dominant oncosphere antigens and represent homologues of known genes: one is known from several taeniid cestodes as a protective antigen containing fibronectin III domains, the second is related to genes of small heat shock proteins. It contains an internal duplication that might be specific for platyhelminths.

Structural transitions in polycytidylic acid: proton buffer capacity data

The pH-dependences of proton buffer capacity of poly(C) were computed on the basis of the literature data. In these curves there were observed four peaks: two narrow and two wide ones. The first narrow peak reflects the process of cooperative formation of double helices, which is induced by protonation of the N3 atom of nucleotide bases. The first wide peak is assigned to noncooperative process of poly(C) double helices protonation at the N3 nitrogen atom. It is proposed that the second wide peak corresponds to noncooperative protonation of the neutral cytosine bases at the oxygen atom. This reaction causes cooperative dissociation of the poly(C) double helices. The second narrow peak reflects the dissociation process.

Translational regulation and RNA localization in Drosophila oocytes and embryos

Translational control is a prevalent means of gene regulation during Drosophila oogenesis and embryogenesis. Multiple maternal mRNAs are localized within the oocyte, and this localization is often coupled to their translational regulation. Subsequently, translational control allows maternally deposited mRNAs to direct the early stages of embryonic development. In this review we outline some general mechanisms of translational regulation and mRNA localization that have been uncovered in various model systems. Then we focus on the posttranscriptional regulation of four maternal transcripts in Drosophila that are localized during oogenesis and are critical for embryonic patterning: bicoid (bcd), nanos (nos), oskar (osk), and gurken (grk). Cis- and trans-acting factors required for the localization and translational control of these mRNAs are discussed along with potential mechanisms for their regulation.

Posttranscriptional regulation of cyclin A1 and cyclin A2 during mouse oocyte meiotic maturation and preimplantation development

A shift from a meiotic cell cycle to a mitotic cell cycle occurs following fertilization. The molecular basis for this transition, however, is poorly understood. Although cyclin A1 is proposed to regulate M phase in the meiotic cell cycle, and cyclin A2 is proposed to regulate S and M phases in the mitotic cell cycle, little is known about changes in the expression levels of cyclin A1 and A2 during meiotic and mitotic cell cycles in mammalian oocytes. We report that the mRNA levels of both cyclins A1 and A2 decrease during oocyte maturation. The amount of cyclin A1 mRNA then increases between the one-cell and blastocyst stages, whereas that of cyclin A2 remains relatively constant. The amount of cyclin A1 protein declines during maturation and is not readily detected from the two-cell to the blastocyst stage. In contrast, cyclin A2 is not readily detected in the oocyte and metaphase II-arrested egg but is detected following fertilization and throughout the subsequent stages of preimplantation development. The appearance of cyclin A2 protein following fertilization positively correlates with an increase in the size of the mRNA. This increase, as well as the increase in the amount of cyclin A2 protein, is prevented by 3'-deoxyadenosine (3'-dA), an inhibitor of polyadenylation. Consistent with a role for cyclin A2 in regulating the G1/S transition, 3'-dA also inhibits DNA replication in treated one-cell embryos. These results suggest that regulation of expression of cyclins A1 and A2 is under posttranscriptional regulation and that the observed changes in their expression may be involved in the transformation of a meiotic cell cycle to a mitotic cell cycle following fertilization.

Regulation of the mRNAs encoding proteins of the BMP signaling pathway during the maternal stages of Xenopus development

Activation of the Xenopus bone morphogenetic protein (BMP) pathway is coincident with the onset of zygotic transcription but requires maternal signaling proteins. The mechanisms controlling the translation of mRNAs that encode proteins of the BMP pathway were investigated by using polysome association as an assay for translational activity. Our results indicate that five different mRNAs encoding proteins of the BMP pathway were translationally regulated during Xenopus development. These mRNAs were either not associated or inefficiently associated with polysomes in oocytes, and each was recruited to polysomes at a different developmental stage. The Smad1 and ALK-2 mRNAs were recruited to polysomes during oocyte maturation, whereas the BMP-7 and XSTK9 mRNAs were recruited during the early stages of embryogenesis. The ALK-3 mRNA was not efficiently associated with polysomes during any maternal stage of development and was efficiently recruited to polysomes only after the onset of zygotic transcription. In general, for all stages except oocytes, polysome recruitment was associated with the presence of a 3' poly(A) tail. However, there was not an obvious correlation between the absolute length of poly(A) and the efficiency of polysome recruitment, indicating that the relationship between poly(A) tail length and translation during early frog embryogenesis is complex. We further focused on the BMP-7 mRNA and demonstrated that sequence elements within the 3'UTR were necessary for recruitment of the BMP-7 mRNA to polysomes and sufficient to direct the addition of poly(A) and activate translation of a reporter during embryogenesis. Interestingly, the BMP-7 mRNA lacks the previously defined eCPE sequences proposed to direct poly(A) addition and translational activation during embryogenesis. The implications of our findings for translational regulation of maternal mRNAs during embryogenesis and for the activation of the BMP pathway are discussed.

