Molecular basis for cytoplasmic RNA surveillance by uridylation-triggered decay in Drosophila

RNA surveillance by the RNA helicase MTR4 determines volume of mouse oocytes

Oocytes are the largest cell type in multicellular animals. Here, we show that mRNA transporter 4 (MTR4) is indispensable for oocyte growth and functions as part of the RNA surveillance mechanism, which is responsible for nuclear waste RNA clearance. MTR4 ensures the normal post-transcriptional processing of maternal RNAs, their nuclear export to the cytoplasm, and the accumulation of properly processed transcripts. Oocytes with Mtr4 knockout fail to accumulate sufficient and normal transcripts in the cytoplasm and cannot grow to normal sizes. MTR4-dependent RNA surveillance has a previously unrecognized function in maintaining a stable nuclear environment for the establishment of non-canonical histone H3 lysine-4 trimethylation and chromatin reorganization, which is necessary to form a nucleolus-like structure in oocytes. In conclusion, MTR4-dependent RNA surveillance activity is a checkpoint that allows oocytes to grow to a normal size, undergo nuclear and cytoplasmic maturation, and acquire developmental competence.

Poly(U) polymerase activity in Caenorhabditis elegans regulates abundance and tailing of sRNA and mRNA

Terminal nucleotidyltransferases add nucleotides to the 3' end of RNA to modify their stability and function. In Caenorhabditis elegans, the terminal uridyltransferases/poly(U) polymerases PUP-1 (aka CID-1, CDE-1), PUP-2, and PUP-3 affect germline identity, survival, and development. Here, we identify small RNA (sRNA) and mRNA targets of these PUPs and of a fourth predicted poly(U) polymerase, F43E2.1/PUP-4. Using genetic and RNA sequencing approaches, we identify RNA targets of each PUP and the U-tail frequency and length of those targets. At the whole organism level, PUP-1 is responsible for most sRNA U-tailing, and other PUPs contribute to modifying discrete subsets of sRNAs. Moreover, the expression of PUP-2, PUP-3, and especially PUP-4 limits uridylation on some sRNAs. The relationship between uridylation status and sRNA abundance suggests that U-tailing can have a negative or positive effect on abundance depending on context. sRNAs modified by PUP activity primarily target mRNAs that are ubiquitously expressed or most highly expressed in the germline. mRNA data obtained with a Nanopore-based method reveal that the addition of U-tails to nonadenylated mRNA is substantially reduced in the absence of PUP-3. Overall, this work identifies PUP RNA targets, defines the effect of uridylation loss on RNA abundance, and reveals the complexity of PUP regulation in C. elegans development.

Post-transcriptional gene regulation: From mechanisms to RNA chemistry and therapeutics

A better understanding of the RNA biology and chemistry is necessary to then develop new RNA therapeutic strategies. This review is the synthesis of a series of conferences that took place during the 6th international course on post-transcriptional gene regulation at Institut Curie. This year, the course made a special focus on RNA chemistry.

DIS3L2 knockdown impairs key oncogenic properties of colorectal cancer cells via the mTOR signaling pathway

DIS3L2 degrades different types of RNAs in an exosome-independent manner including mRNAs and several types of non-coding RNAs. DIS3L2-mediated degradation is preceded by the addition of nontemplated uridines at the 3'end of its targets by the terminal uridylyl transferases 4 and 7. Most of the literature that concerns DIS3L2 characterizes its involvement in several RNA degradation pathways, however, there is some evidence that its dysregulated activity may contribute to cancer development. In the present study, we characterize the role of DIS3L2 in human colorectal cancer (CRC). Using the public RNA datasets from The Cancer Genome Atlas (TCGA), we found higher DIS3L2 mRNA levels in CRC tissues versus normal colonic samples as well as worse prognosis in patients with high DIS3L2 expression. In addition, our RNA deep-sequencing data revealed that knockdown (KD) of DIS3L2 induces a strong transcriptomic disturbance in SW480 CRC cells. Moreover, gene ontology (GO) analysis of significant upregulated transcripts displays enrichment in mRNAs encoding proteins involved in cell cycle regulation and cancer-related pathways, which guided us to evaluate which specific hallmarks of cancer are differentially regulated by DIS3L2. To do so, we employed four CRC cell lines (HCT116, SW480, Caco-2 and HT-29) differing in their mutational background and oncogenicity. We demonstrate that depletion of DIS3L2 results in reduced cell viability of highly oncogenic SW480 and HCT116 CRC cells, but had little or no impact in the more differentiated Caco-2 and HT-29 cells. Remarkably, the mTOR signaling pathway, crucial for cell survival and growth, is downregulated after DIS3L2 KD, whereas AZGP1, an mTOR pathway inhibitor, is upregulated. Furthermore, our results indicate that depletion of DIS3L2 disturbs metastasis-associated properties, such as cell migration and invasion, only in highly oncogenic CRC cells. Our work reveals for the first time a role for DIS3L2 in sustaining CRC cell proliferation and provides evidence that this ribonuclease is required to support the viability and invasive behavior of dedifferentiated CRC cells.

A novel eukaryotic RdRP-dependent small RNA pathway represses antiviral immunity by controlling an ERK pathway component in the black-legged tick

Small regulatory RNAs (sRNAs) are involved in antiviral defense and gene regulation. Although roles of RNA-dependent RNA Polymerases (RdRPs) in sRNA biology are extensively studied in nematodes, plants and fungi, understanding of RdRP homologs in other animals is still lacking. Here, we study sRNAs in the ISE6 cell line, which is derived from the black-legged tick, an important vector of human and animal pathogens. We find abundant classes of ~22nt sRNAs that require specific combinations of RdRPs and sRNA effector proteins (Argonautes or AGOs). RdRP1-dependent sRNAs possess 5'-monophosphates and are mainly derived from RNA polymerase III-transcribed genes and repetitive elements. Knockdown of some RdRP homologs misregulates genes including RNAi-related genes and the regulator of immune response Dsor1. Sensor assays demonstrate that Dsor1 is downregulated by RdRP1 through the 3'UTR that contains a target site of RdRP1-dependent repeat-derived sRNAs. Consistent with viral gene repression by the RNAi mechanism using virus-derived small interfering RNAs, viral transcripts are upregulated by AGO knockdown. On the other hand, RdRP1 knockdown unexpectedly results in downregulation of viral transcripts. This effect is dependent on Dsor1, suggesting that antiviral immunity is enhanced by RdRP1 knockdown through Dsor1 upregulation. We propose that tick sRNA pathways control multiple aspects of immune response via RNAi and regulation of signaling pathways.

