The Cid1 poly(U) polymerase

Template-independent enzymatic synthesis of RNA oligonucleotides

RNA oligonucleotides have emerged as a powerful therapeutic modality to treat disease, yet current manufacturing methods may not be able to deliver on anticipated future demand. Here, we report the development and optimization of an aqueous-based, template-independent enzymatic RNA oligonucleotide synthesis platform as an alternative to traditional chemical methods. The enzymatic synthesis of RNA oligonucleotides is made possible by controlled incorporation of reversible terminator nucleotides with a common 3'-O-allyl ether blocking group using new CID1 poly(U) polymerase mutant variants. We achieved an average coupling efficiency of 95% and demonstrated ten full cycles of liquid phase synthesis to produce natural and therapeutically relevant modified sequences. We then qualitatively assessed the platform on a solid phase, performing enzymatic synthesis of several N + 5 oligonucleotides on a controlled-pore glass support. Adoption of an aqueous-based process will offer key advantages including the reduction of solvent use and sustainable therapeutic oligonucleotide manufacturing.

An update on post-transcriptional regulation of retrotransposons

Retrotransposons, including LINE-1, Alu, SVA, and endogenous retroviruses, are one of the major constituents of human genomic repetitive sequences. Through the process of retrotransposition, some of them occasionally insert into new genomic locations by a copy-paste mechanism involving RNA intermediates. Irrespective of de novo genomic insertions, retrotransposon expression can lead to DNA double-strand breaks and stimulate cellular innate immunity through endogenous patterns. As a result, retrotransposons are tightly regulated by multi-layered regulatory processes to prevent the dangerous effects of their expression. In recent years, significant progress was made in revealing how retrotransposon biology intertwines with general post-transcriptional RNA metabolism. Here, I summarize current knowledge on the involvement of post-transcriptional factors in the biology of retrotransposons, focusing on LINE-1. I emphasize general RNA metabolisms such as methylation of adenine (m⁶ A), RNA 3'-end polyadenylation and uridylation, RNA decay and translation regulation. I discuss the effects of retrotransposon RNP sequestration in cytoplasmic bodies and autophagy. Finally, I summarize how innate immunity restricts retrotransposons and how retrotransposons make use of cellular enzymes, including the DNA repair machinery, to complete their replication cycles.

Terminal Uridylyl Transferase Mediated Site-Directed Access to Clickable Chromatin Employing CRISPR-dCas9

Locus-specific interrogation of target genes employing functional probes such as proteins and small molecules is paramount in decoding the molecular basis of gene function and designing tools to modulate its downstream effects. In this context, CRISPR-based gene editing and targeting technologies have proved tremendously useful, as they can be programmed to target any gene of interest by simply changing the sequence of the single guide RNA (sgRNA). Although these technologies are widely utilized in recruiting genetically encoded functional proteins, display of small molecules using CRISPR system is not well developed due to the lack of adequate techniques. Here, we have devised an innovative technology called sgRNA-Click (sgR-CLK) that harnesses the power of bioorthogonal click chemistry for remodeling guide RNA to display synthetic molecules on target genes. sgR-CLK employs a novel posttranscriptional chemoenzymatic labeling platform wherein a terminal uridylyl transferase (TUTase) was repurposed to generate clickable sgRNA of choice by site-specific tailoring of multiple azide-modified nucleotide analogues at the 3' end. The presence of a minimally invasive azide handle assured that the sgRNAs are indeed functional. Notably, an azide-tailed sgRNA targeting the telomeric repeat served as a Trojan horse on the CRISPR-dCas9 system to guide synthetic tags (biotin) site-specifically on chromatin employing copper-catalyzed or strain-promoted click reactions. Taken together, sgR-CLK presents a significant advancement on the utility of bioorthogonal chemistry, TUTase, and the CRISPR toolbox, which could offer a simplified solution for site-directed display of small molecule probes and diagnostic tools on target genes.

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.

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.

Connections between 3' end processing and DNA damage response: Ten years later

Ten years ago we reviewed how the cellular DNA damage response (DDR) is controlled by changes in the functional and structural properties of nuclear proteins, resulting in a timely coordinated control of gene expression that allows DNA repair. Expression of genes that play a role in DDR is regulated not only at transcriptional level during mRNA biosynthesis but also by changing steady-state levels due to turnover of the transcripts. The 3' end processing machinery, which is important in the regulation of mRNA stability, is involved in these gene-specific responses to DNA damage. Here, we review the latest mechanistic connections described between 3' end processing and DDR, with a special emphasis on alternative polyadenylation, microRNA and RNA binding proteins-mediated deadenylation, and discuss the implications of deregulation of these steps in DDR and human disease. This article is categorized under: RNA Processing > 3' End Processing RNA-Based Catalysis > Miscellaneous RNA-Catalyzed Reactions RNA in Disease and Development > RNA in Disease.