Expression of genes involved in mammalian meiosis during the transition from egg to embryo

The ooplasm of higher eukaryotes provides substances necessary for completing the last stages of meiosis and initiating the first mitotic division. These processes are firmly attuned to other events in the egg and newly formed embryo, such as switching from the use of maternal transcripts to the onset of zygotic transcription. In mammals little is known about the molecular mechanisms guiding this transition, largely due to the lack of information about genes expressed in the egg and early embryos. Studies of yeast mitosis have contributed much of what is known about the vertebrate cell cycle, and recent reports indicate that homologs of yeast DNA repair genes also function during mammalian gametogenesis. To examine whether this conservation can be expanded to include genes operative in oocyte meiosis, we performed a computer-based search for homologs of yeast genes that are induced during sporulation in C. elegans, Drosophila, and mammals. Results from this study suggest that yeast and higher eukaryotes share genes that coordinate the overall process of meiosis. However intriguing differences exist, reflecting the distinctive mechanisms governing the progression of meiosis in each organism. ESTs representing more than half of the mammalian homologs are present in mouse cDNA libraries that contains genes controlling the meiosis/mitosis transition. About 50% of these genes contain potential cis-elements for cytoplasmic polyadenylation in their 3'-UTR, suggesting the importance of controlled translation in the egg and zygote.

Translational control in vertebrate development

Translational control plays a large role in vertebrate oocyte maturation and contributes to the induction of the germ layers. Translational regulation is also observed in the regulation of cell proliferation and differentiation. The features of an mRNA that mediate translational control are found both in the 5' and in the 3' untranslated regions (UTRs). In the 5' UTR, secondary structure, the binding of proteins, and the presence of upstream open reading frames can interfere with the association of initiation factors with the cap, or with scanning of the initiation complex. The 3' UTR can mediate translational activation by directing cytoplasmic polyadenylation and can confer translational repression by interference with the assembly of initiation complexes. Besides mRNA-specific translational control elements, the nonspecific RNA-binding proteins contribute to the modulation of translation in development. This review discusses examples of translational control and their relevance for developmental regulation.

Identification of a novel isoform of poly(A) polymerase, TPAP, specifically present in the cytoplasm of spermatogenic cells

We have identified cDNA clones encoding a testis-specific poly(A) polymerase, termed TPAP, a candidate molecule responsible for cytoplasmic polyadenylation of preexisting mRNAs in male haploid germ cells. The TPAP gene was most abundantly expressed coincident with the additional elongation of mRNA poly(A) tails in round spermatids. The amino acid sequence of TPAP contained 642 residues, and shared a high degree of identity (86%) with that of a nuclear poly(A) polymerase, PAP II. Despite the sequence conservation of functional elements, including three catalytic Asp residues, an ATP-binding site, and an RNA-binding domain, TPAP lacked an approximately 100-residue C-terminal sequence carrying one of two bipartite-type nuclear localization signals, and part of a Ser/Thr-rich domain found in PAP II. Recombinant TPAP produced by an in vitro transcription/translation system was capable of incorporating the AMP moiety from ATP into an oligo(A)(12) RNA primer in the presence of MnCl(2). Moreover, an affinity-purified antibody against the 12-residue C-terminal sequence of TPAP recognized a 70-kDa protein in the cytoplasm of spermatogenic cells. These results suggest that TPAP may participate in the additional extension of mRNA poly(A) tails in the cytoplasm of male germ cells, and may play an important role in spermiogenesis, probably through the stabilization of mRNAs.