TGS1 impacts snRNA 3'-end processing, ameliorates survival motor neuron-dependent neurological phenotypes in vivo and prevents neurodegeneration

Trimethylguanosine synthase 1 (TGS1) is a highly conserved enzyme that converts the 5'-monomethylguanosine cap of small nuclear RNAs (snRNAs) to a trimethylguanosine cap. Here, we show that loss of TGS1 in Caenorhabditis elegans, Drosophila melanogaster and Danio rerio results in neurological phenotypes similar to those caused by survival motor neuron (SMN) deficiency. Importantly, expression of human TGS1 ameliorates the SMN-dependent neurological phenotypes in both flies and worms, revealing that TGS1 can partly counteract the effects of SMN deficiency. TGS1 loss in HeLa cells leads to the accumulation of immature U2 and U4atac snRNAs with long 3' tails that are often uridylated. snRNAs with defective 3' terminations also accumulate in Drosophila Tgs1 mutants. Consistent with defective snRNA maturation, TGS1 and SMN mutant cells also exhibit partially overlapping transcriptome alterations that include aberrantly spliced and readthrough transcripts. Together, these results identify a neuroprotective function for TGS1 and reinforce the view that defective snRNA maturation affects neuronal viability and function.

Terminal modification, sequence, length, and PIWI-protein identity determine piRNA stability

In animals, PIWI-interacting RNAs (piRNAs) silence transposons, fight viral infections, and regulate gene expression. piRNA biogenesis concludes with 3' terminal trimming and 2'-O-methylation. Both trimming and methylation influence piRNA stability. Our biochemical data show that multiple mechanisms destabilize unmethylated mouse piRNAs, depending on whether the piRNA 5' or 3' sequence is complementary to a trigger RNA. Unlike target-directed degradation of microRNAs, complementarity-dependent destabilization of piRNAs in mice and flies is blocked by 3' terminal 2'-O-methylation and does not require base pairing to both the piRNA seed and the 3' sequence. In flies, 2'-O-methylation also protects small interfering RNAs (siRNAs) from complementarity-dependent destruction. By contrast, pre-piRNA trimming protects mouse piRNAs from a degradation pathway unaffected by trigger complementarity. In testis lysate and in vivo, internal or 3' terminal uridine- or guanine-rich tracts accelerate pre-piRNA decay. Loss of both trimming and 2'-O-methylation causes the mouse piRNA pathway to collapse, demonstrating that these modifications collaborate to stabilize piRNAs.

Genome-wide analyses of XRN1-sensitive targets in osteosarcoma cells identify disease-relevant transcripts containing G-rich motifs

XRN1 is a highly conserved exoribonuclease which degrades uncapped RNAs in a 5'-3' direction. Degradation of RNAs by XRN1 is important in many cellular and developmental processes and is relevant to human disease. Studies in D. melanogaster demonstrate that XRN1 can target specific RNAs, which have important consequences for developmental pathways. Osteosarcoma is a malignancy of the bone and accounts for 2% of all pediatric cancers worldwide. Five-year survival of patients has remained static since the 1970s and therefore furthering our molecular understanding of this disease is crucial. Previous work has shown a down-regulation of XRN1 in osteosarcoma cells; however, the transcripts regulated by XRN1 which might promote osteosarcoma remain elusive. Here, we confirm reduced levels of XRN1 in osteosarcoma cell lines and patient samples and identify XRN1-sensitive transcripts in human osteosarcoma cells. Using RNA-seq in XRN1-knockdown SAOS-2 cells, we show that 1178 genes are differentially regulated. Using a novel bioinformatic approach, we demonstrate that 134 transcripts show characteristics of direct post-transcriptional regulation by XRN1. Long noncoding RNAs (lncRNAs) are enriched in this group, suggesting that XRN1 normally plays an important role in controlling lncRNA expression in these cells. Among potential lncRNAs targeted by XRN1 is HOTAIR, which is known to be up-regulated in osteosarcoma and contributes to disease progression. We have also identified G-rich and GU motifs in post-transcriptionally regulated transcripts which appear to sensitize them to XRN1 degradation. Our results therefore provide significant insights into the specificity of XRN1 in human cells which are relevant to disease.

Regulation of RNA stability at the 3' end

RNA homeostasis is regulated by a multitude of cellular pathways. Although the addition of untemplated adenine residues to the 3' end of mRNAs has long been known to affect RNA stability, newly developed techniques for 3'-end sequencing of RNAs have revealed various unexpected RNA modifications. Among these, uridylation is most recognized for its role in mRNA decay but is also a key regulator of numerous RNA species, including miRNAs and tRNAs, with dual roles in both stability and maturation of miRNAs. Additionally, low levels of untemplated guanidine and cytidine residues have been observed as parts of more complex tailing patterns.

A neural m6A/Ythdf pathway is required for learning and memory in Drosophila

Epitranscriptomic modifications can impact behavior. Here, we used Drosophila melanogaster to study N⁶-methyladenosine (m⁶A), the most abundant modification of mRNA. Proteomic and functional analyses confirm its nuclear (Ythdc1) and cytoplasmic (Ythdf) YTH domain proteins as major m⁶A binders. Assays of short term memory in m⁶A mutants reveal neural-autonomous requirements of m⁶A writers working via Ythdf, but not Ythdc1. Furthermore, m⁶A/Ythdf operate specifically via the mushroom body, the center for associative learning. We map m⁶A from wild-type and Mettl3 mutant heads, allowing robust discrimination of Mettl3-dependent m⁶A sites that are highly enriched in 5' UTRs. Genomic analyses indicate that Drosophila m⁶A is preferentially deposited on genes with low translational efficiency and that m⁶A does not affect RNA stability. Nevertheless, functional tests indicate a role for m⁶A/Ythdf in translational activation. Altogether, our molecular genetic analyses and tissue-specific m⁶A maps reveal selective behavioral and regulatory defects for the Drosophila Mettl3/Ythdf pathway.

The TUTase URT1 connects decapping activators and prevents the accumulation of excessively deadenylated mRNAs to avoid siRNA biogenesis

Uridylation is a widespread modification destabilizing eukaryotic mRNAs. Yet, molecular mechanisms underlying TUTase-mediated mRNA degradation remain mostly unresolved. Here, we report that the Arabidopsis TUTase URT1 participates in a molecular network connecting several translational repressors/decapping activators. URT1 directly interacts with DECAPPING 5 (DCP5), the Arabidopsis ortholog of human LSM14 and yeast Scd6, and this interaction connects URT1 to additional decay factors like DDX6/Dhh1-like RNA helicases. Nanopore direct RNA sequencing reveals a global role of URT1 in shaping poly(A) tail length, notably by preventing the accumulation of excessively deadenylated mRNAs. Based on in vitro and in planta data, we propose a model that explains how URT1 could reduce the accumulation of oligo(A)-tailed mRNAs both by favoring their degradation and because 3' terminal uridines intrinsically hinder deadenylation. Importantly, preventing the accumulation of excessively deadenylated mRNAs avoids the biogenesis of illegitimate siRNAs that silence endogenous mRNAs and perturb Arabidopsis growth and development.