RNA surveillance by uridylation-dependent RNA decay in Schizosaccharomyces pombe

Uridylation-dependent RNA decay is a widespread eukaryotic pathway modulating RNA homeostasis. Terminal uridylyltransferases (Tutases) add untemplated uridyl residues to RNA 3'-ends, marking them for degradation by the U-specific exonuclease Dis3L2. In Schizosaccharomyces pombe, Cid1 uridylates a variety of RNAs. In this study, we investigate the prevalence and impact of uridylation-dependent RNA decay in S. pombe by transcriptionally profiling cid1 and dis3L2 deletion strains. We found that the exonuclease Dis3L2 represents a bottleneck in uridylation-dependent mRNA decay, whereas Cid1 plays a redundant role that can be complemented by other Tutases. Deletion of dis3L2 elicits a cellular stress response, upregulating transcription of genes involved in protein folding and degradation. Misfolded proteins accumulate in both deletion strains, yet only trigger a strong stress response in dis3L2 deficient cells. While a deletion of cid1 increases sensitivity to protein misfolding stress, a dis3L2 deletion showed no increased sensitivity or was even protective. We furthermore show that uridylyl- and adenylyltransferases cooperate to generate a 5'-NxAUUAAAA-3' RNA motif on dak2 mRNA. Our studies elucidate the role of uridylation-dependent RNA decay as part of a global mRNA surveillance, and we found that perturbation of this pathway leads to the accumulation of misfolded proteins and elicits cellular stress responses.

Target prediction of candidate miRNAs from Oryza sativa for silencing the RYMV genome

In order to maintain a consistent supply of rice globally, control of pathogens affecting crop production is a matter of due concern. Rice yellow mottle virus(RYMV) is known to cause a variety of symptoms which can result in reduced yield. Four ORFs can be identified in the genome of RYMV encoding for P1 (ORF1), Polyprotein (processed to produce VPg, protease, helicase, RdRp4) (ORF2), putative RdRp (ORF3) and capsid/coat protein (ORF4). This research was aimed at identifying genome encoded miRNAs of O. sativa that are targeted to the genome of Rice Yellow Mottle Virus (RYMV). A consensus of four miRNA target prediction algorithms (RNA22, miRanda, TargetFinder and psRNATarget) was computed, followed by calculation of free energies of miRNA-mRNA duplex formation. A phylogenetic tree was constructed to portray the evolutionary relationships between RYMV strains isolated to date. From the consensus of algorithms used, a total of seven O. sativa miRNAs were predicted and conservation of target site was finally evaluated. Predicted miRNAs can be further evaluated by experiments involving the testing of the success of in vitro gene silencing of RYMV genome; this can pave the way for development of RYMV resistant rice varieties in the future.

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.

Tipping the balance of RNA stability by 3' editing of the transcriptome

BACKGROUND

The regulation of active microRNAs (miRNAs) and maturation of messenger RNAs (mRNAs) that are competent for translation is a crucial point in the control of all cellular processes, with established roles in development and differentiation. Terminal nucleotidyltransferases (TNTases) are potent regulators of RNA metabolism. TNTases promote the addition of single or multiple nucleotides to an RNA transcript that can rapidly alter transcript stability. The well-known polyadenylation promotes transcript stability while the newly discovered but ubiquitious 3'-end polyuridylation marks RNA for degradation. Monoadenylation and uridylation are essential control mechanisms balancing mRNA and miRNA homeostasis.

SCOPE OF REVIEW

This review discusses the multiple functions of non-canonical TNTases, focusing on their substrate range, biological functions, and evolution. TNTases directly control mRNA and miRNA levels, with diverse roles in transcriptome stabilization, maturation, silencing, or degradation. We will summarize the current state of knowledge on non-canonical nucleotidyltransferases and their function in regulating miRNA and mRNA metabolism. We will review the discovery of uridylation as an RNA degradation pathway and discuss the evolution of nucleotidyltransferases along with their use in RNA labeling and future applications as therapeutic targets.

MAJOR CONCLUSIONS

The biochemically and evolutionarily highly related adenylyl- and uridylyltransferases play antagonizing roles in the cell. In general, RNA adenylation promotes stability, while uridylation marks RNA for degradation. Uridylyltransferases evolved from adenylyltransferases in multiple independent evolutionary events by the insertion of a histidine residue into the active site, altering nucleotide, but not RNA specificity.

