Polyadenylation and degradation of incomplete RNA polymerase I transcripts in mammalian cells

Nucleolar organization and ribosomal DNA stability in response to DNA damage

Eukaryotic nuclei are structured into sub-compartments orchestrating various cellular functions. The nucleolus is the largest nuclear organelle: a biomolecular condensate with an architecture composed of immiscible fluids facilitating ribosome biogenesis. The nucleolus forms upon the transcription of the repetitive ribosomal RNA genes (rDNA) that cluster in this compartment. rDNA is intrinsically unstable and prone to rearrangements and copy number variation. Upon DNA damage, a specialized nucleolar-DNA Damage Response (n-DDR) is activated: nucleolar transcription is inhibited, the architecture is rearranged, and rDNA is relocated to the nucleolar periphery. Recent data have highlighted how the composition of nucleoli, its structure, chemical and physical properties, contribute to rDNA stability. In this mini-review we focus on recent data that start to reveal how nucleolar composition and the n-DDR work together to ensure rDNA integrity.

Nuclear mRNA decay: regulatory networks that control gene expression

Proper regulation of mRNA production in the nucleus is critical for the maintenance of cellular homoeostasis during adaptation to internal and environmental cues. Over the past 25 years, it has become clear that the nuclear machineries governing gene transcription, pre-mRNA processing, pre-mRNA and mRNA decay, and mRNA export to the cytoplasm are inextricably linked to control the quality and quantity of mRNAs available for translation. More recently, an ever-expanding diversity of new mechanisms by which nuclear RNA decay factors finely tune the expression of protein-encoding genes have been uncovered. Here, we review the current understanding of how mammalian cells shape their protein-encoding potential by regulating the decay of pre-mRNAs and mRNAs in the nucleus.

MPP6 stimulates both RRP6 and DIS3 to degrade a specified subset of MTR4-sensitive substrates in the human nucleus

Recent in vitro reconstitution analyses have proven that the physical interaction between the exosome core and MTR4 helicase, which promotes the exosome activity, is maintained by either MPP6 or RRP6. However, knowledge regarding the function of MPP6 with respect to in vivo exosome activity remains scarce. Here, we demonstrate a facilitative function of MPP6 that composes a specific part of MTR4-dependent substrate decay by the human exosome. Using RNA polymerase II-transcribed poly(A)+ substrate accumulation as an indicator of a perturbed exosome, we found functional redundancy between RRP6 and MPP6 in the decay of these poly(A)+ transcripts. MTR4 binding to the exosome core via MPP6 was essential for MPP6 to exert its redundancy with RRP6. However, at least for the decay of our identified exosome substrates, MTR4 recruitment by MPP6 was not functionally equivalent to recruitment by RRP6. Genome-wide classification of substrates based on their sensitivity to each exosome component revealed that MPP6 deals with a specific range of substrates and highlights the importance of MTR4 for their decay. Considering recent findings of competitive binding to the exosome between auxiliary complexes, our results suggest that the MPP6-incorporated MTR4-exosome complex is one of the multiple alternative complexes rather than the prevailing one.

Tailer: a pipeline for sequencing-based analysis of nonpolyadenylated RNA 3' end processing

Post-transcriptional trimming and tailing of RNA 3' ends play key roles in the processing and quality control of noncoding RNAs (ncRNAs). However, bioinformatic tools to examine changes in the RNA 3' "tailome" are sparse and not standardized. Here we present Tailer, a bioinformatic pipeline in two parts that allows for robust quantification and analysis of tail information from next-generation sequencing experiments that preserve RNA 3' end information. The first part of Tailer, Tailer-processing, uses genome annotation or reference FASTA gene sequences to quantify RNA 3' ends from SAM-formatted alignment files or FASTQ sequence read files produced from sequencing experiments. The second part, Tailer-analysis, uses the output of Tailer-processing to identify statistically significant RNA targets of trimming and tailing and create graphs for data exploration. We apply Tailer to RNA 3' end sequencing experiments from three published studies and find that it accurately and reproducibly recapitulates key findings. Thus, Tailer should be a useful and easily accessible tool to globally investigate tailing dynamics of nonpolyadenylated RNAs and conditions that perturb them.

9-Aminoacridine Inhibits Ribosome Biogenesis by Targeting Both Transcription and Processing of Ribosomal RNA

Aminoacridines, used for decades as antiseptic and antiparasitic agents, are prospective candidates for therapeutic repurposing and new drug development. Although the mechanisms behind their biological effects are not fully elucidated, they are most often attributed to the acridines’ ability to intercalate into DNA. Here, we characterized the effects of 9-aminoacridine (9AA) on pre-rRNA metabolism in cultured mammalian cells. Our results demonstrate that 9AA inhibits both transcription of the ribosomal RNA precursors (pre-rRNA) and processing of the already synthesized pre-rRNAs, thereby rapidly abolishing ribosome biogenesis. Using a fluorescent intercalator displacement assay, we further show that 9AA can bind to RNA in vitro, which likely contributes to its ability to inhibit post-transcriptional steps in pre-rRNA maturation. These findings extend the arsenal of small-molecule compounds that can be used to block ribosome biogenesis in mammalian cells and have implications for the pharmacological development of new ribosome biogenesis inhibitors.

Soy isoflavone metabolite equol inhibits cancer cell proliferation in a PAP associated domain containing 5-dependent and an estrogen receptor-independent manner

Isoflavone is a species of polyphenol found mainly in soy and soy products. Many studies have demonstrated its estrogen receptor (ER)-dependent action. Equol is an intestinal metabolite of a major soy isoflavone daidzein. We aimed to elucidate the mechanism for ER-independent actions of equol. Equol has been shown to inhibit proliferation of HeLa human cervical cancer cells and mouse melanoma B16 cells in an ER-independent manner. Using functional genetic screening, PAP associated domain containing 5 (PAPD5), which is a non-canonical poly(A) polymerase, was identified as an essential molecule in the ER-independent action. While peroral administration of equol inhibited tumor growth of control B16 cells subcutaneously inoculated in mice, it had little effect on the growth of PAPD5-ablated B16 cells. Intriguingly, equol progressed tumor growth of the PAPD5-ablated human breast cancer MCF-7 cells, which have high ERα expression. Equol has been found to induce polyadenylation of snoRNAs in a PAPD5-depdendent manner. Furthermore, peroral equol administration increased microRNA miR-320a expression in tumors. Together, these results suggest that equol may have a dual effect on ER-positive cancer cells, acting with, antiproliferative activity through PAPD5 and exhibiting proliferative activity via ERα and the former could be associated with miR-320a.