Dis3L2 regulates cell proliferation and tissue growth through a conserved mechanism

Dis3L2 is a highly conserved 3’-5’ exoribonuclease which is mutated in the human overgrowth disorders Perlman syndrome and Wilms’ tumour of the kidney. Using Drosophila melanogaster as a model system, we have generated a new dis3L2 null mutant together with wild-type and nuclease-dead genetic lines in Drosophila to demonstrate that the catalytic activity of Dis3L2 is required to control cell proliferation. To understand the cellular pathways regulated by Dis3L2 to control proliferation, we used RNA-seq on dis3L2 mutant wing discs to show that the imaginal disc growth factor Idgf2 is responsible for driving the wing overgrowth. IDGFs are conserved proteins homologous to human chitinase-like proteins such as CHI3L1/YKL-40 which are implicated in tissue regeneration as well as cancers including colon cancer and non-small cell lung cancer. We also demonstrate that loss of DIS3L2 in human kidney HEK-293T cells results in cell proliferation, illustrating the conservation of this important cell proliferation pathway. Using these human cells, we show that loss of DIS3L2 results in an increase in the PI3-Kinase/AKT signalling pathway, which we subsequently show to contribute towards the proliferation phenotype in Drosophila. Our work therefore provides the first mechanistic explanation for DIS3L2-induced overgrowth in humans and flies and identifies an ancient proliferation pathway controlled by Dis3L2 to regulate cell proliferation and tissue growth.

Regulation of cell proliferation is not only important during development but also required for repair of damaged tissues and during wound healing. Using human kidney cells as well as the fruit fly Drosophila we have recently discovered that cell proliferation can be regulated by a protein named Dis3L2. Depletion or removal of this protein results in excess proliferation. These results are relevant to human disease as DIS3L2 has been shown to be mutated in an overgrowth syndrome (Perlman syndrome) where affected children have abnormal enlargement of organs (e.g. kidneys) and susceptibility to Wilms’ tumour (a kidney cancer). Dis3L2 is an enzyme known to "chew up" mRNA molecules which instruct the cell to make particular proteins. Using state-of-the-art molecular methods in Drosophila, we have discovered that Dis3L2 targets a small subset of mRNAs, including an mRNA encoding a growth factor named 'imaginal disc growth factor 2' (idgf2). For human kidney cells in culture, we have found that depletion of DIS3L2 results in enhanced proliferation, and that this involves a well-known cellular pathway. Our results mean that we have discovered a new way of controlling cell proliferation, which could, in the future, be used in human therapies.

Structure-function analysis of microRNA 3'-end trimming by Nibbler

Nibbler (Nbr) is a 3'-to-5' exoribonuclease whose catalytic 3'-end trimming activity impacts microRNA (miRNA) and PIWI-interacting RNA (piRNA) biogenesis. Here, we report on structural and functional studies to decipher the contributions of Nbr's N-terminal domain (NTD) and exonucleolytic domain (EXO) in miRNA 3'-end trimming. We have solved the crystal structures of the NTD core and EXO domains of Nbr, both in the apo-state. The NTD-core domain of Aedes aegypti Nbr adopts a HEAT-like repeat scaffold with basic patches constituting an RNA-binding surface exhibiting a preference for binding double-strand RNA (dsRNA) over single-strand RNA (ssRNA). Structure-guided functional assays in Drosophila S2 cells confirmed a principal role of the NTD in exonucleolytic miRNA trimming, which depends on basic surface patches. Gain-of-function experiments revealed a potential role of the NTD in recruiting Nbr to Argonaute-bound small RNA substrates. The EXO domain of A. aegypti and Drosophila melanogaster Nbr adopt a mixed α/β-scaffold with a deep pocket lined by a DEDDy catalytic cleavage motif. We demonstrate that Nbr's EXO domain exhibits Mn2+-dependent ssRNA-specific 3'-to-5' exoribonuclease activity. Modeling of a 3' terminal Uridine into the catalytic pocket of Nbr EXO indicates that 2'-O-methylation of the 3'-U would result in a steric clash with a tryptophan side chain, suggesting that 2'-O-methylation protects small RNAs from Nbr-mediated trimming. Overall, our data establish that Nbr requires its NTD as a substrate recruitment platform to execute exonucleolytic miRNA maturation, catalyzed by the ribonuclease EXO domain.

Functions and mechanisms of RNA tailing by metazoan terminal nucleotidyltransferases

Termini often determine the fate of RNA molecules. In recent years, 3' ends of almost all classes of RNA species have been shown to acquire nontemplated nucleotides that are added by terminal nucleotidyltransferases (TENTs). The best-described role of 3' tailing is the bulk polyadenylation of messenger RNAs in the cell nucleus that is catalyzed by canonical poly(A) polymerases (PAPs). However, many other enzymes that add adenosines, uridines, or even more complex combinations of nucleotides have recently been described. This review focuses on metazoan TENTs, which are either noncanonical PAPs or terminal uridylyltransferases with varying processivity. These enzymes regulate RNA stability and RNA functions and are crucial in early development, gamete production, and somatic tissues. TENTs regulate gene expression at the posttranscriptional level, participate in the maturation of many transcripts, and protect cells against viral invasion and the transposition of repetitive sequences. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Processing > 3' End Processing RNA Turnover and Surveillance > Regulation of RNA Stability.

AGO-bound mature miRNAs are oligouridylated by TUTs and subsequently degraded by DIS3L2

MicroRNAs (miRNAs) associated with Argonaute proteins (AGOs) regulate gene expression in mammals. miRNA 3' ends are subject to frequent sequence modifications, which have been proposed to affect miRNA stability. However, the underlying mechanism is not well understood. Here, by genetic and biochemical studies as well as deep sequencing analyses, we find that AGO mutations disrupting miRNA 3' binding are sufficient to trigger extensive miRNA 3' modifications in HEK293T cells and in cancer patients. Comparing these modifications in TUT4, TUT7 and DIS3L2 knockout cells, we find that TUT7 is more robust than TUT4 in oligouridylating mature miRNAs, which in turn leads to their degradation by the DIS3L2 exonuclease. Our findings indicate a decay machinery removing AGO-associated miRNAs with an exposed 3' end. A set of endogenous miRNAs including miR-7, miR-222 and miR-769 are targeted by this machinery presumably due to target-directed miRNA degradation.