The temporal control of Wee1 mRNA translation during Xenopus oocyte maturation is regulated by cytoplasmic polyadenylation elements within the 3'-untranslated region

The Wee1 protein tyrosine kinase is a key regulator of cell cycle progression. Wee1 activity is necessary for the control of the first embryonic cell cycle following the fertilization of meiotically mature Xenopus oocytes. Wee1 mRNA is present in immature oocytes, but Wee1 protein does not accumulate in immature oocytes or during the early stages of progesterone-stimulated maturation. This delay in Wee1 translation is critical since premature Wee1 protein accumulation has been shown to inhibit oocyte maturation. In this study we provide evidence that Wee1 protein accumulation is regulated at the level of mRNA translation. This translational control is directed by sequences within the Wee1 mRNA 3'-untranslated region (3' UTR). Specifically, cytoplasmic polyadenylation element (CPE) sequences within the Wee1 3' UTR are necessary for full translational repression in immature oocytes. Our data further indicate that while CPE-independent mechanisms may regulate the levels of Wee1 protein accumulation during progesterone-stimulated oocyte maturation, the timing of Wee1 mRNA translational induction is directed through a CPE-dependent mechanism.

Timely translation during the mouse oocyte-to-embryo transition

In the mouse, completion of oocyte maturation and the initiation of preimplantation development occur during transcriptional silence and depend on the presence and translation of stored mRNAs transcribed in the growing oocyte. The Spin gene has three transcripts, each with an identical open reading frame and a different 3′ untranslated region (UTR). (Beta)-galactosidase-tagged reporter transcripts containing each of the different Spin 3′UTRs were injected into oocytes and zygotes and (beta)-galactosidase activity was monitored. Results from these experiments suggest that differential polyadenylation and translation occurs at two critical points in the oocyte-to-embryo transition - upon oocyte maturation and fertilization - and is dependent on sequences in the 3′UTR. The stability and mobility shifts of ten other maternal transcripts were monitored by reprobing a northern blot of oocytes and embryos collected at 12 hour intervals after fertilization. Some are more stable than others and the upward mobility shift associated with polyadenylation correlates with the presence of cytoplasmic polyadenylation elements (CPEs) within about 120 nucleotides of the nuclear polyadenylation signal. A survey of the 3′ UTRs of expressed sequence tag clusters from a mouse 2-cell stage cDNA library indicates that about one third contain CPEs. We suggest that differential transcript stability and a translational control program can supply the diversity of protein products necessary for oocyte maturation and the initiation of development.

The expression and structure of TGF-beta2 transcripts in rat muscles

The transforming growth factor-beta2 (TGF-beta2) transcripts expressed in various tissues of rat were characterised by RT-PCR and the nucleotide sequence of the cDNAs determined. A transcript with an 84-nucleotide insert in the latency-associated peptide region, the long form, was found. The long form of TGF-beta2 was detected in the aorta, primary bronchus, uterus, heart, skeletal muscle, sciatic nerve and spinal cord but not in the intestine. The 3' untranslated region of TGF-beta2 contained several putative AU-rich elements and multiple polyadenylation sites, indicating post-transcriptional regulation of TGF-beta2 synthesis. The levels of TGF-beta2 transcripts were estimated using semi-quantitative RT-PCR. They were down-regulated during muscle development and up-regulated after denervation. The long form constituted approximately 6% of the total TGF-beta2 messages in skeletal muscle.

Effects of cyclin A2 noncoding regions on reporter gene translation during early development of Xenopus laevis

The repression of translation of Xenopus cyclin A2 transcripts during early development was examined by analyzing the effects of cyclin A2 noncoding regions using a CAT reporter system. On their own, the 5' and 3' UTRs (untranslated regions) were unable to inhibit reporter translation until approximately the time of the midblastula transition. Transcripts containing the 3' UTR were polyadenylated after fertilization and the midblastula transition. When both noncoding regions flanked a CAT reporter gene, translation was repressed at all stages of development examined in spite of their polyadenylation after fertilization. From these data, we conclude that the 5' and 3' UTRs interact synergically to prevent translation during early development and that the poly(A) tail is insufficient to promote their translation.

Identification of a C-rich element as a novel cytoplasmic polyadenylation element in Xenopus embryos

During Xenopus early development, the length of the poly(A) tail of maternal mRNAs is a key element of translational control. Several sequence elements (cytoplasmic polyadenylation elements) localized in 3' untranslated regions have been shown to be responsible for the cytoplasmic polyadenylation of certain maternal mRNAs. Here, we demonstrate that the mRNA encoding the catalytic subunit of phosphatase 2A is polyadenylated after fertilization of Xenopus eggs. This polyadenylation is mediated by the additive effects of two cis elements, one being similar to already described cytoplasmic polyadenylation elements and the other consisting of a polycytosine motif. Finally, a candidate specificity factor for polycytosine-mediated cytoplasmic polyadenylation has been purified and identified as the Xenopus homologue of human alpha-CP2.