The distinct roles of zinc finger CCHC-type (ZCCHC) superfamily proteins in the regulation of RNA metabolism

The zinc finger CCHC-type (ZCCHC) superfamily proteins, characterized with the consensus sequence C-X₂-C-X₄-H-X₄-C, are accepted to have high-affinity binding to single-stranded nucleic acids, especially single-stranded RNAs. In human beings 25 ZCCHC proteins have been annotated in the HGNC database. Of interest is that among the family, most members are involved in the multiple steps of RNA metabolism. In this review, we focus on the diverged roles of human ZCCHC proteins on RNA transcription, biogenesis, splicing, as well as translation and degradation.

A genome-wide CRISPR screen identifies UFMylation and TRAMP-like complexes as host factors required for hepatitis A virus infection

Hepatitis A virus (HAV) is a positive-sense RNA virus causing acute inflammation of the liver. Here, using a genome-scale CRISPR screen, we provide a comprehensive picture of the cellular factors that are exploited by HAV. We identify genes involved in sialic acid/ganglioside biosynthesis and members of the eukaryotic translation initiation factor complex, corroborating their putative roles for HAV. Additionally, we uncover all components of the cellular machinery for UFMylation, a ubiquitin-like protein modification. We show that HAV translation specifically depends on UFM1 conjugation of the ribosomal protein RPL26. Furthermore, we find that components related to the yeast Trf4/5-Air1/2-Mtr4 polyadenylation (TRAMP) complex are required for viral translation independent of controlling viral poly(A) tails or RNA stability. Finally, we demonstrate that pharmacological inhibition of the TRAMP-like complex decreases HAV replication in hepatocyte cells and human liver organoids, thus providing a strategy for host-directed therapy of HAV infection.

To identify host factors required for the infection with hepatitis A virus, Kulsuptrakul et al. conducted a genome-wide CRISPR knockout screen in human hepatocytes. They reveal that UFMylation of the ribosomal protein RPL26 as well as the polyadenylation activity of a TRAMP-like complex enhance viral translation.

Out or decay: fate determination of nuclear RNAs

In eukaryotes, RNAs newly synthesized by RNA polymerase II (RNAPII) undergo several processing steps prior to transport to the cytoplasm. It has long been known that RNAs with defects in processing or export are removed in the nucleus. Recent studies revealed that RNAs without apparent defects are also subjected to nuclear degradation, indicating that nuclear RNA fate is determined in a more complex and dynamic way than previously thought. Nuclear RNA sorting directly determines the quality and quantity of RNA pools for future translation and thus is of significant importance. In this essay, we will summarize recent studies on this topic, mainly focusing on findings in mammalian system, and discuss about important remaining questions and possible biological relevance for nuclear RNA fate determination.

Host RNA quality control as a hepatitis B antiviral target

Inhibition of the host RNA polyadenylating polymerases, PAPD5 and PAPD7 (PAPD5/7), with dihydroquinolizinone, a small orally available, molecule, results in a rapid and selective degradation of hepatitis B virus (HBV) RNA, and hence reduction in the amounts of viral gene products. DHQ, is a first in class investigational agent and could represent an entirely new category of HBV antivirals. PAPD5 and PAPD7 are non-canonical, cell specified, polyadenylating polymerases, also called terminal nucleotidyl transferases 4B and 4A (TENT4B/A), respectively. They are involved in the degradation of poor-quality cell transcripts, mostly non-coding RNAs and in the maturation of a sub-set of transcripts. They also appear to play a role in shielding some mRNA from degradation. The results of studies with DHQ, along with other recent findings, provide evidence that repression of the PAPD5/7 arm of the cell "RNA quality control" pathway, causes a profound (multi-fold) reduction rather than increase, in the amount of HBV pre-genomic, pre-core and HBsAg mRNA levels in tissue culture and animal models, as well. In this review we will briefly discuss the need for new HBV therapeutics and provide background about HBV transcription. We also discuss cellular degradation of host transcripts, as it relates to a new family of anti-HBV drugs that interfere with these processes. Finally, since HBV mRNA maturation appears to be selectively sensitive to PAPD5/7 inhibition in hepatocytes, we discuss the possibility of targeting host RNA "quality control" as an antiviral strategy.

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.

mRNAs are sorted for export or degradation before passing through nuclear speckles

A significant fraction of mRNAs are degraded by the nuclear exosome in normal cells. Here, we studied where and when these exosome target mRNAs are sorted away from properly exported ones in the cells. We show that upon exosome inactivation, polyA RNAs are apparently accumulated in nuclear foci that are distinct from nuclear speckles (NSs), and provide several lines of evidence supporting that these polyA RNAs mainly correspond to accumulating exosome target mRNAs. These results suggest that exosomal mRNA degradation mostly occurs outside of NSs. In support of this possibility, targeting exosome target mRNAs to NSs stabilizes them by preventing exosomal degradation. Furthermore, inhibiting mRNA release from NSs does not attenuate exosomal degradation in normal cells, and results in polyA RNA accumulation both inside and outside of NSs in exosome inactivated cells, suggesting that passage through NSs is not required for sorting mRNAs for degradation or export. Indeed, exosome target mRNAs that normally do not enter NSs are exported upon exosome inactivation. Together, our data suggest that exosome target mRNAs are mainly degraded in the nucleoplasm before entering NSs and rapid removal of these mRNAs is important for preventing their nuclear export.

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.

Drosophila Trf4-1 involves in mRNA and primary miRNA transcription

Drosophila Trf4-1 (DmTrf4-1) is a polyadenylation polymerase or terminal nucleotidyl transferase (PAP/TENT) that has been reported to add poly(A) tails to snRNAs in nucleus or mRNAs in cytoplasm. Here, we found that the loss of Trf4-1 resulted in the reduction of mRNAs and primary miRNAs (pri-miRNAs) in both Drosophila S2 cells and adult flies. Interestingly, the role of Trf4-1 in transcription is independent of its PAP/TENT activity. Moreover, using the chromatin immunoprecipitation assay, we uncovered that the loss of Trf4-1 led to abnormal RNA polymerase II accumulation and reduced H3K4me3 binding in promoter regions. Thus, our study indicates a positive role of Trf4-1 in the transcription of mRNAs and pri-miRNAs.