A tale of non-canonical tails: gene regulation by post-transcriptional RNA tailing

RNA tailing, or the addition of non-templated nucleotides to the 3' end of RNA, is the most frequent and conserved type of RNA modification. The addition of tails and their composition reflect RNA maturation stages and have important roles in determining the fate of the modified RNAs. Apart from canonical poly(A) polymerases, which add poly(A) tails to mRNAs in a transcription-coupled manner, a family of terminal nucleotidyltransferases (TENTs), including terminal uridylyltransferases (TUTs), modify RNAs post-transcriptionally to control RNA stability and activity. The human genome encodes 11 different TENTs with distinct substrate specificity, intracellular localization and tissue distribution. In this Review, we discuss recent advances in our understanding of non-canonical RNA tails, with a focus on the functions of human TENTs, which include uridylation, mixed tailing and post-transcriptional polyadenylation of mRNAs, microRNAs and other types of non-coding RNA.

The Perlman syndrome DIS3L2 exoribonuclease safeguards endoplasmic reticulum-targeted mRNA translation and calcium ion homeostasis

DIS3L2-mediated decay (DMD) is a surveillance pathway for certain non-coding RNAs (ncRNAs) including ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nuclear RNAs (snRNAs), and RMRP. While mutations in DIS3L2 are associated with Perlman syndrome, the biological significance of impaired DMD is obscure and pathological RNAs have not been identified. Here, by ribosome profiling (Ribo-seq) we find specific dysregulation of endoplasmic reticulum (ER)-targeted mRNA translation in DIS3L2-deficient cells. Mechanistically, DMD functions in the quality control of the 7SL ncRNA component of the signal recognition particle (SRP) required for ER-targeted translation. Upon DIS3L2 loss, sustained 3’-end uridylation of aberrant 7SL RNA impacts ER-targeted translation and causes ER calcium leakage. Consequently, elevated intracellular calcium in DIS3L2-deficient cells activates calcium signaling response genes and perturbs ESC differentiation. Thus, DMD is required to safeguard ER-targeted mRNA translation, intracellular calcium homeostasis, and stem cell differentiation.

The DIS3L2 exonuclease degrades aberrant 7SL RNAs tagged by an oligouridine 3′-tail. Here the authors analyze DIS3L2 knockout mouse embryonic stem cells and suggest that DIS3L2-mediated quality control of 7SL RNA is important for ER-mediated translation and calcium ion homeostasis.

RNA Exosomes and Their Cofactors

The RNA exosome is a ribonucleolytic multiprotein complex that is conserved and essential in all eukaryotes. Although we tend to speak of "the" exosome complex, it should be more correctly viewed as several different subtypes that share a common core. Subtypes of the exosome complex are present in the cytoplasm, the nucleus and the nucleolus of all eukaryotic cells, and carry out the 3'-5' processing and/or degradation of a wide range of RNA substrates.Because the substrate specificity of the exosome complex is determined by cofactors, the system is highly adaptable, and different organisms have adjusted the machinery to their specific needs. Here, we present an overview of exosome complexes and their cofactors that have been described in different eukaryotes.

Structural insights into a unique preference for 3' terminal guanine of mirtron in Drosophila TUTase tailor

Terminal uridylyl transferase (TUTase) is one type of enzyme that modifies RNA molecules by facilitating the post-transcriptional addition of uridyl ribonucleotides to their 3' ends. Recent researches have reported that Drosophila TUTase, Tailor, exhibits an intrinsic preference for RNA substrates ending in 3'G, distinguishing it from any other known TUTases. Through this unique feature, Tailor plays a crucial role as the repressor in the biogenesis pathway of splicing-derived mirtron pre-miRNAs. Here we describe crystal structures of core catalytic domain of Tailor and its complexes with RNA stretches 5'-AGU-3' and 5'-AGUU-3'. We demonstrate that R327 and N347 are two key residues contributing cooperatively to Tailor's preference for 3'G, and R327 may play an extra role in facilitating the extension of polyuridylation chain. We also demonstrate that conformational stability of the exit of RNA-binding groove also contributes significantly to Tailor's activity. Overall, our work reveals useful insights to explain why Drosophila Tailor can preferentially select RNA substrates ending in 3'G and provides important values for further understanding the biological significances of biogenesis pathway of mirtron in flies.

Time-Resolved Small RNA Sequencing Unravels the Molecular Principles of MicroRNA Homeostasis

Argonaute-bound microRNAs silence mRNA expression in a dynamic and regulated manner to control organismal development, physiology, and disease. We employed metabolic small RNA sequencing for a comprehensive view on intracellular microRNA kinetics in Drosophila. Based on absolute rate of biogenesis and decay, microRNAs rank among the fastest produced and longest-lived cellular transcripts, disposing up to 105 copies per cell at steady-state. Mature microRNAs are produced within minutes, revealing tight intracellular coupling of biogenesis that is selectively disrupted by pre-miRNA-uridylation. Control over Argonaute protein homeostasis generates a kinetic bottleneck that cooperates with non-coding RNA surveillance to ensure faithful microRNA loading. Finally, regulated small RNA decay enables the selective rapid turnover of Ago1-bound microRNAs, but not of Ago2-bound small interfering RNAs (siRNAs), reflecting key differences in the robustness of small RNA silencing pathways. Time-resolved small RNA sequencing opens new experimental avenues to deconvolute the timescales, molecular features, and regulation of small RNA silencing pathways in living cells.

Structural basis for acceptor RNA substrate selectivity of the 3' terminal uridylyl transferase Tailor

Non-templated 3′-uridylation of RNAs has emerged as an important mechanism for regulating the processing, stability and biological function of eukaryotic transcripts. In Drosophila, oligouridine tailing by the terminal uridylyl transferase (TUTase) Tailor of numerous RNAs induces their degradation by the exonuclease Dis3L2, which serves functional roles in RNA surveillance and mirtron RNA biogenesis. Tailor preferentially uridylates RNAs terminating in guanosine or uridine nucleotides but the structural basis underpinning its RNA substrate selectivity is unknown. Here, we report crystal structures of Tailor bound to a donor substrate analog or mono- and oligouridylated RNA products. These structures reveal specific amino acid residues involved in donor and acceptor substrate recognition, and complementary biochemical assays confirm the critical role of an active site arginine in conferring selectivity toward 3′-guanosine terminated RNAs. Notably, conservation of these active site features suggests that other eukaryotic TUTases, including mammalian TUT4 and TUT7, might exhibit similar, hitherto unknown, substrate selectivity. Together, these studies provide critical insights into the specificity of 3′-uridylation in eukaryotic post-transcriptional gene regulation.