Smaug, a novel and conserved protein, contributes to repression of nanos mRNA translation in vitro

Proper deployment of Nanos protein at the posterior of the Drosophila embryo, where it directs posterior development, requires a combination of RNA localization and translational controls. These controls ensure that only the posteriorly-localized nanos mRNA is translated, whereas unlocalized nanos mRNA is translationally repressed. Here we describe cloning of the gene encoding Smaug, an RNA-binding protein that interacts with the sequences, SREs, in the nanos mRNA that mediate translational repression. Using an in vitro translation assay, we demonstrate that SRE-dependent repression occurs in extracts from early stage embryos. Immunodepletion of Smaug from the extracts eliminates repression, consistent with the notion that Smaug is involved. Smaug is a novel gene and the existence of potential mammalian Smaug homologs raises the possibility that Smaug represents a new class of conserved translational repressor.

Translational control of nuclear lamin B1 mRNA during oogenesis and early development of Xenopus

Cytoplasmic polyadenylation of specific mRNAs is commonly correlated with their translational activation during development. A canonical nuclear polyadenylation element AAUAAA (NPE) and cytoplasmic polyadenylation element(s) (CPE) are necessary and sufficient for polyadenylation during egg maturation. We have characterized cis-acting sequences of Xenopus nuclear lamin B1 mRNA that mediate translational regulation. By injection of synthetic RNAs into oocytes we show that the two CPE-like elements found in the 3'-untranslated region of B1 mRNA act as translational repressors in oocytes. The same CPEs in conjunction with the NPE confer transient polyadenylation and translational activation during egg maturation. Poly(A) length determination of the endogenous lamin B1 mRNA reveals a gradual increase of poly(A) tail length in early development up to mid-blastula, and a shortening of poly(A) tails during gastrulation and neurulation. The same kinetic and extent of polyadenylation and poly(A) tail shortening is observed with synthetic RNAs injected into fertilized eggs. Polyadenylation and translational activation of these RNAs is independent of the two CPEs and a NPE during early development. While translational regulation of lamin B1 mRNA functions in parts via established mechanisms, the pattern of polyadenylation and deadenylation during early development points to a novel mode of translational regulation.

Characterization of Xenopus RalB and its involvement in F-actin control during early development

We describe the characterization and a functional analysis in Xenopus development of RalB, a small G protein. RalB RNA and protein are detectable during oogenesis and early development, but the gene is expressed only weakly in adult tissues. The RalB transcripts are processed by poly(A) extension during oocyte maturation and up to the gastrulation stage. Microinjection of wild-type or mutant RalB RNAs was performed in fertilized eggs in order to gain insight into the function of RalB during development. We show that during cleavage stages the activated GTP form of RalB specifically induces a cortical reaction that affects the localization of pigment granules. The use of different drugs suggests that this reaction is dependent on the outer cortical actin array. The relation between F-actin and RalB was shown by confocal analysis. Injection of mRNAs encoding the mutated activated form of RalB leads, at dependent doses, to a blocking of gastrulation or defects in closing of neural folding structures. In contrast, the inactivated form blocks only the closing of neural tube. Altogether, these observations suggest that RalB is part of a regulatory pathway that may affect the blastomere cytoskeleton and take part in early development.

CPEB-mediated cytoplasmic polyadenylation and the regulation of experience-dependent translation of alpha-CaMKII mRNA at synapses

Long-term changes in synaptic efficacy may require the regulated translation of dendritic mRNAs. While the basis of such regulation is unknown, it seemed possible that some features of translational control in development could be recapitulated in neurons. Polyadenylation-induced translation of oocyte mRNAs requires the cis-acting CPE sequence and the CPE-binding protein CPEB. CPEB is also present in the dendritic layers of the hippocampus, at synapses in cultured neurons, and in postsynaptic densities of adult brain. alpha-CaMKII mRNA, which is localized in dendrites and is necessary for synaptic plasticity and LTP, contains two CPEs. These CPEs are bound by CPEB and mediate polyadenylation-induced translation in injected Xenopus oocytes. In the intact brain, visual experience induces alpha-CaMKII mRNA polyadenylation and translation, suggesting that this process likely occurs at synapses.