GENERAL SIGNIFICANCE

Understanding the mechanisms regulating RNA stability in the cell and controlling the transcriptome is essential for efforts aiming to influence cellular fate. Selectively enhancing or reducing RNA stability allows for alterations in the transcriptome, proteome, and downstream cellular processes. Genetic, biochemical, and clinical data suggest TNTases are potent targets for chemotherapeutics and have been exploited for RNA labeling applications. This article is part of a Special Issue entitled "Biochemistry of Synthetic Biology - Recent Developments" Guest Editor: Dr. Ilka Heinemann and Dr. Patrick O'Donoghue.

The polyadenylation complex of Trypanosoma brucei: Characterization of the functional poly(A) polymerase

The generation of mature mRNA in the protozoan parasite Trypanosoma brucei requires coupled polyadenylation and trans splicing. In contrast to other eukaryotes, we still know very little on components, mechanisms, and dynamics of the 3' end-processing machinery in trypanosomes. To characterize the catalytic core of the polyadenylation complex in T. brucei, we first identified the poly(A) polymerase [Tb927.7.3780] as the major functional, nuclear-localized enzyme in trypanosomes. In contrast, another poly(A) polymerase, encoded by an intron-containing gene [Tb927.3.3160], localizes mainly in the cytoplasm and appears not to be functional in general 3' end processing of mRNAs. Based on tandem-affinity purification with tagged CPSF160 and mass spectrometry, we identified ten associated components of the trypanosome polyadenylation complex, including homologues to all four CPSF subunits, Fip1, CstF50/64, and Symplekin, as well as two hypothetical proteins. RNAi-mediated knockdown revealed that most of these factors are essential for growth and required for both in vivo polyadenylation and trans splicing, arguing for a general coupling of these two mRNA-processing reactions.

Constructive edge of uridylation-induced RNA degradation

RNA uridylation is a significant transcriptome-shaping factor in protists, fungi, metazoans, and plants. The 3' U-additions are catalyzed by terminal uridyltransferases (TUTases), a diverse group of enzymes that along with non-canonical poly(A) polymerases form a distinct group in the superfamily of DNA polymerase β-like nucleotidyl transferases. Within and across studied organisms and subcellular compartments, TUTases differ in nucleotide triphosphate selectivity, interacting partners, and RNA targets. A general premise linking RNA uridylation to 3'-5' degradation received support from several studies of small RNAs and mRNA turnover. However, recent work on kinetoplastid protists typified by Trypanosoma brucei provides evidence that RNA uridylation may play a more nuanced role in generating functional small RNAs. In this pathogen's mitochondrion, most mRNAs are internally edited by U-insertions and deletions, and subjected to 3' adenylation/uridylation; guide RNAs (gRNAs) required for editing are U-tailed. The prominent role of uridylation in mitochondrial RNA metabolism stimulated identification of the first TUTase, RNA editing TUTase 1 (RET1). Here we discuss functional studies of mitochondrial uridylation in trypanosomes that have revealed an unorthodox pathway of small RNA biogenesis. The current model accentuates physical coupling of RET1 and 3'-5' RNase II/RNB-type exonuclease DSS1 within a stable complex termed the mitochondrial 3' processome (MPsome). In the confines of this complex, RET1 initially uridylates a long precursor to activate its 3'-5' degradation by DSS1, and then uridylates trimmed guide RNA to disengage the processing complex from the mature molecule. We also discuss a potential role of antisense transcription in the MPsome pausing at a fixed distance from gRNA's 5' end. This step likely defines the mature 3' end by enabling kinetic competition between TUTase and exonuclease activities.

RNA Editing TUTase 1: structural foundation of substrate recognition, complex interactions and drug targeting

Terminal uridyltransferases (TUTases) execute 3′ RNA uridylation across protists, fungi, metazoan and plant species. Uridylation plays a particularly prominent role in RNA processing pathways of kinetoplastid protists typified by the causative agent of African sleeping sickness, Trypanosoma brucei. In mitochondria of this pathogen, most mRNAs are internally modified by U-insertion/deletion editing while guide RNAs and rRNAs are U-tailed. The founding member of TUTase family, RNA editing TUTase 1 (RET1), functions as a subunit of the 3′ processome in uridylation of gRNA precursors and mature guide RNAs. Along with KPAP1 poly(A) polymerase, RET1 also participates in mRNA translational activation. RET1 is divergent from human TUTases and is essential for parasite viability in the mammalian host and the insect vector. Given its robust in vitro activity, RET1 represents an attractive target for trypanocide development. Here, we report high-resolution crystal structures of the RET1 catalytic core alone and in complex with UTP analogs. These structures reveal a tight docking of the conserved nucleotidyl transferase bi-domain module with a RET1-specific C2H2 zinc finger and RNA recognition (RRM) domains. Furthermore, we define RET1 region required for incorporation into the 3′ processome, determinants for RNA binding, subunit oligomerization and processive UTP incorporation, and predict druggable pockets.