NRDE2 negatively regulates exosome functions by inhibiting MTR4 recruitment and exosome interaction

The exosome functions in the degradation of diverse RNA species, yet how it is negatively regulated remains largely unknown. Here, we show that NRDE2 forms a 1:1 complex with MTR4, a nuclear exosome cofactor critical for exosome recruitment, via a conserved MTR4-interacting domain (MID). Unexpectedly, NRDE2 mainly localizes in nuclear speckles, where it inhibits MTR4 recruitment and RNA degradation, and thereby ensures efficient mRNA nuclear export. Structural and biochemical data revealed that NRDE2 interacts with MTR4's key residues, locks MTR4 in a closed conformation, and inhibits MTR4 interaction with the exosome as well as proteins important for MTR4 recruitment, such as the cap-binding complex (CBC) and ZFC3H1. Functionally, MID deletion results in the loss of self-renewal of mouse embryonic stem cells. Together, our data pinpoint NRDE2 as a nuclear exosome negative regulator that ensures mRNA stability and nuclear export.

Terminal nucleotidyl transferases (TENTs) in mammalian RNA metabolism

In eukaryotes, almost all RNA species are processed at their 3′ ends and most mRNAs are polyadenylated in the nucleus by canonical poly(A) polymerases. In recent years, several terminal nucleotidyl transferases (TENTs) including non-canonical poly(A) polymerases (ncPAPs) and terminal uridyl transferases (TUTases) have been discovered. In contrast to canonical polymerases, TENTs' functions are more diverse; some, especially TUTases, induce RNA decay while others, such as cytoplasmic ncPAPs, activate translationally dormant deadenylated mRNAs. The mammalian genome encodes 11 different TENTs. This review summarizes the current knowledge about the functions and mechanisms of action of these enzymes.

This article is part of the theme issue ‘5′ and 3′ modifications controlling RNA degradation’.

The multitasking polyA tail: nuclear RNA maturation, degradation and export

A polyA (pA) tail is an essential modification added to the 3' ends of a wide range of RNAs at different stages of their metabolism. Here, we describe the main sources of polyadenylation and outline their underlying biochemical interactions within the nuclei of budding yeast Saccharomyces cerevisiae, human cells and, when relevant, the fission yeast Schizosaccharomyces pombe Polyadenylation mediated by the S. cerevisiae Trf4/5 enzymes, and their human homologues PAPD5/7, typically leads to the 3'-end trimming or complete decay of non-coding RNAs. By contrast, the primary function of canonical pA polymerases (PAPs) is to produce stable and nuclear export-competent mRNAs. However, this dichotomy is becoming increasingly blurred, at least in S. pombe and human cells, where polyadenylation mediated by canonical PAPs may also result in transcript decay.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

R1 retrotransposons in the nucleolar organizers of Drosophila melanogaster are transcribed by RNA polymerase I upon heat shock

The ribosomal RNA genes (rDNA) of Drosophila melanogaster reside within centromere-proximal nucleolar organizers on both the X and Y chromosomes. Each locus contains between 200-300 tandem repeat rDNA units that encode 18S, 5.8S, 2S, and 28S ribosomal RNAs (rRNAs) necessary for ribosome biogenesis. In arthropods like Drosophila, about 60% of the rDNA genes have R1 and/or R2 retrotransposons inserted at specific sites within their 28S regions; these units likely fail to produce functional 28S rRNA. We showed earlier that R2 expression increases upon nucleolar stress caused by the loss of the ribosome assembly factor, Nucleolar Phosphoprotein of 140 kDa (Nopp140). Here we show that R1 expression is selectively induced by heat shock. Actinomycin D, but not α-amanitin, blocked R1 expression in S2 cells upon heat shock, indicating that R1 elements are transcribed by Pol I. A series of RT-PCRs established read-through transcription by Pol I from the 28S gene region into R1. Sequencing the RT-PCR products confirmed the 28S-R1 RNA junction and the expression of R1 elements within nucleolar rDNA rather than R1 elements known to reside in centromeric heterochromatin. Using a genome-wide precision run-on sequencing (PRO-seq) data set available at NCBI-GEO, we show that Pol I activity on R1 elements is negligible under normal non-heat shock conditions but increases upon heat shock. We propose that prior to heat shock Pol I pauses within the 5' end of R1 where we find a consensus "pause button", and that heat shock releases Pol I for read-through transcription farther into R1.

Tales around the clock: Poly(A) tails in circadian gene expression

Circadian rhythms are ubiquitous time-keeping processes in eukaryotes with a period of ~24 hr. Light is perhaps the main environmental cue (zeitgeber) that affects several aspects of physiology and behaviour, such as sleep/wake cycles, orientation of birds and bees, and leaf movements in plants. Temperature can serve as the main zeitgeber in the absence of light cycles, even though it does not lead to rhythmicity through the same mechanism as light. Additional cues include feeding patterns, humidity, and social rhythms. At the molecular level, a master oscillator orchestrates circadian rhythms and organizes molecular clocks located in most cells. The generation of the 24 hr molecular clock is based on transcriptional regulation, as it drives intrinsic rhythmic changes based on interlocked transcription/translation feedback loops that synchronize expression of genes. Thus, processes and factors that determine rhythmic gene expression are important to understand circadian rhythms. Among these, the poly(A) tails of RNAs play key roles in their stability, translational efficiency and degradation. In this article, we summarize current knowledge and discuss perspectives on the role and significance of poly(A) tails and associating factors in the context of the circadian clock. This article is categorized under: RNA Turnover and Surveillance > Regulation of RNA Stability RNA Processing > 3' End Processing.

