Anteroposterior patterning of the Drosophila embryo depends on a gradient of Nanos protein arising from the posterior pole. This gradient results from both nanos mRNA translational repression in the bulk of the embryo and translational activation of nanos mRNA localized at the posterior pole. Two mechanisms of nanos translational repression have been described, at the initiation step and after this step. Here we identify a novel level of nanos translational control. We show that the Smaug protein bound to the nanos 3Ј UTR recruits the deadenylation complex CCR4-NOT, leading to rapid deadenylation and subsequent decay of nanos mRNA. Inhibition of deadenylation causes stabilization of nanos mRNA, ectopic synthesis of Nanos protein and head defects. Therefore, deadenylation is essential for both translational repression and decay of nanos mRNA. We further propose a mechanism for translational activation at the posterior pole. Translation of nanos mRNA at the posterior pole depends on oskar function. We show that Oskar prevents the rapid deadenylation of nanos mRNA by precluding its binding to Smaug, thus leading to its stabilization and translation. This study provides insights into molecular mechanisms of regulated deadenylation by specific proteins and demonstrates its importance in development.

Translational control of maternal mRNA through regulation of poly(A) tail length is crucial during early development. The nuclear poly(A) binding protein, PABP2, was identified biochemically from its role in nuclear polyadenylation. Here, we analyze the in vivo function of PABP2 in Drosophila. PABP2 is required in vivo for polyadenylation, and Pabp2 function, including poly(A) polymerase stimulation, is essential for viability. We also demonstrate an unanticipated cytoplasmic function for PABP2 during early development. In contrast to its role in nuclear polyadenylation, cytoplasmic PABP2 acts to shorten the poly(A) tails of specific mRNAs. PABP2, together with the deadenylase CCR4, regulates the poly(A) tails of oskar and cyclin B mRNAs, both of which are also controlled by cytoplasmic polyadenylation. Both Cyclin B protein levels and embryonic development depend upon this regulation. These results identify a regulator of maternal mRNA poly(A) tail length and highlight the importance of this mode of translational control.

SummaryTranslational regulation plays an essential role in Drosophila ovarian germline stem cell (GSC) biology. GSC self-renewal requires two translational repressors, Nanos (Nos) and Pumilio (Pum), which repress the expression of differentiation factors in the stem cells. The molecular mechanisms underlying this translational repression remain unknown. Here, we show that the CCR4 deadenylase is required for GSC self-renewal and that Nos and Pum act through its recruitment onto specific mRNAs. We identify mei-P26 mRNA as a direct and major target of Nos/Pum/CCR4 translational repression in the GSCs. mei-P26 encodes a protein of the Trim-NHL tumor suppressor family that has conserved functions in stem cell lineages. We show that fine-tuning Mei-P26 expression by CCR4 plays a key role in GSC self-renewal. These results identify the molecular mechanism of Nos/Pum function in GSC self-renewal and reveal the role of CCR4-NOT-mediated deadenylation in regulating the balance between GSC self-renewal and differentiation.

The I factor, a transposable element related to mammalian LINEs, controls the I‐R system of hybrid dysgenesis in Drosophila melanogaster. It transposes at high frequency in the germ‐line of the female progeny of crosses between females of the reactive class of strains and males of the inducer class. The structure and DNA sequence of the I factor suggest that it transposes by reverse transcription of an RNA intermediate. Northern blot and S1 mapping experiments show that a full‐length RNA of the I factor is synthesized specifically in the conditions of which I factors transpose. This RNA has all characteristics of a transposition intermediate. It is only found in the ovaries of dysgenic females suggesting that I factor activity is restricted to this tissue because of regulation at the level of the initiation of transcription or RNA stability.

RNA interference (RNAi) requires RNA-dependent RNA polymerases (RdRPs) in many eukaryotes, and RNAi amplification constitutes the only known function for eukaryotic RdRPs. Yet in animals, classical model organisms can elicit RNAi without possessing RdRPs, and only nematode RNAi was shown to require RdRPs. Here we show that RdRP genes are much more common in animals than previously thought, even in insects, where they had been assumed not to exist. RdRP genes were present in the ancestors of numerous clades, and they were subsequently lost at a high frequency. In order to probe the function of RdRPs in a deuterostome (the cephalochordate Branchiostoma lanceolatum ), we performed high-throughput analyses of small RNAs from various Branchiostoma developmental stages. Our results show that Branchiostoma RdRPs do not appear to participate in RNAi: we did not detect any candidate small RNA population exhibiting classical siRNA length or sequence features. Our results show that RdRPs have been independently lost in dozens of animal clades, and even in a clade where they have been conserved (cephalochordates) their function in RNAi amplification is not preserved. Such a dramatic functional variability reveals an unexpected plasticity in RNA silencing pathways.