Nucleotide specificity of the human terminal nucleotidyltransferase Gld2 (TUT2)

The nontemplated addition of single or multiple nucleotides to RNA transcripts is an efficient means to control RNA stability and processing. Cytoplasmic RNA adenylation and the less well-known uridylation are post-transcriptional mechanisms regulating RNA maturation, activity, and degradation. Gld2 is a member of the noncanonical poly(A) polymerases, which include enzymes with varying nucleotide specificity, ranging from strictly ATP to ambiguous to exclusive UTP adding enzymes. Human Gld2 has been associated with transcript stabilizing miRNA monoadenylation and cytoplasmic mRNA polyadenylation. Most recent data have uncovered an unexpected miRNA uridylation activity, which promotes miRNA maturation. These conflicting data raise the question of Gld2 nucleotide specificity. Here, we biochemically characterized human Gld2 and demonstrated that it is a bona fide adenylyltransferase with only weak activity toward other nucleotides. Despite its sequence similarity with uridylyltransferases (TUT4, TUT7), Gld2 displays an 83-fold preference of ATP over UTP. Gld2 is a promiscuous enzyme, with activity toward miRNA, pre-miRNA, and polyadenylated RNA substrates. Apo-Gld2 activity is restricted to adding single nucleotides and processivity likely relies on additional RNA-binding proteins. A phylogeny of the PAP/TUTase superfamily suggests that uridylyltransferases, which are derived from distinct adenylyltransferase ancestors, arose multiple times during evolution via insertion of an active site histidine. A corresponding histidine insertion into the Gld2 active site alters substrate specificity from ATP to UTP.

Widespread 3'-end uridylation in eukaryotic RNA viruses

RNA 3′ uridylation occurs pervasively in eukaryotes, but is poorly characterized in viruses. In this study, we demonstrate that a broad array of RNA viruses, including mycoviruses, plant viruses and animal viruses, possess a novel population of RNA species bearing nontemplated oligo(U) or (U)-rich tails, suggesting widespread 3′ uridylation in eukaryotic viruses. Given the biological relevance of 3′ uridylation to eukaryotic RNA degradation, we propose a conserved but as-yet-unknown mechanism in virus-host interaction.

Degradation of oligouridylated histone mRNAs: see UUUUU and goodbye

During the cell cycle the expression of replication-dependent histones is tightly coupled to DNA synthesis. Histone messenger RNA (mRNA) levels strongly increase during early S-phase and rapidly decrease at the end of it. Here, we review the degradation of replication-dependent histone mRNAs, a paradigm of post-transcriptional gene regulation, in the context of processing, translation, and oligouridylation. Replication-dependent histone transcripts are characterized by the absence of introns and by the presence of a stem-loop structure at the 3' end of a very short 3' untranslated region (UTR). These features, together with a need for active translation, are a prerequisite for their rapid decay. The degradation is induced by 3' end additions of untemplated uridines, performed by terminal uridyl transferases. Such 3' oligouridylated transcripts are preferentially bound by the heteroheptameric LSM1-7 complex, which also interacts with the 3'→5' exonuclease ERI1 (also called 3'hExo). Presumably in cooperation with LSM1-7 and aided by the helicase UPF1, ERI1 degrades through the stem-loop of oligouridylated histone mRNAs in repeated rounds of partial degradation and reoligouridylation. Although histone mRNA decay is now known in some detail, important questions remain open: How is ceasing nuclear DNA replication relayed to the cytoplasmic histone mRNA degradation? Why is translation important for this process? Recent research on factors such as SLIP1, DBP5, eIF3, CTIF, CBP80/20, and ERI1 has provided new insights into the 3' end formation, the nuclear export, and the translation of histone mRNAs. We discuss how these results fit with the preparation of histone mRNAs for degradation, which starts as early as these transcripts are generated.

Detection of 3'-end RNA uridylation with a protein nanopore

Post-transcriptional modifications of the 3'-ends of RNA molecules have a profound impact on their stability and processing in the cell. Uridylation, the addition of uridines to 3'-ends, has recently been found to be an important regulatory signal to stabilize the tagged molecules or to direct them toward degradation. Simple and cost-effective methods for the detection of this post-transcriptional modification are not yet available. Here, we demonstrate the selective and transient binding of 3'-uridylated ssRNAs inside the β barrel of the staphylococcal α-hemolysin (αHL) nanopore and investigate the molecular basis of uridine recognition by the pore. We show the discrimination of 3'-oligouridine tails on the basis of their lengths and propose the αHL nanopore as a useful sensor for this biologically relevant RNA modification.