Nuclear mRNA Surveillance Mechanisms: Function and Links to Human Disease

Production of export-competent mRNAs involves transcription and a series of dynamic processing and modification events of pre-messenger RNAs in the nucleus. Mutations in the genes encoding the transcription and mRNP processing machinery and the complexities involved in the biogenesis events lead to the formation of aberrant messages. These faulty transcripts are promptly eliminated by the nuclear RNA exosome and its cofactors to safeguard the cells and organisms from genetic catastrophe. Mutations in the components of the core nuclear exosome and its cofactors lead to the tissue-specific dysfunction of exosomal activities, which are linked to diverse human diseases and disorders. In this article, we examine the structure and function of both the yeast and human RNA exosome complex and its cofactors, discuss the nature of the various altered amino acid residues implicated in these diseases with the speculative mechanisms of the mutation-induced disorders and project the frontier and prospective avenues of the future research in this field.

RNA surveillance by the nuclear RNA exosome: mechanisms and significance

The nuclear RNA exosome is an essential and versatile machinery that regulates maturation and degradation of a huge plethora of RNA species. The past two decades have witnessed remarkable progress in understanding the whole picture of its RNA substrates and the structural basis of its functions. In addition to the exosome itself, recent studies focusing on associated co-factors have been elucidating how the exosome is directed towards specific substrates. Moreover, it has been gradually realized that loss-of-function of exosome subunits affect multiple biological processes such as the DNA damage response, R-loop resolution, maintenance of genome integrity, RNA export, translation and cell differentiation. In this review, we summarize the current knowledge of the mechanisms of nuclear exosome-mediated RNA metabolism and discuss their physiological significance.

Essential role for non-canonical poly(A) polymerase GLD4 in cytoplasmic polyadenylation and carbohydrate metabolism

Regulation of gene expression at the level of cytoplasmic polyadenylation is important for many biological phenomena including cell cycle progression, mitochondrial respiration, and learning and memory. GLD4 is one of the non-canonical poly(A) polymerases that regulates cytoplasmic polyadenylation-induced translation, but its target mRNAs and role in cellular physiology is not well known. To assess the full panoply of mRNAs whose polyadenylation is controlled by GLD4, we performed an unbiased whole genome-wide screen using poy(U) chromatography and thermal elution. We identified hundreds of mRNAs regulated by GLD4, several of which are involved in carbohydrate metabolism including GLUT1, a major glucose transporter. Depletion of GLD4 not only reduced GLUT1 poly(A) tail length, but also GLUT1 protein. GLD4-mediated translational control of GLUT1 mRNA is dependent of an RNA binding protein, CPEB1, and its binding elements in the 3΄ UTR. Through regulating GLUT1 level, GLD4 affects glucose uptake into cells and lactate levels. Moreover, GLD4 depletion impairs glucose deprivation-induced GLUT1 up-regulation. In addition, we found that GLD4 affects glucose-dependent cellular phenotypes such as migration and invasion in glioblastoma cells. Our observations delineate a novel post-transcriptional regulatory network involving carbohydrate metabolism and glucose homeostasis mediated by GLD4.

Poly(A)-specific ribonuclease is a nuclear ribosome biogenesis factor involved in human 18S rRNA maturation

The poly-A specific ribonuclease (PARN), initially characterized for its role in mRNA catabolism, supports the processing of different types of non-coding RNAs including telomerase RNA. Mutations in PARN are linked to dyskeratosis congenita and pulmonary fibrosis. Here, we show that PARN is part of the enzymatic machinery that matures the human 18S ribosomal RNA (rRNA). Consistent with its nucleolar steady-state localization, PARN is required for 40S ribosomal subunit production and co-purifies with 40S subunit precursors. Depletion of PARN or expression of a catalytically-compromised PARN mutant results in accumulation of 3΄ extended 18S rRNA precursors. Analysis of these processing intermediates reveals a defect in 3΄ to 5΄ trimming of the internal transcribed spacer 1 (ITS1) region, subsequent to endonucleolytic cleavage at site E. Consistent with a function of PARN in exonucleolytic trimming of 18S-E pre-rRNA, recombinant PARN can process the corresponding ITS1 RNA fragment in vitro. Trimming of 18S-E pre-rRNA by PARN occurs in the nucleus, upstream of the final endonucleolytic cleavage by the endonuclease NOB1 in the cytoplasm. These results identify PARN as a new component of the ribosome biogenesis machinery in human cells. Defects in ribosome biogenesis could therefore underlie the pathologies linked to mutations in PARN.

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.

Inhibition of post-transcriptional steps in ribosome biogenesis confers cytoprotection against chemotherapeutic agents in a p53-dependent manner

The p53-mediated nucleolar stress response associated with inhibition of ribosomal RNA transcription was previously shown to potentiate killing of tumor cells. Here, we asked whether targeting of ribosome biogenesis can be used as the basis for selective p53-dependent cytoprotection of nonmalignant cells. Temporary functional inactivation of the 60S ribosome assembly factor Bop1 in a 3T3 cell model markedly increased cell recovery after exposure to camptothecin or methotrexate. This was due, at least in part, to reversible pausing of the cell cycle preventing S phase associated DNA damage. Similar cytoprotective effects were observed after transient shRNA-mediated silencing of Rps19, but not several other tested ribosomal proteins, indicating distinct cellular responses to the inhibition of different steps in ribosome biogenesis. By temporarily inactivating Bop1 function, we further demonstrate selective killing of p53-deficient cells with camptothecin while sparing isogenic p53-positive cells. Thus, combining cytotoxic treatments with inhibition of select post-transcriptional steps of ribosome biogenesis holds potential for therapeutic targeting of cells that have lost p53.

Exosome cofactor hMTR4 competes with export adaptor ALYREF to ensure balanced nuclear RNA pools for degradation and export

The exosome is a key RNA machine that functions in the degradation of unwanted RNAs. Here, we found that significant fractions of precursors and mature forms of mRNAs and long noncoding RNAs are degraded by the nuclear exosome in normal human cells. Exosome-mediated degradation of these RNAs requires its cofactor hMTR4. Significantly, hMTR4 plays a key role in specifically recruiting the exosome to its targets. Furthermore, we provide several lines of evidence indicating that hMTR4 executes this role by directly competing with the mRNA export adaptor ALYREF for associating with ARS2, a component of the cap-binding complex (CBC), and this competition is critical for determining whether an RNA is degraded or exported to the cytoplasm. Together, our results indicate that the competition between hMTR4 and ALYREF determines exosome recruitment and functions in creating balanced nuclear RNA pools for degradation and export.