Exonuclease requirements for mammalian ribosomal RNA biogenesis and surveillance

Ribosomal RNA (rRNA) biogenesis is a multistep process requiring several nuclear and cytoplasmic exonucleases. The exact processing steps for mammalian 5.8S rRNA remain obscure. Here, using loss-of-function approaches in mouse embryonic stem cells and deep sequencing of rRNA intermediates, we investigate at nucleotide resolution the requirements of exonucleases known to be involved in 5.8S maturation, and explore the role of the Perlman syndrome-associated 3’-5’ exonuclease Dis3l2 in rRNA processing. We uncover a novel cytoplasmic intermediate that we name ‘7S_B’ rRNA that is generated through sequential processing by distinct exosome complexes. 7S_B rRNA can be oligoadenylated by an unknown enzyme and/or oligouridylated by TUT4/7 and subsequently processed by Dis3l2 and Eri1. Moreover, exosome depletion triggers Dis3l2-mediated decay (DMD) as a surveillance pathway for rRNAs. Our data identify previously unknown 5.8S rRNA processing steps and provide nucleotide level insight into the exonuclease requirements for mammalian rRNA processing.

Analysis of Expression Pattern of snoRNAs in Different Cancer Types with Machine Learning Algorithms

Small nucleolar RNAs (snoRNAs) are a new type of functional small RNAs involved in the chemical modifications of rRNAs, tRNAs, and small nuclear RNAs. It is reported that they play important roles in tumorigenesis via various regulatory modes. snoRNAs can both participate in the regulation of methylation and pseudouridylation and regulate the expression pattern of their host genes. This research investigated the expression pattern of snoRNAs in eight major cancer types in TCGA via several machine learning algorithms. The expression levels of snoRNAs were first analyzed by a powerful feature selection method, Monte Carlo feature selection (MCFS). A feature list and some informative features were accessed. Then, the incremental feature selection (IFS) was applied to the feature list to extract optimal features/snoRNAs, which can make the support vector machine (SVM) yield best performance. The discriminative snoRNAs included HBII-52-14, HBII-336, SNORD123, HBII-85-29, HBII-420, U3, HBI-43, SNORD116, SNORA73B, SCARNA4, HBII-85-20, etc., on which the SVM can provide a Matthew’s correlation coefficient (MCC) of 0.881 for predicting these eight cancer types. On the other hand, the informative features were fed into the Johnson reducer and repeated incremental pruning to produce error reduction (RIPPER) algorithms to generate classification rules, which can clearly show different snoRNAs expression patterns in different cancer types. The analysis results indicated that extracted discriminative snoRNAs can be important for identifying cancer samples in different types and the expression pattern of snoRNAs in different cancer types can be partly uncovered by quantitative recognition rules.

Analysis of 3' End Modifications in microRNAs by High-Throughput Sequencing

MicroRNAs are ~22 nt small, non-coding RNAs that direct posttranscriptional silencing of gene expression to regulate animal development, physiology, and disease. An emerging mechanism that controls the biogenesis of microRNAs is the addition of non-templated nucleotides, predominantly uridine, to the 3' end of precursor-microRNAs, in a process that is commonly referred to as tailing. Here, we describe methods that enable the systematic characterization of tailing events in mature microRNAs and their precursors. We report protocols for untargeted and targeted cDNA library preparation procedures, as exemplified in the context of the model organism Drosophila melanogaster and focusing on precursor-microRNAs. We also refer to a dedicated computational framework for the subsequent analysis of untemplated nucleotide additions in cDNA libraries. The described methods for the systematic characterization of posttranscriptional modifications in gene regulatory small RNAs and their precursors will be instrumental in clarifying regulatory concepts that control posttranscriptional gene silencing.

Unraveling 3'-end RNA uridylation at nucleotide resolution

Post-transcriptional modification of RNA, the so-called 'Epitranscriptome', can regulate RNA structure, stability, localization, and function. Numerous modifications have been identified in virtually all classes of RNAs, including messenger RNAs (mRNAs), transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), microRNAs (miRNAs), and other noncoding RNAs (ncRNAs). These modifications may occur internally (by base or sugar modifications) and include RNA methylation at different nucleotide positions, or by the addition of various nucleotides at the 3'-end of certain transcripts by a family of terminal nucleotidylyl transferases. Developing methods to specifically and accurately detect and map these modifications is essential for understanding the molecular function(s) of individual RNA modifications and also for identifying and characterizing the proteins that may read, write, or erase them. Here, we focus on the characterization of RNA species targeted by 3' terminal uridylyl transferases (TUTases) (TUT4/7, also known as Zcchc11/6) and a 3'-5' exoribonuclease, Dis3l2, in the recently identified Dis3l2-mediated decay (DMD) pathway - a dedicated quality control pathway for a subset of ncRNAs. We describe the detailed methods used to precisely identify 3'-end modifications at nucleotide level resolution with a particular focus on the U1 and U2 small nuclear RNA (snRNA) components of the Spliceosome. These tools can be applied to investigate any RNA of interest and should facilitate studies aimed at elucidating the functional relevance of 3'-end modifications.

Terminal Uridylyltransferases Execute Programmed Clearance of Maternal Transcriptome in Vertebrate Embryos

During the maternal-to-zygotic transition (MZT), maternal RNAs are actively degraded and replaced by newly synthesized zygotic transcripts in a highly coordinated manner. However, it remains largely unknown how maternal mRNA decay is triggered in early vertebrate embryos. Here, through genome-wide profiling of RNA abundance and 3' modification, we show that uridylation is induced at the onset of maternal mRNA clearance. The temporal control of uridylation is conserved in vertebrates. When the homologs of terminal uridylyltransferases TUT4 and TUT7 (TUT4/7) are depleted in zebrafish and Xenopus, maternal mRNA clearance is significantly delayed, leading to developmental defects during gastrulation. Short-tailed mRNAs are selectively uridylated by TUT4/7, with the highly uridylated transcripts degraded faster during the MZT than those with unmodified poly(A) tails. Our study demonstrates that uridylation plays a crucial role in timely mRNA degradation, thereby allowing the progression of early development.

Regulation of RNA decay and cellular function by 3'-5' exoribonuclease DIS3L2

DIS3L2, in which mutations have been linked to Perlman syndrome, is an RNA-binding protein with 3'-5' exoribonuclease activity. It contains two CSD domains and one S1 domain, all of which are RNA-binding domains, and one RNB domain that is responsible for the exoribonuclease activity. The 3' polyuridine of RNA substrates can serve as a degradation signal for DIS3L2. Because DIS3L2 is predominantly localized in the cytoplasm, it can recognize, bind, and mediate the degradation of cytoplasmic uridylated RNA, including pre-microRNA, mature microRNA, mRNA, and some other non-coding RNAs. Therefore, DIS3L2 plays an important role in cytoplasmic RNA surveillance and decay. DIS3L2 is involved in multiple biological and physiological processes such as cell division, proliferation, differentiation, and apoptosis. Nonetheless, the function of DIS3L2, especially its association with cancer, remains largely unknown. We summarize here the RNA substrates degraded by DIS3L2 with its exonucleolytic activity, together with the corresponding biological functions it is implicated in. Furthermore, we discuss whether DIS3L2 can function independently of its 3'-5' exoribonuclease activity, as well as its potential tumor-suppressive or oncogenic roles during cancer progression.