Cytoplasmic RNA: a case of the tail wagging the dog

The addition of poly(A) tails to eukaryotic nuclear mRNAs promotes their stability, export to the cytoplasm and translation. Subsequently, the balance between exonucleolytic deadenylation and selective re-establishment of translation-competent poly(A) tails by cytoplasmic poly(A) polymerases is essential for the appropriate regulation of gene expression from oocytes to neurons. In recent years, surprising roles for cytoplasmic poly(A) polymerase-related enzymes that add uridylyl, rather than adenylyl, residues to RNA 3' ends have also emerged. These terminal uridylyl transferases promote the turnover of certain mRNAs but also modify microRNAs, their precursors and other small RNAs to modulate their stability or biological functions.

Gradual processing of the ITS1 from the nucleolus to the cytoplasm during synthesis of the human 18S rRNA

Defects in ribosome biogenesis trigger stress response pathways, which perturb cell proliferation and differentiation in several genetic diseases. In Diamond–Blackfan anemia (DBA), a congenital erythroblastopenia, mutations in ribosomal protein genes often interfere with the processing of the internal transcribed spacer 1 (ITS1), the mechanism of which remains elusive in human cells. Using loss-of-function experiments and extensive RNA analysis, we have defined the precise position of the endonucleolytic cleavage E in the ITS1, which generates the 18S-E intermediate, the last precursor to the 18S rRNA. Unexpectedly, this cleavage is followed by 3′–5′ exonucleolytic trimming of the 18S-E precursor during nuclear export of the pre-40S particle, which sets a new mechanism for 18S rRNA formation clearly different from that established in yeast. In addition, cleavage at site E is also followed by 5′–3′ exonucleolytic trimming of the ITS1 by exonuclease XRN2. Perturbation of this step on knockdown of the large subunit ribosomal protein RPL26, which was recently associated to DBA, reveals the putative role of a highly conserved cis-acting sequence in ITS1 processing. These data cast new light on the original mechanism of ITS1 elimination in human cells and provide a mechanistic framework to further study the interplay of DBA-linked ribosomal proteins in this process.

RNA decay via 3' uridylation

The post-transcriptional addition of non-templated nucleotides to the 3' ends of RNA molecules can have a profound impact on their stability and biological function. Evidence accumulated over the past few decades has identified roles for polyadenylation in RNA stabilisation, degradation and, in the case of eukaryotic mRNAs, translational competence. By contrast, the biological significance of RNA 3' modification by uridylation has only recently started to become apparent. The evolutionary origin of eukaryotic RNA terminal uridyltransferases can be traced to an ancestral poly(A) polymerase. Here we review what is currently known about the biological roles of these enzymes, the ways in which their activity is regulated and the consequences of this covalent modification for the target RNA molecule, with a focus on those instances where uridylation has been found to contribute to RNA degradation. Roles for uridylation have been identified in the turnover of mRNAs, pre-microRNAs, piwi-interacting RNAs and the products of microRNA-directed mRNA cleavage; many mature microRNAs are also modified by uridylation, though the consequences in this case are currently less well understood. In the case of piwi-interacting RNAs, modification of the 3'-terminal nucleotide by the HEN1 methyltransferase blocks uridylation and so stabilises the small RNA. The extent to which other uridylation-dependent mechanisms of RNA decay are similarly regulated awaits further investigation. This article is part of a Special Issue entitled: RNA Decay mechanisms.

mRNAs containing the histone 3' stem-loop are degraded primarily by decapping mediated by oligouridylation of the 3' end

Metazoan replication-dependent histone mRNAs are only present in S-phase, due partly to changes in their stability. These mRNAs end in a unique stem-loop (SL) that is required for both translation and cell-cycle regulation. Previous studies showed that histone mRNA degradation occurs through both 5'→3' and 3'→5' processes, but the relative contributions are not known. The 3' end of histone mRNA is oligouridylated during its degradation, although it is not known whether this is an essential step. We introduced firefly luciferase reporter mRNAs containing the histone 3' UTR SL (Luc-SL) and either a normal or hDcp2-resistant cap into S-phase HeLa cells. Both mRNAs were translated, and translation initially protected the mRNAs from degradation, but there was a lag of ∼40 min with the uncleavable cap compared to ∼8 min for the normal cap before rapid decay. Knockdown of hDcp2 resulted in a similar longer lag for Luc-SL containing a normal cap, indicating that 5'→3' decay is important in this system. Inhibition of DNA replication with hydroxyurea accelerated the degradation of Luc-SL. Knockdown of terminal uridyltransferase (TUTase) 4 but not TUTase 3 slowed the decay process, but TUTase 4 knockdown had no effect on destabilization of the mRNA by hydroxyurea. Both Luc-SL and its 5' decay intermediates were oligouridylated. Preventing oligouridylation by 3'-deoxyadenosine (cordycepin) addition to the mRNA slowed degradation, in the presence or absence of hydroxyurea, suggesting oligouridylation initiates degradation. The spectrum of oligouridylated fragments suggests the 3'→5' degradation machinery stalls during initial degradation, whereupon reuridylation occurs.