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.

Diurnal Oscillations in Liver Mass and Cell Size Accompany Ribosome Assembly Cycles

The liver plays a pivotal role in metabolism and xenobiotic detoxification, processes that must be particularly efficient when animals are active and feed. A major question is how the liver adapts to these diurnal changes in physiology. Here, we show that, in mice, liver mass, hepatocyte size, and protein levels follow a daily rhythm, whose amplitude depends on both feeding-fasting and light-dark cycles. Correlative evidence suggests that the daily oscillation in global protein accumulation depends on a similar fluctuation in ribosome number. Whereas rRNA genes are transcribed at similar rates throughout the day, some newly synthesized rRNAs are polyadenylated and degraded in the nucleus in a robustly diurnal fashion with a phase opposite to that of ribosomal protein synthesis. Based on studies with cultured fibroblasts, we propose that rRNAs not packaged into complete ribosomal subunits are polyadenylated by the poly(A) polymerase PAPD5 and degraded by the nuclear exosome.

Ribosomes Need Straight A's to Sleep

Like falling asleep and waking up, many biological processes in mammals cycle in a diurnal fashion. Now, Sinturel et al. demonstrate that diurnal size changes in the liver require eating during a mouse's normal awake time and that these size changes are controlled by a nuclear mechanism that modulates ribosome production.

Attacked from All Sides: RNA Decay in Antiviral Defense

The innate immune system has evolved a number of sensors that recognize viral RNA (vRNA) to restrict infection, yet the full spectrum of host-encoded RNA binding proteins that target these foreign RNAs is still unknown. The RNA decay machinery, which uses exonucleases to degrade aberrant RNAs largely from the 5′ or 3′ end, is increasingly recognized as playing an important role in antiviral defense. The 5′ degradation pathway can directly target viral messenger RNA (mRNA) for degradation, as well as indirectly attenuate replication by limiting specific pools of endogenous RNAs. The 3′ degradation machinery (RNA exosome) is emerging as a downstream effector of a diverse array of vRNA sensors. This review discusses our current understanding of the roles of the RNA decay machinery in controlling viral infection.

Identification of a Nuclear Exosome Decay Pathway for Processed Transcripts

The RNA exosome is fundamental for the degradation of RNA in eukaryotic nuclei. Substrate targeting is facilitated by its co-factor Mtr4p/hMTR4, which links to RNA-binding protein adaptors. One example is the trimeric human nuclear exosome targeting (NEXT) complex, which is composed of hMTR4, the Zn-finger protein ZCCHC8, and the RNA-binding factor RBM7. NEXT primarily targets early and unprocessed transcripts, which demands a rationale for how the nuclear exosome recognizes processed RNAs. Here, we describe the poly(A) tail exosome targeting (PAXT) connection, which comprises the ZFC3H1 Zn-knuckle protein as a central link between hMTR4 and the nuclear poly(A)-binding protein PABPN1. Individual depletion of ZFC3H1 and PABPN1 results in the accumulation of common transcripts that are generally both longer and more extensively polyadenylated than NEXT substrates. Importantly, ZFC3H1/PABPN1 and ZCCHC8/RBM7 contact hMTR4 in a mutually exclusive manner, revealing that the exosome targets nuclear transcripts of different maturation status by substituting its hMTR4-associating adaptors.

Insight into the RNA Exosome Complex Through Modeling Pontocerebellar Hypoplasia Type 1b Disease Mutations in Yeast

Pontocerebellar hypoplasia type 1b (PCH1b) is an autosomal recessive disorder that causes cerebellar hypoplasia and spinal motor neuron degeneration, leading to mortality in early childhood. PCH1b is caused by mutations in the RNA exosome subunit gene, EXOSC3 The RNA exosome is an evolutionarily conserved complex, consisting of nine different core subunits, and one or two 3'-5' exoribonuclease subunits, that mediates several RNA degradation and processing steps. The goal of this study is to assess the functional consequences of the amino acid substitutions that have been identified in EXOSC3 in PCH1b patients. To analyze these EXOSC3 substitutions, we generated the corresponding amino acid substitutions in the Saccharomyces cerevisiae ortholog of EXOSC3, Rrp40 We find that the rrp40 variants corresponding to EXOSC3-G31A and -D132A do not affect yeast function when expressed as the sole copy of the essential Rrp40 protein. In contrast, the rrp40-W195R variant, corresponding to EXOSC3-W238R in PCH1b patients, impacts cell growth and RNA exosome function when expressed as the sole copy of Rrp40 The rrp40-W195R protein is unstable, and does not associate efficiently with the RNA exosome in cells that also express wild-type Rrp40 Consistent with these findings in yeast, the levels of mouse EXOSC3 variants are reduced compared to wild-type EXOSC3 in a neuronal cell line. These data suggest that cells possess a mechanism for optimal assembly of functional RNA exosome complex that can discriminate between wild-type and variant exosome subunits. Budding yeast can therefore serve as a useful tool to understand the molecular defects in the RNA exosome caused by PCH1b-associated amino acid substitutions in EXOSC3, and potentially extending to disease-associated substitutions in other exosome subunits.

The new (dis)order in RNA regulation

RNA-binding proteins play a key role in the regulation of all aspects of RNA metabolism, from the synthesis of RNA to its decay. Protein-RNA interactions have been thought to be mostly mediated by canonical RNA-binding domains that form stable secondary and tertiary structures. However, a number of pioneering studies over the past decades, together with recent proteome-wide data, have challenged this view, revealing surprising roles for intrinsically disordered protein regions in RNA binding. Here, we discuss how disordered protein regions can mediate protein-RNA interactions, conceptually grouping these regions into RS-rich, RG-rich, and other basic sequences, that can mediate both specific and non-specific interactions with RNA. Disordered regions can also influence RNA metabolism through protein aggregation and hydrogel formation. Importantly, protein-RNA interactions mediated by disordered regions can influence nearly all aspects of co- and post-transcriptional RNA processes and, consequently, their disruption can cause disease. Despite growing interest in disordered protein regions and their roles in RNA biology, their mechanisms of binding, regulation, and physiological consequences remain poorly understood. In the coming years, the study of these unorthodox interactions will yield important insights into RNA regulation in cellular homeostasis and disease.