Beyond transcription factors: roles of mRNA decay in regulating gene expression in plants

Gene expression is typically quantified as RNA abundance, which is influenced by both synthesis (transcription) and decay. Cytoplasmic decay typically initiates by deadenylation, after which decay can occur through any of three cytoplasmic decay pathways. Recent advances reveal several mechanisms by which RNA decay is regulated to control RNA abundance. mRNA can be post-transcriptionally modified, either indirectly through secondary structure or through direct modifications to the transcript itself, sometimes resulting in subsequent changes in mRNA decay rates. mRNA abundances can also be modified by tapping into pathways normally used for RNA quality control. Regulated mRNA decay can also come about through post-translational modification of decapping complex subunits. Likewise, mRNAs can undergo changes in subcellular localization (for example, the deposition of specific mRNAs into processing bodies, or P-bodies, where stabilization and destabilization occur in a transcript- and context-dependent manner). Additionally, specialized functions of mRNA decay pathways were implicated in a genome-wide mRNA decay analysis in Arabidopsis. Advances made using plants are emphasized in this review, but relevant studies from other model systems that highlight RNA decay mechanisms that may also be conserved in plants are discussed.

The role of 3' end uridylation in RNA metabolism and cellular physiology

Most eukaryotic RNAs are posttranscriptionally modified. The majority of modifications promote RNA maturation, others may regulate function and stability. The 3' terminal non-templated oligouridylation is a widespread modification affecting many cellular RNAs at some stage of their life cycle. It has diverse roles in RNA metabolism. The most prevalent is the regulation of stability and quality control. On the cellular and organismal level, it plays a critical role in a number of pathways, such as cell cycle regulation, cell death, development or viral infection. Defects in uridylation have been linked to several diseases. This review summarizes the current knowledge about the role of the 3' terminal oligo(U)-tailing in biology of various RNAs in eukaryotes and describes key factors involved in these pathways.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

RNA uridylation and decay in plants

RNA uridylation consists of the untemplated addition of uridines at the 3' extremity of an RNA molecule. RNA uridylation is catalysed by terminal uridylyltransferases (TUTases), which form a subgroup of the terminal nucleotidyltransferase family, to which poly(A) polymerases also belong. The key role of RNA uridylation is to regulate RNA degradation in a variety of eukaryotes, including fission yeast, plants and animals. In plants, RNA uridylation has been mostly studied in two model species, the green algae Chlamydomonas reinhardtii and the flowering plant Arabidopsis thaliana Plant TUTases target a variety of RNA substrates, differing in size and function. These RNA substrates include microRNAs (miRNAs), small interfering silencing RNAs (siRNAs), ribosomal RNAs (rRNAs), messenger RNAs (mRNAs) and mRNA fragments generated during post-transcriptional gene silencing. Viral RNAs can also get uridylated during plant infection. We describe here the evolutionary history of plant TUTases and we summarize the diverse molecular functions of uridylation during RNA degradation processes in plants. We also outline key points of future research.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

Target RNAs Strike Back on MicroRNAs

MicroRNAs are extensively studied regulatory non-coding small RNAs that silence animal genes throughout most biological processes, typically doing so by binding to partially complementary sequences within target RNAs. A plethora of studies has described detailed mechanisms for microRNA biogenesis and function, as well as their temporal and spatial regulation during development. By inducing translational repression and/or degradation of their target RNAs, microRNAs can contribute to achieve highly specific cell- or tissue-specific gene expression, while their aberrant expression can lead to disease. Yet an unresolved aspect of microRNA biology is how such small RNA molecules are themselves cleared from the cell, especially under circumstances where fast microRNA turnover or specific degradation of individual microRNAs is required. In recent years, it was unexpectedly found that binding of specific target RNAs to microRNAs with extensive complementarity can reverse the outcome, triggering degradation of the bound microRNAs. This emerging pathway, named TDMD for Target RNA-Directed MicroRNA Degradation, leads to microRNA 3'-end tailing by the addition of A/U non-templated nucleotides, trimming or shortening from the 3' end, and highly specific microRNA loss, providing a new layer of microRNA regulation. Originally described in flies and known to be triggered by viral RNAs, novel endogenous instances of TDMD have been uncovered and are now starting to be understood. Here, we review our current knowledge of this pathway and its potential role in the control and diversification of microRNA expression patterns.

Major 3'-5' Exoribonucleases in the Metabolism of Coding and Non-coding RNA

3'-5' exoribonucleases are key enzymes in the degradation of superfluous or aberrant RNAs and in the maturation of precursor RNAs into their functional forms. The major bacterial 3'-5' exoribonucleases responsible for both these activities are PNPase, RNase II and RNase R. These enzymes are of ancient nature with widespread distribution. In eukaryotes, PNPase and RNase II/RNase R enzymes can be found in the cytosol and in mitochondria and chloroplasts; RNase II/RNase R-like enzymes are also found in the nucleus. Humans express one PNPase (PNPT1) and three RNase II/RNase R family members (Dis3, Dis3L and Dis3L2). These enzymes take part in a multitude of RNA surveillance mechanisms that are critical for translation accuracy. Although active against a wide range of both coding and non-coding RNAs, the different 3'-5' exoribonucleases exhibit distinct substrate affinities. The latest studies on these RNA degradative enzymes have contributed to the identification of additional homologue proteins, the uncovering of novel RNA degradation pathways, and to a better comprehension of several disease-related processes and response to stress, amongst many other exciting findings. Here, we provide a comprehensive and up-to-date overview on the function, structure, regulation and substrate preference of the key 3'-5' exoribonucleases involved in RNA metabolism.