Cytoplasmic mRNA 3' tagging in eukaryotes: does it spell the end?

Although functional RNA is generally protected against degradation, defects or irregularity during RNA biogenesis lead to rapid degradation. Cellular surveillance mechanisms therefore need to distinguish aberrant, erroneous, damaged or aging transcripts from normal RNAs in order to maintain fidelity and control of gene expression. The detection of defects seems to be primarily based on functionality or aberrant rates of a given step in RNA biogenesis, allowing efficient detection of many different errors without recognition of their specific nature. We propose that the addition of non-templated nucleotides to the 3' end of mRNAs and small non-coding RNAs, 3' tagging, is the primary means by which malfunctioning RNAs are labelled, promoting their functional repression and degradation. However, the addition of non-templated nucleotides to transcripts can have diverse effects which vary with location, length, substrate and sequence.

Optimization of enzymatic reaction conditions for generating representative pools of cDNA from small RNA

Small regulatory RNA repertoires in biological samples are heterogeneous mixtures that may include species arising from varied biosynthetic pathways and modification events. Small RNA profiling and discovery approaches ought to capture molecules in a way that is representative of expression level. It follows that the effects of RNA modifications on representation should be minimized. The collection of high-quality, representative data, therefore, will be highly dependent on bias-free sample manipulation in advance of quantification. We examined the impact of 2'-O-methylation of the 3'-terminal nucleotide of small RNA on key enzymatic reactions of standard front-end manipulation schemes. Here we report that this common modification negatively influences the representation of these small RNA species. Deficits occurred at multiple steps as determined by gel analysis of synthetic input RNA and by quantification and sequencing of derived cDNA pools. We describe methods to minimize the effects of 2'-O-methyl modification of small RNA 3'-termini using T4 RNA ligase 2 truncated, and other optimized reaction conditions, demonstrating their use by quantifying representation of miRNAs and piRNAs in cDNA pools prepared from biological samples.

Next-generation sequencing identifies the natural killer cell microRNA transcriptome

Natural killer (NK) cells are innate lymphocytes important for early host defense against infectious pathogens and surveillance against malignant transformation. Resting murine NK cells regulate the translation of effector molecule mRNAs (e.g., granzyme B, GzmB) through unclear molecular mechanisms. MicroRNAs (miRNAs) are small noncoding RNAs that post-transcriptionally regulate the translation of their mRNA targets, and are therefore candidates for mediating this control process. While the expression and importance of miRNAs in T and B lymphocytes have been established, little is known about miRNAs in NK cells. Here, we used two next-generation sequencing (NGS) platforms to define the miRNA transcriptomes of resting and cytokine-activated primary murine NK cells, with confirmation by quantitative real-time PCR (qRT-PCR) and microarrays. We delineate a bioinformatics analysis pipeline that identified 302 known and 21 novel mature miRNAs from sequences obtained from NK cell small RNA libraries. These miRNAs are expressed over a broad range and exhibit isomiR complexity, and a subset is differentially expressed following cytokine activation. Using these miRNA NGS data, miR-223 was identified as a mature miRNA present in resting NK cells with decreased expression following cytokine activation. Furthermore, we demonstrate that miR-223 specifically targets the 3' untranslated region of murine GzmB in vitro, indicating that this miRNA may contribute to control of GzmB translation in resting NK cells. Thus, the sequenced NK cell miRNA transcriptome provides a valuable framework for further elucidation of miRNA expression and function in NK cell biology.

Exonucleases and endonucleases involved in polyadenylation-assisted RNA decay

RNA polyadenylation occurs in most forms of life, excluding a small number of biological systems. This posttranscriptional modification undertakes two roles, both of which influence the stability of the polyadenylated transcript. One is associated with the mature 3' ends of nucleus-encoded mRNAs in eukaryotic cells and is important for nuclear exit, translatability, and longevity. The second form of RNA polyadenylation assumes an almost opposite role; it is termed 'transient' and serves to mediate the degradation of RNA. Poly(A)-assisted RNA decay pathways were once thought to occur only in prokaryotes/organelles but are now known to be a common phenomenon, present in bacteria, organelles, archaea, and the nucleus and cytoplasm of eukaryotic cells, regardless of the fact that in some of these systems, stable polyadenylation exists as well. This article will summarize the current knowledge of polyadenylation and degradation factors involved in poly(A)-assisted RNA decay in the domains of life, focusing mainly on that which occurs in prokaryotes and organelles. In addition, it will offer an evolutionary view of the development of RNA polyadenylation and degradation and the cellular machinery that is involved.