A subset of poly(A) polymerase is concentrated at sites of RNA synthesis and is associated with domains enriched in splicing factors and poly(A) RNA

We have performed a detailed study of the spatial distribution of a set of mRNA 3' processing factors in human T24 cells. A key enzyme in RNA 3' processing, poly(A) polymerase (PAP), was found in the cytoplasm and throughout the nucleus in a punctated pattern. A subset of the various isoforms of PAP was specifically concentrated at sites of RNA synthesis in the nucleoplasm. Additionally, the other factors necessary for RNA 3' processing, such as CstF, CPSF, and PABII, were also found at these transcription sites. Our data show that the set of 3' processing factors that are presumed to be necessary for most RNA 3' cleavage and polyadenylation is indeed found at sites of RNA synthesis in the nucleoplasm. Furthermore, sites of RNA synthesis that are particularly enriched in both PAP and PABII are found at the periphery of irregularly shaped domains, called speckles, which are known to contain high concentrations of splicing factors and poly(A) RNA. Disruption of RNA 3' processing by the drug 9-beta-D-arabinofuranosyladenine caused the speckles to break up into smaller structures. These findings indicate that there is a spatial and structural relationship between 3' processing and the nuclear speckles. Our studies reveal a complex and distinct organization of the RNA 3' processing machinery in the mammalian cell nucleus.

Cloning of a complementary DNA encoding an Ambystoma mexicanum metallothionein, AmMT, and expression of the gene during early development

We have used a polymerase chain reaction strategy to isolate a metallothionein (MT) cDNA from the amphibian Ambystoma mexicanum (axolotl). This cDNA is 875-bp long and encodes a 60 amino acid protein, AmMT, typical for family 1 MTs. It contains 20 cysteine (Cys) residues that can be aligned with those of other vertebrate MTs. The overall structure of the protein is unique among vertebrates in having only two amino acid residues before the first Cys at the amino-terminal end. Northern analyses showed that AmMT is expressed throughout embryogenesis, giving rise to three mRNA species of 650, 750, and 1,600 nucleotides (nt). The 750 and 1,600 nt transcripts appear to result from differential use of polyadenylation signals, whereas the 650 nt RNA could arise from deadenylation of the 750-nt transcript. Both the 750- and 1,600-nt RNAs were presented in embryos before the mid-blastula transition (MBT). After the MBT, the 750-nt RNA was replaced by the 650-nt RNA which was gradually degraded to undetectable levels in post-neurulation embryos. Levels of the 1,600-nt transcript increased at gastrulation and reach a maximum in Stage 30 embryos. In adult animals, levels of the 750-nt RNA were high in liver and testes, and very low in lung, gut, skin, and oviducts, whereas levels of the 1,600-nt transcript were similar and moderately elevated in all tissues examined. In contrast, in Xenopus laevis, Northern analysis did not detect XIMT-A mRNA in embryos before late neurulation (Stage 24). XIMT-A mRNA levels then increased sharply in Stage 36 hatched embryos at levels similar to those found in adult livers. These results show that AmMT presents a unique expression pattern among metazoans being transcribed as two transcripts differing in the length of their 3' untranslated regions, the levels of which vary during embryogenesis and in adult tissues.

Polyadenylation-mediated translational regulation of maternal P450(11beta) mRNA in frog oocytes

Northern blot analysis of bullfrog tissues using a cDNA probe of cytochrome P450(11beta) showed that a large amount of message was present in the ovary as well as in the adrenal tissue. Two kinds of mRNA of different sizes were found in the ovary. Sequence determination of the two cDNAs and analysis by reverse-transcription polymerase chain reaction indicated that the protein encoded by the larger mRNA was identical to the adrenal enzyme, while the protein encoded by the smaller had a truncated sequence lacking an extension peptide necessary for the protein transport to the mitochondria. The mRNAs were present in the oocytes but not in the follicular cells, and their content in an oocyte varied little during its maturation. Immunoblot analyses of the mitochondrial fraction of oocytes failed to demonstrate the presence of P450(11beta) protein. In contrast the eggs were found to contain a large amount of enzymatically active protein. Interestingly the mRNA has a cis-element called cytoplasmic polyadenylation element at its 3' untranslated region. When poly(A) tails of the message prepared from eggs and oocytes were examined by RNase H digestion or reverse-transcription polymerase chain reaction, those of eggs were about 150 nucleotides longer than those of oocytes. These results suggest that translation of the message is stimulated during the oocyte maturation as a result of enhanced polyadenylation at its 3'-end. Finally a finding is presented that progesterone was converted to 11beta-hydroxyprogesterone by the frog P450(11beta), implying that the enzyme expressed in eggs may control a level of progesterone which is needed to initiate the oocyte maturation.