Regulation of cytoplasmic RNA stability: Lessons from Drosophila

The process of RNA degradation is a critical level of regulation contributing to the control of gene expression. In the last two decades a number of studies have shown the specific and targeted nature of RNA decay and its importance in maintaining homeostasis. The key players within the pathways of RNA decay are well conserved with their mutation or disruption resulting in distinct phenotypes as well as human disease. Model organisms including Drosophila melanogaster have played a substantial role in elucidating the mechanisms conferring control over RNA stability. A particular advantage of this model organism is that the functions of ribonucleases can be assessed in the context of natural cells within tissues in addition to individual immortalized cells in culture. Drosophila RNA stability research has demonstrated how the cytoplasmic decay machines, such as the exosome, Dis3L2 and Xrn1, are responsible for regulating specific processes including apoptosis, proliferation, wound healing and fertility. The work discussed here has begun to identify specific mRNA transcripts that appear sensitive to specific decay pathways representing mechanisms through which the ribonucleases control mRNA stability. Drosophila research has also contributed to our knowledge of how specific RNAs are targeted to the ribonucleases including AU rich elements, miRNA targeting and 3' tailing. Increased understanding of these mechanisms is critical to elucidating the control elicited by the cytoplasmic ribonucleases which is relevant to human disease. This article is categorized under: RNA in Disease and Development > RNA in Development RNA Turnover and Surveillance > Regulation of RNA Stability RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms.

Loss of Dis3l2 partially phenocopies Perlman syndrome in mice and results in up-regulation of Igf2 in nephron progenitor cells

Loss of function of the DIS3L2 exoribonuclease is associated with Wilms tumor and the Perlman congenital overgrowth syndrome. Here, Hunter et al.’s analysis of Dis3l2-null nephron progenitor cells reveals up-regulation of Igf2, a growth-promoting gene strongly associated with Wilms tumorigenesis.

Loss of function of the DIS3L2 exoribonuclease is associated with Wilms tumor and the Perlman congenital overgrowth syndrome. LIN28, a Wilms tumor oncoprotein, triggers the DIS3L2-mediated degradation of the precursor of let-7, a microRNA that inhibits Wilms tumor development. These observations have led to speculation that DIS3L2-mediated tumor suppression is attributable to let-7 regulation. Here we examine new DIS3L2-deficient cell lines and mouse models, demonstrating that DIS3L2 loss has no effect on mature let-7 levels. Rather, analysis of Dis3l2-null nephron progenitor cells, a potential cell of origin of Wilms tumors, reveals up-regulation of Igf2, a growth-promoting gene strongly associated with Wilms tumorigenesis. These findings nominate a new potential mechanism underlying the pathology associated with DIS3L2 deficiency.

A Network of Noncoding Regulatory RNAs Acts in the Mammalian Brain

Noncoding RNAs (ncRNAs) play increasingly appreciated gene-regulatory roles. Here, we describe a regulatory network centered on four ncRNAs-a long ncRNA, a circular RNA, and two microRNAs-using gene editing in mice to probe the molecular consequences of disrupting key components of this network. The long ncRNA Cyrano uses an extensively paired site to miR-7 to trigger destruction of this microRNA. Cyrano-directed miR-7 degradation is much more effective than previously described examples of target-directed microRNA degradation, which come primarily from studies of artificial and viral RNAs. By reducing miR-7 levels, Cyrano prevents repression of miR-7-targeted mRNAs and enables accumulation of Cdr1as, a circular RNA known to regulate neuronal activity. Without Cyrano, excess miR-7 causes cytoplasmic destruction of Cdr1as in neurons, in part through enhanced slicing of Cdr1as by a second miRNA, miR-671. Thus, several types of ncRNAs can collaborate to establish a sophisticated regulatory network.

Biochemical and structural bioinformatics studies of fungal CutA nucleotidyltransferases explain their unusual specificity toward CTP and increased tendency for cytidine incorporation at the 3'-terminal positions of synthesized tails

Noncanonical RNA nucleotidyltransferases (NTases), including poly(A), poly(U) polymerases (PAPs/PUPs), and C/U-adding enzymes, modify 3'-ends of different transcripts affecting their functionality and stability. They contain PAP/OAS1 substrate-binding domain (SBD) with inserted NTase domain. Aspergillus nidulans CutA (AnCutA), synthesizes C/U-rich 3'-terminal extensions in vivo. Here, using high-throughput sequencing of the 3'-RACE products for tails generated by CutA proteins in vitro in the presence of all four NTPs, we show that even upon physiological ATP excess synthesized tails indeed contain an unprecedented number of cytidines interrupted by uridines and stretches of adenosines, and that the majority end with two cytidines. Strikingly, processivity assays documented that in the presence of CTP as a sole nucleotide, the enzyme terminates after adding two cytidines only. Comparison of our CutA 3D model to selected noncanonical NTases of known structures revealed substantial differences in the nucleotide recognition motif (NRM) within PAP/OAS1 SBD. We demonstrate that CutA specificity toward CTP can be partially changed to PAP or PUP by rational mutagenesis within NRM and, analogously, Cid1 PUP can be converted into a C/U-adding enzyme. Collectively, we suggest that a short cluster of amino acids within NRM is a determinant of NTases' substrate preference, which may allow us to predict their specificity.

Noncoding RNA Surveillance: The Ends Justify the Means

Numerous surveillance pathways sculpt eukaryotic transcriptomes by degrading unneeded, defective, and potentially harmful noncoding RNAs (ncRNAs). Because aberrant and excess ncRNAs are largely degraded by exoribonucleases, a key characteristic of these RNAs is an accessible, protein-free 5' or 3' end. Most exoribonucleases function with cofactors that recognize ncRNAs with accessible 5' or 3' ends and/or increase the availability of these ends. Noncoding RNA surveillance pathways were first described in budding yeast, and there are now high-resolution structures of many components of the yeast pathways and significant mechanistic understanding as to how they function. Studies in human cells are revealing the ways in which these pathways both resemble and differ from their yeast counterparts, and are also uncovering numerous pathways that lack equivalents in budding yeast. In this review, we describe both the well-studied pathways uncovered in yeast and the new concepts that are emerging from studies in mammalian cells. We also discuss the ways in which surveillance pathways compete with chaperone proteins that transiently protect nascent ncRNA ends from exoribonucleases, with partner proteins that sequester these ends within RNPs, and with end modification pathways that protect the ends of some ncRNAs from nucleases.