I factors are responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. They belong to the LINE class of mobile elements, which transpose via reverse transcription of a full-length RNA intermediate. I factors are active members of the I element family, which also contains defective I elements that are immobilized within peri-centromeric heterochromatin and represent very old components of the genome. Active I factors have recently invaded natural populations of Drosophila melanogaster, giving rise to inducer strains. Reactive strains, devoid of active I factors, derive from old laboratory stocks established before the invasion. Transposition of I factors is activated at very high frequencies in the germline of hybrid females issued from crosses between females from reactive strains and males from inducer strains. It results in the production of high rates of mutations and chromosomal rearrangements as well as in a particular syndrome of sterility. The frequency of transposition of I factors is dependent on the amount of full-length RNA that is synthesized from an internal promoter. This full-length RNA serves both as an intermediate of transposition and presumably as a messenger for protein synthesis. Regulators of transposition apparently affect transcription initiation from the internal promoter. The data presented here lead to the proposal of a tentative model for transposition.

Long interspersed repetitive elements (LINEs) are transposable elements present in many species. In mammals they are difficult to study because most of them are defective and their transposition frequency is low. The I factor of Drosophila melanogaster is a LINE element that is particularly interesting because its transposition occurs at high frequency during I-R hybrid dysgenesis. This phenomenon occurs when males from the class of inducer strains are crossed with females from the class of reactive strains. Inducer strains contain several complete 5.4-kilobase I factors at various sites on the chromosomal arms. Reactive strains are devoid of complete I factors. Many results indicate that active I factors have invaded the D. melanogaster genome recently. To study the evolutionary history of I elements, we have cloned and sequenced a potentially active I factor from Drosophila teissieri. It is flanked by a target-site duplication and terminates at the 3' end by tandem repeats of the sequence TAA. When introduced into the germ line of a reactive strain of D. melanogaster by P element-mediated transformation, it is able to transpose and induces hybrid dysgenesis. This strengthens the hypothesis of a recent reinvasion of the D. melanogaster genome by active I factors giving rise to the inducer strains. They could have originated by horizontal transfer from another species. Such events also could occur for other LINE elements and might explain the spread of new variants in mammalian genomes. Moreover, the results give a further insight into I factor functional organization.

Non-long terminal repeat retrotransposons, widespread among eukaryotic genomes, transpose by reverse transcription of an RNA intermediate. Some of them, like L1 in the human, terminate at the 3-end with a poly(dA) stretch whereas others, like the I factor in Drosophila melanogaster, have instead a short sequence repeated in tandem. This suggests different requirements for the initiation of reverse transcription. Here, we have used an RNA circularization/reverse transcription-PCR technique to analyze the 5-and 3-ends of the full-length transcripts produced by the I factor at the time of active retrotransposition. These transcripts are capped and polyadenylated similar to conventional messenger RNAs. We have analyzed the 3-ends of transcripts and transposed copies produced by I elements mutated at the 3-ends. Transcripts devoid of tandem UAA repeats, although capable of building the components of the retrotransposition machinery, are inefficiently used as retrotransposition intermediates. Such transcripts produce rare new integrated copies issued from the inaccurate initiation of reverse transcription near the 3-end of the element. The tandem UAA repeats at the 3-end of the transcripts of I are required for the efficient and precise initiation of reverse transcription. This strong specificity of the I factor reverse transcriptase for its own transcript has implications for the impact of I factor retrotransposition on the host genome.Very little is known about the mechanism of retrotransposition of non-LTR 1 elements that are widespread among eukaryotic genomes. Most current knowledge comes from in vitro studies of the site-specific R2 elements from Bombyx mori. These studies indicate that reverse transcription of a full-length RNA intermediate of transposition occurs at the site of integration, using a 3Ј-hydroxyl group generated by endonucleolytic cleavage of the genomic DNA to prime synthesis of the first cDNA strand (1). This target-primed reverse transcription process is mediated by endonuclease and reverse transcriptase activities encoded by the single open reading frame (ORF) of R2 elements (1-3). Many non-LTR retrotransposons, including L1 in the human and the I factor in Drosophila, differ from R2 in that they possess an additional ORF encoding a protein probably involved in ribonucleoparticle formation (4 -6) and maybe also in reverse transcription (7). In addition, their endonucleases have similarities with apurinic/apyrimidinic endonucleases (8, 9), whereas the endonuclease of R2 elements is related to bacterial restriction endonucleases (10). Nevertheless, although direct experimental evidence is lacking, many observations are consistent with the idea that other non-LTR elements also use the target-primed reverse transcription mechanism of retrotransposition (1, 11). The 3Ј-end of the RNA intermediate of transposition is crucial in this mechanism because it contains the site of initiation of reverse transcription. Studies of human L1 indicate that the poly(A) tail at the 3Ј-end of the transcript is use...