In mammals, a single Dicer participates in biogenesis of small RNAs in microRNA (miRNA) and RNAi pathways. In mice, endogenous RNAi is highly active in oocytes, but not in somatic cells, which we ascribe here to an oocyte-specific Dicer isoform (Dicer(O)). Dicer(O) lacks the N-terminal DExD helicase domain and has higher cleavage activity than the full-length Dicer in somatic cells (Dicer(S)). Unlike Dicer(S), Dicer(O) efficiently produces small RNAs from long double-stranded (dsRNA) substrates. Expression of the Dicer(O) isoform is driven by an intronic MT-C retrotransposon promoter, deletion of which causes loss of Dicer(O) and female sterility. Oocytes from females lacking the MT-C element show meiotic spindle defects and increased levels of endogenous small interfering RNA (endo-siRNA) targets, phenocopying the maternal Dicer null phenotype. The alternative Dicer isoform, whose phylogenetic origin demonstrates evolutionary plasticity of RNA-silencing pathways, is the main determinant of endogenous RNAi activity in the mouse female germline.

Summary MicroRNAs (miRNAs) are small endogenous RNAs, which typically imperfectly base-pair with 3′UTRs and mediate translational repression and mRNA degradation. Dicer, an RNase III generating small RNAs in the miRNA and RNAi pathways, is essential for meiotic maturation of mouse oocytes. We found that 3′UTRs of transcripts up-regulated in Dicer1−/− oocytes are not enriched in miRNA binding sites implicating a weak impact of miRNAs on the maternal transcriptome. Therefore, we tested the ability of endogenous miRNAs to mediate RNA-like cleavage or translational repression of reporter mRNAs. In contrast to somatic cells, endogenous miRNAs in fully-grown GV oocytes poorly repressed translation of mRNA reporters whereas their RNAi-like activity was much less affected. In addition, reporter mRNA carrying let-7-binding sites failed to localize to P-body-like structures in oocytes. Our data suggest that normal miRNA function is down-regulated during oocyte development and this idea is further supported by normal meiotic maturation of oocytes lacking Dgcr8, which is required for the miRNA but not the RNAi pathway [Suh et al.]. We propose that suppression of miRNA function during oocyte growth is an early event in reprogramming gene expression during the transition of a differentiated oocyte into pluripotent blastomeres of the embryo.

In primary cells, overexpression of oncogenes such as RasV12 induces premature senescence rather than transformation. Senescence is an irreversible form of G1 arrest that requires the p19ARF/p53 and p16INK4a/pRB pathways and may suppress tumorigenesis in vivo. Here we show that the transcription factor C/EBPβ is required for RasV12‐induced senescence. C/EBPβ−/− mouse embryo fibroblasts (MEFs) expressing RasV12 continued to proliferate despite unimpaired induction of p19ARF and p53, and lacked morphological features of senescent fibroblasts. Enforced C/EBPβ expression inhibited proliferation of wild‐type MEFs and also slowed proliferation of p19Arf−/− and p53−/− cells, indicating that C/EBPβ acts downstream or independently of p19ARF/p53 to suppress growth. C/EBPβ was unable to inhibit proliferation of MEFs lacking all three RB family proteins or wild‐type cells expressing dominant negative E2F‐1 and, instead, stimulated their growth. C/EBPβ decreased expression of several E2F target genes and was associated with their promoters in chromatin immunoprecipitation assays, suggesting that C/EBPβ functions by repressing genes required for cell cycle progression. C/EBPβ is therefore a novel component of the RB:E2F‐dependent senescence program activated by oncogenic stress in primary cells.

The main role of the translation initiation factor 3 (eIF3) is to orchestrate formation of 43S-48S preinitiation complexes (PICs). Until now, most of our knowledge on eIF3 functional contribution to regulation of gene expression comes from yeast studies. Hence, here we developed several novel in vivo assays to monitor the integrity of the 13-subunit human eIF3 complex, defects in assembly of 43S PICs, efficiency of mRNA recruitment, and postassembly events such as AUG recognition. We knocked down expression of the PCI domain-containing eIF3c and eIF3a subunits and of eIF3j in human HeLa and HEK293 cells and analyzed the functional consequences. Whereas eIF3j downregulation had barely any effect and eIF3a knockdown disintegrated the entire eIF3 complex, eIF3c knockdown produced a separate assembly of the a, b, g, and i subunits (closely resembling the yeast evolutionary conserved eIF3 core), which preserved relatively high 40S binding affinity and an ability to promote mRNA recruitment to 40S subunits and displayed defects in AUG recognition. Both eIF3c and eIF3a knockdowns also severely reduced protein but not mRNA levels of many other eIF3 subunits and indeed shut off translation. We propose that eIF3a and eIF3c control abundance and assembly of the entire eIF3 and thus represent its crucial scaffolding elements critically required for formation of PICs.

Retrotransposons are "copy-and-paste" insertional mutagens that substantially contribute to mammalian genome content. Retrotransposons often carry long terminal repeats (LTRs) for retrovirus-like reverse transcription and integration into the genome. We report an extraordinary impact of a group of LTRs from the mammalian endogenous retrovirus-related ERVL retrotransposon class on gene expression in the germline and beyond. In mouse, we identified more than 800 LTRs from ORR1, MT, MT2, and MLT families, which resemble mobile gene-remodeling platforms that supply promoters and first exons. The LTR-mediated gene remodeling also extends to hamster, human, and bovine oocytes. The LTRs function in a stagespecific manner during the oocyte-to-embryo transition by activating transcription, altering protein-coding sequences, producing noncoding RNAs, and even supporting evolution of new protein-coding genes. These functions result, for example, in recycling processed pseudogenes into mRNAs or lncRNAs with regulatory roles. The functional potential of the studied LTRs is even higher, because we show that dormant LTR promoter activity can rescue loss of an essential upstream promoter. We also report a novel protein-coding gene evolution-D6Ertd527e-in which an MT LTR provided a promoter and the 5

The fully grown mammalian oocyte is transcriptionally quiescent and utilizes only transcripts synthesized and stored during early development. However, we find that an abundant RNA population is retained in the oocyte nucleus and contains specific mRNAs important for meiotic progression. Here we show that during the first meiotic division, shortly after nuclear envelope breakdown, translational hotspots develop in the chromosomal area and in a region that was previously surrounded the nucleus. These distinct translational hotspots are separated by endoplasmic reticulum and Lamin, and disappear following polar body extrusion. Chromosomal translational hotspots are controlled by the activity of the mTOR–eIF4F pathway. Here we reveal a mechanism that—following the resumption of meiosis—controls the temporal and spatial translation of a specific set of transcripts required for normal spindle assembly, chromosome alignment and segregation.

No abstract

No abstract