Stimulation of translation and cytoplasmic polyadenylation by the Xenopus c-mycI 3'-untranslated region

Expression of messenger RNA (mRNA) in both embryonic and adult cells may be profoundly influenced by untranslated sequences in the 3'-end. Elements in the 3'-untranslated regions (UTRs) of messengers are known to influence messenger stability, polyadenylation, and translation. We have examined the effects of the 3'-UTR of Xenopus laevis c-mycI (either alone or in combination with the 5'-first exon) on the expression of a chloramphenicol acetyltransferase (CAT) reporter in Xenopus embryos. The Xenopus c-mycI 3'-UTR enhanced messenger translation independent of the 5'-UTR. RNase H analysis indicated that the Xenopus c-mycI 3'-UTR can promote the cytoplasmic polyadenylation of CAT mRNA in embryos. The result suggests that the post-fertilization enhancement of translation caused by the c-mycI 3'-UTR may be a consequence of cytoplasmic polyadenylation. A uridine (U)-rich sequence in the Xenopus c-mycI 3'-UTR that may be responsible for polyadenylation is similar to an element that destabilizes mammalian c-myc transcripts. We discuss the possibility that U-rich sequences may play a dual role by destabilizing growth-related transcripts in adult cells and stimulating their polyadenylation during development, and we propose that a switch in the role of such sequences in adult cells could lead to stabilization of these messengers, increased translation, and abnormal growth control.

Laminin chain-specific gene expression during mouse oocyte maturation

Expression patterns of laminin chain mRNAs (A, B1, and B2) during mouse oocyte maturation were examined using the competitive reverse transcription-polymerase chain reaction (RT-PCR) method. Total and poly (A)-rich mRNAs isolated from various stages of maturing oocytes in vitro were subjected to RT-PCR and the precise amount of laminin chain-specific mRNA transcripts was estimated by adding externally known amounts of in vitro transcribed mutant cRNA transcripts as an internal control. The estimated copy numbers for A, B1, and B2 chain mRNAs in a single germinal vesicle-stage oocyte were 1.34 +/- 0.19 x 10(5), 6.95 +/- 0.32 x 10(6), and 2.0 +/- 0.56 x 10(5), respectively. Although notable changes of all laminin chain mRNA levels were not observed at any stage of meiotic maturation in total mRNA preparation, chain- and meiotic stage-dependent alterations of poly (A)-tailed mRNA quantities were observed in poly (A)-rich mRNA preparation. A potent RNA synthesis blocker, alpha-amanitin did not influence the changes of mRNA levels, implying the presence of posttranscriptional regulation mechanism in the expression of laminin chains during mouse oocyte maturation. Discrete and time-dependent deadenylation of A and B1 chain, but not B2 chain mRNA, was observed during oocyte maturation by a rapid amplification of cDNA ends (RACE)-PCR. In germinal vesicle (GV)-stage oocytes, only B1 chain was found to be present in a highly polyadenylated state and subsequent deadenylation was observed as meiosis progressed. The poly (A) tail modification was dependent on the initiation of meiotic resumption. Although all laminin chain mRNAs were found in fully grown and meiotically competent mouse oocytes, Western blot analysis detected the B1 chain polypeptide only in GV- and polar body (PB)-stage eggs. These results suggest that the expression of laminin B1 chain in mouse oocytes may be due to its large amount of mRNA transcripts and/or high level of polyadenylation state that is efficient for translational activation.