Oligoadenylation of 3' decay intermediates promotes cytoplasmic mRNA degradation in Drosophila cells

Post-transcriptional 3' end addition of nucleotides is important in a variety of RNA decay pathways. We have examined the 3' end addition of nucleotides during the decay of the Hsp70 mRNA and a corresponding reporter RNA in Drosophila S2 cells by conventional sequencing of cDNAs obtained after mRNA circularization and by deep sequencing of dedicated libraries enriched for 3' decay intermediates along the length of the mRNA. Approximately 5%-10% of 3' decay intermediates carried nonencoded oligo(A) tails with a mean length of 2-3 nucleotides. RNAi experiments showed that the oligoadenylated RNA fragments were intermediates of exosomal decay and the noncanonical poly(A) polymerase Trf4-1 was mainly responsible for A addition. A hot spot of A addition corresponded to an intermediate of 3' decay that accumulated upon inhibition of decapping, and knockdown of Trf4-1 increased the abundance of this intermediate, suggesting that oligoadenylation facilitates 3' decay. Oligoadenylated 3' decay intermediates were found in the cytoplasmic fraction in association with ribosomes, and fluorescence microscopy revealed a cytoplasmic localization of Trf4-1. Thus, oligoadenylation enhances exosomal mRNA degradation in the cytoplasm.

Mutations in Mtr4 Structural Domains Reveal Their Important Role in Regulating tRNAiMet Turnover in Saccharomyces cerevisiae and Mtr4p Enzymatic Activities In Vitro

RNA processing and turnover play important roles in the maturation, metabolism and quality control of a large variety of RNAs thereby contributing to gene expression and cellular health. The TRAMP complex, composed of Air2p, Trf4p and Mtr4p, stimulates nuclear exosome-dependent RNA processing and degradation in Saccharomyces cerevisiae. The Mtr4 protein structure is composed of a helicase core and a novel so-called arch domain, which protrudes from the core. The helicase core contains highly conserved helicase domains RecA-1 and 2, and two structural domains of unclear functions, winged helix domain (WH) and ratchet domain. How the structural domains (arch, WH and ratchet domain) coordinate with the helicase domains and what roles they are playing in regulating Mtr4p helicase activity are unknown. We created a library of Mtr4p structural domain mutants for the first time and screened for those defective in the turnover of TRAMP and exosome substrate, hypomodified tRNA_i^Met. We found these domains regulate Mtr4p enzymatic activities differently through characterizing the arch domain mutants K700N and P731S, WH mutant K904N, and ratchet domain mutant R1030G. Arch domain mutants greatly reduced Mtr4p RNA binding, which surprisingly did not lead to significant defects on either in vivo tRNA_i^Met turnover, or in vitro unwinding activities. WH mutant K904N and Ratchet domain mutant R1030G showed decreased tRNA_i^Met turnover in vivo, as well as reduced RNA binding, ATPase and unwinding activities of Mtr4p in vitro. Particularly, K904 was found to be very important for steady protein levels in vivo. Overall, we conclude that arch domain plays a role in RNA binding but is largely dispensable for Mtr4p enzymatic activities, however the structural domains in the helicase core significantly contribute to Mtr4p ATPase and unwinding activities.

Genome-Wide Analysis of PAPS1-Dependent Polyadenylation Identifies Novel Roles for Functionally Specialized Poly(A) Polymerases in Arabidopsis thaliana

The poly(A) tail at 3’ ends of eukaryotic mRNAs promotes their nuclear export, stability and translational efficiency, and changes in its length can strongly impact gene expression. The Arabidopsis thaliana genome encodes three canonical nuclear poly(A) polymerases, PAPS1, PAPS2 and PAPS4. As shown by their different mutant phenotypes, these three isoforms are functionally specialized, with PAPS1 modifying organ growth and suppressing a constitutive immune response. However, the molecular basis of this specialization is largely unknown. Here, we have estimated poly(A)-tail lengths on a transcriptome-wide scale in wild-type and paps1 mutants. This identified categories of genes as particularly strongly affected in paps1 mutants, including genes encoding ribosomal proteins, cell-division factors and major carbohydrate-metabolic proteins. We experimentally verified two novel functions of PAPS1 in ribosome biogenesis and redox homoeostasis that were predicted based on the analysis of poly(A)-tail length changes in paps1 mutants. When overlaying the PAPS1-dependent effects observed here with coexpression analysis based on independent microarray data, the two clusters of transcripts that are most closely coexpressed with PAPS1 show the strongest change in poly(A)-tail length and transcript abundance in paps1 mutants in our analysis. This suggests that their coexpression reflects at least partly the preferential polyadenylation of these transcripts by PAPS1 versus the other two poly(A)-polymerase isoforms. Thus, transcriptome-wide analysis of poly(A)-tail lengths identifies novel biological functions and likely target transcripts for polyadenylation by PAPS1. Data integration with large-scale co-expression data suggests that changes in the relative activities of the isoforms are used as an endogenous mechanism to co-ordinately modulate plant gene expression.

The poly(A) tail of eukaryotic mRNAs promotes export from the nucleus, translation in the cytoplasm and stability of the mRNA, and changes in poly(A)-tail length can strongly impact on gene expression. The Arabidopsis thaliana genome encodes three nuclear canonical poly(A) polymerases (PAPS1, PAPS2, PAPS4) that fulfill different functions, presumably by preferentially polyadenylating certain subpopulations of pre-mRNAs. Here, we use a fractionation-based technique to assess the transcriptome-wide impact of reduced PAPS1 activity and identify functional classes of transcripts that are particularly sensitive to reduced PAPS1 activity. Analysis of these transcripts identifies two novel biological functions for PAPS1 in ribosome biogenesis and in redox homeostasis that we confirm experimentally. By overlaying our results with information about genome-wide co-expression, we demonstrate that genes co-expressed with PAPS1 are the most strongly affected in terms of poly(A)-tail length and total-abundance changes in the paps1 mutants. This provides strong evidence that the co-expression of these genes with PAPS1 that is seen across thousands of microarrays is at least partly caused by altered activity of the PAPS1 isoform, suggesting that the plant indeed uses modulation of the balance of isoform activities to coordinately regulate the expression of groups of genes.