NMR solution structure of poliovirus uridylyated peptide linked to the genome (VPgpU)

Picornaviruses have a 22-24 amino acid peptide, VPg, bound covalently at the 5' end of their RNA, that is essential for replication. VPgs are uridylylated at a conserved tyrosine to form VPgpU, the primer of RNA synthesis by the viral polymerase. This first complete structure for any uridylylated VPg, of poliovirus type 1 (PV1)-VPgpU, shows that conserved amino acids in VPg stabilize the bound UMP, with the uridine atoms involved in base pairing and chain elongation projected outward. Comparing this structure to PV1-VPg and partial structures of VPg/VPgpU from other picornaviruses suggests that enteroviral polymerases require a more stable VPg structure than does the distantly related aphthovirus, foot and mouth disease virus (FMDV). The glutamine residue at the C-terminus of PV1-VPgpU lies in back of the uridine base and may stabilize its position during chain elongation and/or contribute to base specificity. Under in vivo-like conditions with the authentic cre(2C) hairpin RNA and Mg(2+), 5-methylUTP cannot compete with UTP for VPg uridylyation in an in vitro uridylyation assay, but both nucleotides are equally incorporated by PV1-polymerase with Mn(2+) and a poly-A RNA template. This indicates the 5 position is recognized under in vivo conditions. The compact VPgpU structure docks within the active site cavity of the PV-polymerase, close to the position seen for the fragment of FMDV-VPgpU with its polymerase. This structure could aid in design of novel enterovirus inhibitors, and stabilization upon uridylylation may also be pertinent for post-translational uridylylation reactions that underlie other biological processes.

Recognition of adenosine residues by the active site of poly(A)-specific ribonuclease

Poly(A)-specific ribonuclease (PARN) is a mammalian 3'-exoribonuclease that degrades poly(A) with high specificity. To reveal mechanisms by which poly(A) is recognized by the active site of PARN, we have performed a kinetic analysis using a large repertoire of trinucleotide substrates. Our analysis demonstrated that PARN harbors specificity for adenosine recognition in its active site and that the nucleotides surrounding the scissile bond are critical for adenosine recognition. We propose that two binding pockets, which interact with the nucleotides surrounding the scissile bond, play a pivotal role in providing specificity for the recognition of adenosine residues by the active site of PARN. In addition, we show that PARN, besides poly(A), also quite efficiently degrades poly(U), approximately 10-fold less efficiently than poly(A). The poly(U)-degrading property of PARN could be of biological significance as oligo(U) tails recently have been proposed to play a role in RNA stabilization and destabilization.

A single RNA-dependent RNA polymerase assembles with mutually exclusive nucleotidyl transferase subunits to direct different pathways of small RNA biogenesis

Members of the conserved family of eukaryotic RNA-dependent RNA polymerases (Rdrs) synthesize double-stranded RNA (dsRNA) intermediates in diverse pathways of small RNA (sRNA) biogenesis and RNA-mediated silencing. Rdr-dependent pathways of sRNA production are poorly characterized relative to Rdr-independent pathways, and the Rdr enzymes themselves are poorly characterized relative to their viral RNA-dependent RNA polymerase counterparts. We previously described a physical and functional coupling of the Tetrahymena thermophila Rdr, Rdr1, and a Dicer enzyme, Dcr2, in the production of approximately 24-nucleotide (nt) sRNA in vitro. Here we characterize the endogenous complexes that harbor Rdr1, termed RDRCs. Distinct RDRCs assemble to contain Rdr1 and subsets of the total of four tightly Rdr1-associated proteins. Of particular interest are two RDRC subunits, Rdn1 and Rdn2, which possess noncanonical ribonucleotidyl transferase motifs. We show that the two Rdn proteins are uridine-specific polymerases of separate RDRCs. Two additional RDRC subunits, Rdf1 and Rdf2, are present only in RDRCs containing Rdn1. Rdr1 catalytic activity is retained in RDRCs purified from cell extracts lacking any of the nonessential RDRC subunits (Rdn2, Rdf1, Rdf2) or if the RDRC harbors a catalytically inactive Rdn. However, specific disruption of each RDRC imposes distinct loss-of-function consequences at the cellular level and has a differential impact on the accumulation of specific 23-24-nt sRNA sequences in vivo. The biochemical and biological phenotypes of RDRC subunit disruption reveal a previously unanticipated complexity of Rdr-dependent sRNA biogenesis in vivo.