Isolation and characterization of a cDNA encoding a Xenopus immunoglobulin binding protein, BiP (grp78)

We have isolated a full-length cDNA clone encoding a Xenopus laevis immunoglobulin binding protein (BiP; also called glucose-regulated protein or grp78). The Bip cDNA sequence includes an open reading frame of 1,965 bp encoding a 655 amino acid protein with an N-terminal hydrophobic leader sequence and a C-terminal KDEL tetrapeptide which has been found in other lumenal proteins of the endoplasmic reticulum. The 3' untranslated region contains a polyadenylation and an adenylation control element (ACE) as well as a putative mRNA instability sequence. The Xenopus BiP amino acid sequence displayed high identity with BiP from other vertebrates including chicken (91.3%), rat (90.7%), and human (89.9%). Northern hybridization analysis demonstrated that BiP mRNA was present constitutively in the Xenopus A6 kidney epithelial cell line and that BiP mRNA levels could be enhanced by treatment of the cells with galactose-free media, 2-deoxyglucose, 2-deoxygalactose, glucosamine, tunicamycin, heat shock, dithiothreitol, and the calcium ionophore, A23187. Finally, while BiP mRNA was detected in all of the adult tissues examined, the relative level of BiP mRNA differed dramatically between organs. For example, relatively high levels of BiP mRNA were detected in liver with moderate levels in testis, ovary and heart and reduced levels in eye and muscle tissue.

Spindlin, a major maternal transcript expressed in the mouse during the transition from oocyte to embryo

Timely translation of maternal transcripts and post-translational modification of their gene products control the initial development of preimplantation-stage embryos. We have isolated and characterized a gene encoding a stage-specific embryonic protein. This novel gene, spindlin (Spin), is an abundant maternal transcript present in the unfertilized egg and 2-cell, but not 8-cell, stage embryo. Spin exhibits high homology to a multicopy gene, Y-linked spermiogenesis-specific transcript (Ssty), and together they form a new gene family expressed during gametogenesis. We find that spindlin associates with the meiotic spindle and is modified by phosphorylation in a cell-cycle-dependent fashion. Furthermore, it comigrates with the previously described 30x10(3) Mr metaphase complex which is posttranslationally modified during the first mitotic cell cycle. Our data suggest that spindlin plays a role in cell-cycle regulation during the transition from gamete to embryo.

Mechanisms of translational control in early development

Oocytes accumulate a dowry of maternal mRNAs in preparation for embryogenesis. These maternal transcripts are kept dormant until late oogenesis or early embryogenesis when their translation is activated. In recent years, three types of translational control acting on maternal mRNAs have emerged: translational activation by cytoplasmic polyadenylation, translational activation by RNA localization, and regulated translational repression. In each case, translational control depends on the binding of trans-acting factors to sequences in the 3' untranslated region (3'UTR). Identification of these trans-acting factors is beginning to shed light on the molecular mechanisms that mediate translational control.

Translational regulation of maternal mRNAs

Translational regulation of maternal mRNAs serves to constrain their activities in both time and space. Both types of constraint might be expected to be critical for normal development and the rather short list of maternal mRNAs for which this has been shown to be true has expanded considerably over the past year. Substantial progress has also been reported in the identification and characterization of the cis-acting elements and trans-acting factors that mediate translational regulation and their interactions with one another.

CPEB controls the cytoplasmic polyadenylation of cyclin, Cdk2 and c-mos mRNAs and is necessary for oocyte maturation in Xenopus

Cytoplasmic polyadenylation is a key mechanism controlling maternal mRNA translation in early development. In most cases, mRNAs that undergo poly(A) elongation are translationally activated; those that undergo poly(A) shortening are deactivated. Poly(A) elongation is regulated by two cis-acting sequences in the 3'-untranslated region (UTR) of responding mRNAs, the polyadenylation hexanucleotide AAUAAA and the U-rich cytoplasmic polyadenylation element (CPE). Previously, we cloned and characterized the Xenopus oocyte CPE binding protein (CPEB), showing that it was essential for the cytoplasmic polyadenylation of B4 RNA. Here, we show that CPEB also binds the CPEs of G10, c-mos, cdk2, cyclins A1, B1 and B2 mRNAs. We find that CPEB is necessary for polyadenylation of these RNAs in egg extracts, suggesting that this protein is required for polyadenylation of most RNAs during oocyte maturation. Our data demonstrate that the complex timing and extent of polyadenylation are partially controlled by CPEB binding to multiple target sites in the 3' UTRs of responsive mRNAs. Finally, injection of CPEB antibody into oocytes not only inhibits polyadenylation in vivo, but also blocks progesterone-induced maturation. This is due to inhibition of polyadenylation and translation of c-mos mRNA, suggesting that CPEB is critical for early development.