Reduced expression of the mouse ribosomal protein Rpl17 alters the diversity of mature ribosomes by enhancing production of shortened 5.8S rRNA

Processing of rRNA during ribosome assembly can proceed through alternative pathways but it is unclear whether this could affect the structure of the ribosome. Here, we demonstrate that shortage of a ribosomal protein can change pre-rRNA processing in a way that over time alters ribosome diversity in the cell. Reducing the amount of Rpl17 in mouse cells led to stalled 60S subunit maturation, causing degradation of most of the synthesized precursors. A fraction of pre-60S subunits, however, were able to complete maturation, but with a 5'-truncated 5.8S rRNA, which we named 5.8SC. The 5' exoribonuclease Xrn2 is involved in the generation of both 5.8S(C) and the canonical long form of 5.8S rRNA. Ribosomes containing 5.8S(C) rRNA are present in various mouse and human cells and engage in translation. These findings uncover a previously undescribed form of mammalian 5.8S rRNA and demonstrate that perturbations in ribosome assembly can be a source of heterogeneity in mature ribosomes.

Defective structural RNA processing in relapsing-remitting multiple sclerosis

BACKGROUND

Surveillance of integrity of the basic elements of the cell including DNA, RNA, and proteins is a critical element of cellular physiology. Mechanisms of surveillance of DNA and protein integrity are well understood. Surveillance of structural RNAs making up the vast majority of RNA in a cell is less well understood. Here, we sought to explore integrity of processing of structural RNAs in relapsing remitting multiple sclerosis (RRMS) and other inflammatory diseases.

RESULTS

We employed mononuclear cells obtained from subjects with RRMS and cell lines. We used quantitative-PCR and whole genome RNA sequencing to define defects in structural RNA surveillance and siRNAs to deplete target proteins. We report profound defects in surveillance of structural RNAs in RRMS exemplified by elevated levels of poly(A) + Y1-RNA, poly(A) + 18S rRNA and 28S rRNAs, elevated levels of misprocessed 18S and 28S rRNAs and levels of the U-class of small nuclear RNAs. Multiple sclerosis is also associated with genome-wide defects in mRNA splicing. Ro60 and La proteins, which exist in ribonucleoprotein particles and play different roles in quality control of structural RNAs, are also deficient in RRMS. In cell lines, silencing of the genes encoding Ro60 and La proteins gives rise to these same defects in surveillance of structural RNAs.

CONCLUSIONS

Our results establish that profound defects in structural RNA surveillance exist in RRMS and establish a causal link between Ro60 and La proteins and integrity of structural RNAs.

Identification of a recently active mammalian SINE derived from ribosomal RNA

Complex eukaryotic genomes are riddled with repeated sequences whose derivation does not coincide with phylogenetic history and thus is often unknown. Among such sequences, the capacity for transcriptional activity coupled with the adaptive use of reverse transcription can lead to a diverse group of genomic elements across taxa, otherwise known as selfish elements or mobile elements. Short interspersed nuclear elements (SINEs) are nonautonomous mobile elements found in eukaryotic genomes, typically derived from cellular RNAs such as tRNAs, 7SL or 5S rRNA. Here, we identify and characterize a previously unknown SINE derived from the 3'-end of the large ribosomal subunit (LSU or 28S rDNA) and transcribed via RNA polymerase III. This new element, SINE28, is represented in low-copy numbers in the human reference genome assembly, wherein we have identified 27 discrete loci. Phylogenetic analysis indicates these elements have been transpositionally active within primate lineages as recently as 6 MYA while modern humans still carry transcriptionally active copies. Moreover, we have identified SINE28s in all currently available assembled mammalian genome sequences. Phylogenetic comparisons indicate that these elements are frequently rederived from the highly conserved LSU rRNA sequences in a lineage-specific manner. We propose that this element has not been previously recognized as a SINE given its high identity to the canonical LSU, and that SINE28 likely represents one of possibly many unidentified, active transposable elements within mammalian genomes.

The human nuclear exosome targeting complex is loaded onto newly synthesized RNA to direct early ribonucleolysis

The RNA exosome complex constitutes the major nuclear eukaryotic 3'-5' exonuclease. Outside of nucleoli, the human nucleoplasmic exosome is directed to some of its substrates by the nuclear exosome targeting (NEXT) complex. How NEXT targets RNA has remained elusive. Using an in vivo crosslinking approach, we report global RNA binding sites of RBM7, a key component of NEXT. RBM7 associates broadly with RNA polymerase II-derived RNA, including pre-mRNA and short-lived exosome substrates such as promoter upstream transcripts (PROMPTs), enhancer RNAs (eRNAs), and 3'-extended products from snRNA and replication-dependent histone genes. Within pre-mRNA, RBM7 accumulates at the 3' ends of introns, and pulse-labeling experiments demonstrate that RBM7/NEXT defines an early exosome-targeting pathway for 3'-extended snoRNAs derived from such introns. We propose that RBM7 is generally loaded onto newly synthesized RNA to accommodate exosome action in case of available unprotected RNA 3' ends.

An overview of pre-ribosomal RNA processing in eukaryotes

Ribosomal RNAs are the most abundant and universal noncoding RNAs in living organisms. In eukaryotes, three of the four ribosomal RNAs forming the 40S and 60S subunits are borne by a long polycistronic pre-ribosomal RNA. A complex sequence of processing steps is required to gradually release the mature RNAs from this precursor, concomitant with the assembly of the 79 ribosomal proteins. A large set of trans-acting factors chaperone this process, including small nucleolar ribonucleoparticles. While yeast has been the gold standard for studying the molecular basis of this process, recent technical advances have allowed to further define the mechanisms of ribosome biogenesis in animals and plants. This renewed interest for a long-lasting question has been fueled by the association of several genetic diseases with mutations in genes encoding both ribosomal proteins and ribosome biogenesis factors, and by the perspective of new anticancer treatments targeting the mechanisms of ribosome synthesis. A consensus scheme of pre-ribosomal RNA maturation is emerging from studies in various kinds of eukaryotic organisms. However, major differences between mammalian and yeast pre-ribosomal RNA processing have recently come to light. WIREs RNA 2015, 6:225–242. doi: 10.1002/wrna.1269