Novel TUTase associates with an editosome-like complex in mitochondria of Trypanosoma brucei

Expression of mitochondrial genomes in Kinetoplastida protists requires massive uracil insertion/deletion mRNA editing. The cascade of editing reactions is accomplished by a multiprotein complex, the 20S editosome, and is directed by trans-acting guide RNAs. Two distinct RNA terminal uridylyl transferases (TUTases), RNA Editing TUTase 1 (RET1) and RNA Editing TUTase 2 (RET2), catalyze 3' uridylylation of guide RNAs and U-insertions into the mRNAs, respectively. RET1 is also involved in mitochondrial mRNA turnover and participates in numerous heterogeneous complexes; RET2 is an integral part of the 20S editosome, in which it forms a U-insertion subcomplex with zinc finger protein MP81 and RNA editing ligase REL2. Here we report the identification of a third mitochondrial TUTase from Trypanosoma brucei. The mitochondrial editosome-like complex associated TUTase (MEAT1) interacts with a 20S editosome-like particle, effectively substituting the U-insertion subcomplex. MEAT1 and RET2 are mutually exclusive in their respective complexes, which otherwise share several components. Similarly to RET2, MEAT1 is exclusively U-specific in vitro and is active on gapped double-stranded RNA resembling editing substrates. However, MEAT1 does not require a 5' phosphate group on the 3' mRNA cleavage fragment produced by editing endonucleases. The functional RNAi complementation experiments showed that MEAT1 is essential for viability of bloodstream and insect parasite forms. The growth inhibition phenotype in the latter can be rescued by coexpressing an RNAi-resistant gene with double-stranded RNA targeting the endogenous transcript. However, preliminary RNA analysis revealed no gross effects on RNA editing in MEAT1-depleted cells and indicated its possible role in regulating the mitochondrial RNA stability.

Decapping is preceded by 3' uridylation in a novel pathway of bulk mRNA turnover

Both end structures of eukaryotic mRNAs, namely the 5′ cap and 3′ poly(A) tail, are necessary for transcript stability, and loss of either is sufficient to stimulate decay. mRNA turnover is classically thought to be initiated by deadenylation, as has been particularly well described in Saccharomyces cerevisiae. Here we describe two additional, parallel decay pathways in the fission yeast Schizosaccharomyces pombe. First, in fission yeast mRNA decapping is frequently independent of deadenylation. Second, Cid1-dependent uridylation of polyadenylated mRNAs, such as act1, hcn1 and urg1, appears to stimulate decapping as part of a novel mRNA turnover pathway. Accordingly, urg1 mRNA is stabilized in cid1∆ cells. Uridylation and deadenylation act redundantly to stimulate decapping, and our data suggest that uridylation-dependent decapping is mediated by the Lsm1-7 complex. As human cells contain Cid1 orthologs, uridylation may form the basis of a widespread, conserved mechanism of mRNA decay.

The many pathways of RNA degradation

From the earliest comparisons of RNA production with steady-state levels, it has been clear that cells transcribe more RNA than they accumulate, implying the existence of active RNA degradation systems. In general, RNA is degraded at the end of its useful life, which is long for a ribosomal RNA but very short for excised introns or spacer fragments, and is closely regulated for most mRNA species. RNA molecules with defects in processing, folding, or assembly with proteins are identified and rapidly degraded by the surveillance machinery. Because RNA degradation is ubiquitous in all cells, it is clear that it must be carefully controlled to accurately recognize target RNAs. How this is achieved is perhaps the most pressing question in the field.

Meta-analysis of small RNA-sequencing errors reveals ubiquitous post-transcriptional RNA modifications

Recent advances in DNA-sequencing technology have made it possible to obtain large datasets of small RNA sequences. Here we demonstrate that not all non-perfectly matched small RNA sequences are simple technological sequencing errors, but many hold valuable biological information. Analysis of three small RNA datasets originating from Oryza sativa and Arabidopsis thaliana small RNA-sequencing projects demonstrates that many single nucleotide substitution errors overlap when aligning homologous non-identical small RNA sequences. Investigating the sites and identities of substitution errors reveal that many potentially originate as a result of post-transcriptional modifications or RNA editing. Modifications include N1-methyl modified purine nucleotides in tRNA, potential deamination or base substitutions in micro RNAs, 3′ micro RNA uridine extensions and 5′ micro RNA deletions. Additionally, further analysis of large sequencing datasets reveal that the combined effects of 5′ deletions and 3′ uridine extensions can alter the specificity by which micro RNAs associate with different Argonaute proteins. Hence, we demonstrate that not all sequencing errors in small RNA datasets are technical artifacts, but that these actually often reveal valuable biological insights to the sites of post-transcriptional RNA modifications.