RNA uridylation: a key posttranscriptional modification shaping the coding and noncoding transcriptome

RNA uridylation is a potent and widespread posttranscriptional regulator of gene expression. RNA uridylation has been detected in a range of eukaryotes including trypanosomes, animals, plants, and fungi, but with the noticeable exception of budding yeast. Virtually all classes of eukaryotic RNAs can be uridylated and uridylation can also tag viral RNAs. The untemplated addition of a few uridines at the 3' end of a transcript can have a decisive impact on RNA's fate. In rare instances, uridylation is an intrinsic step in the maturation of noncoding RNAs like for the U6 spliceosomal RNA or mitochondrial guide RNAs in trypanosomes. Uridylation can also switch specific miRNA precursors from a degradative to a processing mode. This switch depends on the number of uridines added which is regulated by the cellular context. Yet, the typical consequence of uridylation on mature noncoding RNAs or their precursors is to accelerate decay. Importantly, mRNAs are also tagged by uridylation. In fact, the advent of novel high throughput sequencing protocols has recently revealed the pervasiveness of mRNA uridylation, from plants to humans. As for noncoding RNAs, the main function to date for mRNA uridylation is to promote degradation. Yet, additional roles begin to be ascribed to U-tailing such as the control of mRNA deadenylation, translation control and possibly storage. All these new findings illustrate that we are just beginning to appreciate the diversity of roles played by RNA uridylation and its full temporal and spatial implication in regulating gene expression. WIREs RNA 2018, 9:e1440. doi: 10.1002/wrna.1440 This article is categorized under: RNA Processing > 3' End Processing RNA Processing > RNA Editing and Modification RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms.

Comparison of preribosomal RNA processing pathways in yeast, plant and human cells - focus on coordinated action of endo- and exoribonucleases

Proper regulation of ribosome biosynthesis is mandatory for cellular adaptation, growth and proliferation. Ribosome biogenesis is the most energetically demanding cellular process, which requires tight control. Abnormalities in ribosome production have severe consequences, including developmental defects in plants and genetic diseases (ribosomopathies) in humans. One of the processes occurring during eukaryotic ribosome biogenesis is processing of the ribosomal RNA precursor molecule (pre-rRNA), synthesized by RNA polymerase I, into mature rRNAs. It must not only be accurate but must also be precisely coordinated with other phenomena leading to the synthesis of functional ribosomes: RNA modification, RNA folding, assembly with ribosomal proteins and nucleocytoplasmic RNP export. A multitude of ribosome biogenesis factors ensure that these events take place in a correct temporal order. Among them are endo- and exoribonucleases involved in pre-rRNA processing. Here, we thoroughly present a wide spectrum of ribonucleases participating in rRNA maturation, focusing on their biochemical properties, regulatory mechanisms and substrate specificity. We also discuss cooperation between various ribonucleolytic activities in particular stages of pre-rRNA processing, delineating major similarities and differences between three representative groups of eukaryotes: yeast, plants and humans.

Characterization of a TUTase/RNase complex required for Drosophila gametogenesis

Post-transcriptional regulatory strategies that involve coupling between terminal uridyltransferase (TUTase) and exoribonuclease enzymes have recently been characterized in diverse species. Of note, the 3' exoribonuclease Dis3L2 has received substantial attention as a factor that metabolizes uridylated substrates in contexts such as general mRNA degradation, turnover of specific miRNAs, and quality control of noncoding RNAs. To date, most studies of Dis3L2 have focused on fungi and mammalian cells. Here, we introduce Drosophila as a system that permits analysis of molecular mechanisms as well as the ability to interrogate organismal phenotypes. We started with a structure-function analysis of the Drosophila TUTase Tailor, which we recently identified to inhibit biogenesis of splicing-derived miRNA hairpins. Next, we show that Tailor/Dis3L2 form a complex via N-terminal domains in the respective proteins that are distinct from their catalytic domains. In vitro, Dis3L2 has nuclease activity, but substrate oligouridylation by Tailor stimulates their degradation by Dis3L2, especially for structured substrates. We analyzed mutants of Tailor and Dis3L2, which are viable and lack overt morphological defects. Instead, these mutants exhibit defects in female and male fertility, implying specific requirements in the germline. Dis3L2 defects are more severe than Tailor, and their requirements appear stronger in males than in females. In particular, loss of Dis3L2 completely blocks productive spermatogenesis, causing male sterility. RNA-seq analysis from single- and double-mutant testes reveals aberrant gene expression programs and suggests that noncoding RNAs may be preferentially affected by Dis3L2. Overall, our studies of a new tailing/trimming complex reveal unexpectedly specific requirements during gametogenesis.

A common class of transcripts with 5'-intron depletion, distinct early coding sequence features, and N1-methyladenosine modification

Introns are found in 5' untranslated regions (5'UTRs) for 35% of all human transcripts. These 5'UTR introns are not randomly distributed: Genes that encode secreted, membrane-bound and mitochondrial proteins are less likely to have them. Curiously, transcripts lacking 5'UTR introns tend to harbor specific RNA sequence elements in their early coding regions. To model and understand the connection between coding-region sequence and 5'UTR intron status, we developed a classifier that can predict 5'UTR intron status with >80% accuracy using only sequence features in the early coding region. Thus, the classifier identifies transcripts with 5' proximal-intron-minus-like-coding regions ("5IM" transcripts). Unexpectedly, we found that the early coding sequence features defining 5IM transcripts are widespread, appearing in 21% of all human RefSeq transcripts. The 5IM class of transcripts is enriched for non-AUG start codons, more extensive secondary structure both preceding the start codon and near the 5' cap, greater dependence on eIF4E for translation, and association with ER-proximal ribosomes. 5IM transcripts are bound by the exon junction complex (EJC) at noncanonical 5' proximal positions. Finally, N¹-methyladenosines are specifically enriched in the early coding regions of 5IM transcripts. Taken together, our analyses point to the existence of a distinct 5IM class comprising ∼20% of human transcripts. This class is defined by depletion of 5' proximal introns, presence of specific RNA sequence features associated with low translation efficiency, N¹-methyladenosines in the early coding region, and enrichment for noncanonical binding by the EJC.

Molecular basis for cytoplasmic RNA surveillance by uridylation-triggered decay in Drosophila

The posttranscriptional addition of nucleotides to the 3' end of RNA regulates the maturation, function, and stability of RNA species in all domains of life. Here, we show that in flies, 3' terminal RNA uridylation triggers the processive, 3'-to-5' exoribonucleolytic decay via the RNase II/R enzyme CG16940, a homolog of the human Perlman syndrome exoribonuclease Dis3l2. Together with the TUTase Tailor, dmDis3l2 forms the cytoplasmic, terminal RNA uridylation-mediated processing (TRUMP) complex that functionally cooperates in the degradation of structured RNA RNA immunoprecipitation and high-throughput sequencing reveals a variety of TRUMP complex substrates, including abundant non-coding RNA, such as 5S rRNA, tRNA, snRNA, snoRNA, and the essential RNase MRP Based on genetic and biochemical evidence, we propose a key function of the TRUMP complex in the cytoplasmic quality control of RNA polymerase III transcripts. Together with high-throughput biochemical characterization of dmDis3l2 and bacterial RNase R, our results imply a conserved molecular function of RNase II/R enzymes as "readers" of destabilizing posttranscriptional marks-uridylation in eukaryotes and adenylation in prokaryotes-that play important roles in RNA surveillance.