Two orthogonal cleavages separate subunit RNAs in mouse ribosome biogenesis

Ribosome biogenesis is a dynamic multistep process, many features of which are still incompletely documented. Here, we show that changes in this pathway can be captured and annotated by means of a graphic set of pre-rRNA ratios, a technique we call Ratio Analysis of Multiple Precursors (RAMP). We find that knocking down a ribosome synthesis factor produces a characteristic RAMP profile that exhibits consistency across a range of depletion levels. This facilitates the inference of affected steps and simplifies comparative analysis. We applied RAMP to examine how endonucleolytic cleavages of the mouse pre-rRNA transcript in the internal transcribed spacer 1 (ITS1) are affected by depletion of factors required for maturation of the small ribosomal subunit (Rcl1, Fcf1/Utp24, Utp23) and the large subunit (Pes1, Nog1). The data suggest that completion of early maturation in a subunit triggers its release from the common pre-rRNA transcript by stimulating cleavage at the proximal site in ITS1. We also find that splitting of pre-rRNA in the 3' region of ITS1 is prevalent in adult mouse tissues and quiescent cells, as it is in human cells. We propose a model for subunit separation during mammalian ribosome synthesis and discuss its implications for understanding pre-rRNA processing pathways.

Poly(A) polymerase (PAP) diversity in gene expression--star-PAP vs canonical PAP

Almost all eukaryotic mRNAs acquire a poly(A) tail at the 3'-end by a concerted RNA processing event: cleavage and polyadenylation. The canonical PAP, PAPα, was considered the only nuclear PAP involved in general polyadenylation of mRNAs. A phosphoinositide-modulated nuclear PAP, Star-PAP, was then reported to regulate a select set of mRNAs in the cell. In addition, several non-canonical PAPs have been identified with diverse cellular functions. Further, canonical PAP itself exists in multiple isoforms thus illustrating the diversity of PAPs. In this review, we compare two nuclear PAPs, Star-PAP and PAPα with a general overview of PAP diversity in the cell. Emerging evidence suggests distinct niches of target pre-mRNAs for the two PAPs and that modulation of these PAPs regulates distinct cellular functions.

Certain adenylated non-coding RNAs, including 5' leader sequences of primary microRNA transcripts, accumulate in mouse cells following depletion of the RNA helicase MTR4

RNA surveillance plays an important role in posttranscriptional regulation. Seminal work in this field has largely focused on yeast as a model system, whereas exploration of RNA surveillance in mammals is only recently begun. The increased transcriptional complexity of mammalian systems provides a wider array of targets for RNA surveillance, and, while many questions remain unanswered, emerging data suggest the nuclear RNA surveillance machinery exhibits increased complexity as well. We have used a small interfering RNA in mouse N2A cells to target the homolog of a yeast protein that functions in RNA surveillance (Mtr4p). We used high-throughput sequencing of polyadenylated RNAs (PA-seq) to quantify the effects of the mMtr4 knockdown (KD) on RNA surveillance. We demonstrate that overall abundance of polyadenylated protein coding mRNAs is not affected, but several targets of RNA surveillance predicted from work in yeast accumulate as adenylated RNAs in the mMtr4KD. microRNAs are an added layer of transcriptional complexity not found in yeast. After Drosha cleavage separates the pre-miRNA from the microRNA's primary transcript, the byproducts of that transcript are generally thought to be degraded. We have identified the 5′ leading segments of pri-miRNAs as novel targets of mMtr4 dependent RNA surveillance.

The roles of SSU processome components and surveillance factors in the initial processing of human ribosomal RNA

During eukaryotic ribosome biogenesis, three of the mature ribosomal (r)RNAs are released from a single precursor transcript (pre-rRNA) by an ordered series of endonucleolytic cleavages and exonucleolytic processing steps. Production of the 18S rRNA requires the removal of the 5' external transcribed spacer (5'ETS) by endonucleolytic cleavages at sites A0 and A1/site 1. In metazoans, an additional cleavage in the 5'ETS, at site A', upstream of A0, has also been reported. Here, we have investigated how A' processing is coordinated with assembly of the early preribosomal complex. We find that only the tUTP (UTP-A) complex is critical for A' cleavage, while components of the bUTP (UTP-B) and U3 snoRNP are important, but not essential, for efficient processing at this site. All other factors involved in the early stages of 18S rRNA processing that were tested here function downstream from this processing step. Interestingly, we show that the RNA surveillance factors XRN2 and MTR4 are also involved in A' cleavage in humans. A' cleavage is largely bypassed when XRN2 is depleted, and we also discover that A' cleavage is not always the initial processing event in all cell types. Together, our data suggest that A' cleavage is not a prerequisite for downstream pre-rRNA processing steps and may, in fact, represent a quality control step for initial pre-rRNA transcripts. Furthermore, we show that components of the RNA surveillance machinery, including the exosome and TRAMP complexes, also play key roles in the recycling of excised spacer fragments and degradation of aberrant pre-rRNAs in human cells.

Specificity factors in cytoplasmic polyadenylation

Poly(A) tail elongation after export of an messenger RNA (mRNA) to the cytoplasm is called cytoplasmic polyadenylation. It was first discovered in oocytes and embryos, where it has roles in meiosis and development. In recent years, however, has been implicated in many other processes, including synaptic plasticity and mitosis. This review aims to introduce cytoplasmic polyadenylation with an emphasis on the factors and elements mediating this process for different mRNAs and in different animal species. We will discuss the RNA sequence elements mediating cytoplasmic polyadenylation in the 3' untranslated regions of mRNAs, including the CPE, MBE, TCS, eCPE, and C-CPE. In addition to describing the role of general polyadenylation factors, we discuss the specific RNA binding protein families associated with cytoplasmic polyadenylation elements, including CPEB (CPEB1, CPEB2, CPEB3, and CPEB4), Pumilio (PUM2), Musashi (MSI1, MSI2), zygote arrest (ZAR2), ELAV like proteins (ELAVL1, HuR), poly(C) binding proteins (PCBP2, αCP2, hnRNP-E2), and Bicaudal C (BICC1). Some emerging themes in cytoplasmic polyadenylation will be highlighted. To facilitate understanding for those working in different organisms and fields, particularly those who are analyzing high throughput data, HUGO gene nomenclature for the human orthologs is used throughout. Where human orthologs have not been clearly identified, reference is made to protein families identified in man.