SummaryCircular RNAs (circRNAs) are abundant and evolutionarily conserved RNAs of largely unknown function. Here, we show that a subset of circRNAs is translated in vivo. By performing ribosome footprinting from fly heads, we demonstrate that a group of circRNAs is associated with translating ribosomes. Many of these ribo-circRNAs use the start codon of the hosting mRNA, are bound by membrane-associated ribosomes, and have evolutionarily conserved termination codons. In addition, we found that a circRNA generated from the muscleblind locus encodes a protein, which we detected in fly head extracts by mass spectrometry. Next, by performing in vivo and in vitro translation assays, we show that UTRs of ribo-circRNAs (cUTRs) allow cap-independent translation. Moreover, we found that starvation and FOXO likely regulate the translation of a circMbl isoform. Altogether, our study provides strong evidence for translation of circRNAs, revealing the existence of an unexplored layer of gene activity.

AbstractmiRNA-mediated repression in animals is dependent on the GW182 protein family. GW182 proteins are recruited to the miRNA repression complex through direct interaction with Argonaute proteins, and they function downstream to repress target mRNA. Here we demonstrate that in human and Drosophila melanogaster cells, the critical repressive features of both the N-terminal and C-terminal effector domains of GW182 proteins are Gly/Ser/Thr-Trp (G/S/TW) or Trp-Gly/ Ser/Thr (WG/S/T) motifs. These motifs, which are dispersed across both domains and act in an additive manner, function by recruiting components of the CCR 4-NOT deadenylation complex. A heterologous yeast polypeptide with engineered WG/S/T motifs acquired the ability to repress tethered mRNA and to interact with the CCR 4-NOT complex. These results identify previously unknown effector motifs functioning as important mediators of miRNA-induced silencing in both species, and they reveal that recruitment of the CCR 4-NOT complex by tryptophan-containing motifs acts downstream of GW182 to repress mRNA s, including inhibiting translation independently of deadenylation.MicroRNAs (miRNAs) are small, ~21-nt-long RNAs that post-transcriptionally regulate gene expression in eukaryotes. In animals, miRNAs bind to partially complementary sites in mRNAs, leading to translational repression and mRNA deadenylation and degradation [1][2][3][4] . miRNAs function as part of ribonucleoprotein complexes, miRNPs, with Argonaute (AGO) and GW182 family proteins being the crucial components. GW182s interact directly with AGO proteins and function downstream as effectors mediating mRNA repression. Hence, understanding the function of GW182 proteins is critical for understanding miRNAmediated repression.GW182 functional regions have been mapped in D. melanogaster and mammalian proteins. In D. melanogaster, three regions were found to repress tethered mRNA to a similar extent 5

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Prior to reaching the posterior pole of the Drosophila oocyte, oskar mRNA is translationally silenced by Bruno binding to BREs in the 3' untranslated region. The eIF4E binding protein Cup interacts with Bruno and inhibits oskar translation. Validating current models, we directly demonstrate the mechanism proposed for Cup-mediated repression: inhibition of small ribosomal subunit recruitment to oskar mRNA. However, 43S complex recruitment remains inhibited in the absence of functional Cup, uncovering a second Bruno-dependent silencing mechanism. This mechanism involves mRNA oligomerization and formation of large (50S-80S) silencing particles that cannot be accessed by ribosomes. Bruno-dependent mRNA oligomerization into silencing particles emerges as a mode of translational control that may be particularly suited to coupling with mRNA transport.

Protein subcellular localization is fundamental to the establishment of the body axis, cell migration, synaptic plasticity, and a vast range of other biological processes. Protein localization occurs through three mechanisms: protein transport, mRNA localization, and local translation. However, the relative contribution of each process to neuronal polarity remains unknown. Using neurons differentiated from mouse embryonic stem cells, we analyze protein and RNA expression and translation rates in isolated cell bodies and neurites genome-wide. We quantify 7323 proteins and the entire transcriptome, and identify hundreds of neurite-localized proteins and locally translated mRNAs. Our results demonstrate that mRNA localization is the primary mechanism for protein localization in neurites that may account for half of the neurite-localized proteome. Moreover, we identify multiple neurite-targeted non-coding RNAs and RNA-binding proteins with potential regulatory roles. These results provide further insight into the mechanisms underlying the establishment of neuronal polarity.