Termination of cryptic unstable transcripts is directed by yeast RNA-binding proteins Nrd1 and Nab3

Interplay between LncRNAs and autophagy-related pathways in leukemia: mechanisms and clinical implications

Autophagy is a conserved catabolic process that removes protein clumps and defective organelles, thereby promoting cell equilibrium. Growing data suggest that dysregulation of the autophagic pathway is linked to several cancer hallmarks. Long non-coding RNAs (lncRNAs), which are key parts of gene transcription, are increasingly recognized for their significant roles in various biological processes. Recent studies have uncovered a strong connection between the mutational landscape and altered expression of lncRNAs in the tumor formation and development, including leukemia. Research over the past few years has emphasized the role of lncRNAs as important regulators of autophagy-related gene expression. These RNAs can influence key leukemia characteristics, such as apoptosis, proliferation, epithelial-mesenchymal transition (EMT), migration, and angiogenesis, by modulating autophagy-associated signaling pathways. With altered lncRNA expression observed in leukemia cells and tissues, they hold promise as diagnostic biomarkers and therapeutic targets. The current review focuses on the regulatory function of lncRNAs in autophagy and their involvement in leukemia, potentially uncovering valuable therapeutic targets for leukemia treatment.

Overlapping and distinct functions of SPT6, PNUTS, and PCF11 in regulating transcription termination

The histone chaperone and transcription elongation factor SPT6 is integral to RNA polymerase II (RNAPII) activity. SPT6 also plays a crucial role in regulating transcription termination, although the mechanisms involved are largely unknown. In an attempt to identify the pathways employed by SPT6 in this regulation, we found that, while SPT6 and its partner IWS1 interact and co-localize with RNAPII, their functions diverge significantly at gene termination sites. Depletion of SPT6, but not of IWS1, results in extensive readthrough transcription, indicating that SPT6 independently regulates transcription termination. Further analysis identified that the cleavage and polyadenylation factor PCF11 and the phosphatase regulatory protein PNUTS collaborate with SPT6 in this process. These findings suggest that SPT6 may facilitate transcription termination by recruiting PNUTS and PCF11 to RNAPII. Additionally, SPT6 and PNUTS jointly restrict promoter upstream transcripts (PROMPTs), whereas PCF11 presence is essential for their accumulation in the absence of SPT6 at hundreds of genes. Thus, SPT6, PCF11, and PNUTS have both distinct and overlapping functions in transcription termination. Our data highlight the pivotal role of SPT6 in ensuring proper transcription termination at the 5' and 3'-ends of genes.

Mechanism of polyadenylation-independent RNA polymerase II termination

The mechanisms underlying the initiation and elongation of RNA polymerase II (Pol II) transcription are well-studied, whereas termination remains poorly understood. Here we analyze the mechanism of polyadenylation-independent Pol II termination mediated by the yeast Sen1 helicase. Cryo-electron microscopy structures of two pretermination intermediates show that Sen1 binds to Pol II and uses its adenosine triphosphatase activity to pull on exiting RNA in the 5' direction. This is predicted to push Pol II forward, induce an unstable hypertranslocated state and destabilize the transcription bubble, thereby facilitating termination. This mechanism of transcription termination may be widely used because it is conceptually conserved in the bacterial transcription system.

ZC3H4/Restrictor Exerts a Stranglehold on Pervasive Transcription

The regulation of transcription by RNA polymerase II (RNAPII) underpins all cellular processes and is perturbed in thousands of diseases. In humans, RNAPII transcribes ∼20000 protein-coding genes and engages in apparently futile non-coding transcription at thousands of other sites. Despite being so ubiquitous, this transcription is usually attenuated soon after initiation and the resulting products are immediately degraded by the nuclear exosome. We and others have recently described a new complex, "Restrictor", which appears to control such unproductive transcription. Underpinned by the RNA binding protein, ZC3H4, Restrictor curtails unproductive/pervasive transcription genome-wide. Here, we discuss these recent discoveries and speculate on some of the many unknowns regarding Restrictor function and mechanism.

Multiple Forms and Functions of Premature Termination by RNA Polymerase II

Eukaryotic genomes are widely transcribed by RNA polymerase II (pol II) both within genes and in intergenic regions. POL II elongation complexes comprising the polymerase, the DNA template and nascent RNA transcript must be extremely processive in order to transcribe the longest genes which are over 1 megabase long and take many hours to traverse. Dedicated termination mechanisms are required to disrupt these highly stable complexes. Transcription termination occurs not only at the 3' ends of genes once a full length transcript has been made, but also within genes and in promiscuously transcribed intergenic regions. Termination at these latter positions is termed "premature" because it is not triggered in response to a specific signal that marks the 3' end of a gene, like a polyA site. One purpose of premature termination is to remove polymerases from intergenic regions where they are "not wanted" because they may interfere with transcription of overlapping genes or the progress of replication forks. Premature termination has recently been appreciated to occur at surprisingly high rates within genes where it is speculated to serve regulatory or quality control functions. In this review I summarize current understanding of the different mechanisms of premature termination and its potential functions.

Sen1: The Varied Virtues of a Multifaceted Helicase

Several machineries concurrently work on the DNA, but among them RNA Polymerases (RNAPs) are the most widespread and active users. The homeostasis of such a busy genomic environment relies on the existence of mechanisms that allow limiting transcription to a functional level, both in terms of extent and rate. Sen1 is a central player in this sense: using its translocase activity this protein has evolved the specific function of dislodging RNAPs from the DNA template, thus ending the transcription cycle. Over the years, studies have shown that Sen1 uses this same mechanism in a multitude of situations, allowing termination of all three eukaryotic RNAPs in different contexts. In virtue of its helicase activity, Sen1 has also been proposed to have a prominent function in the resolution of co-transcriptional genotoxic R-loops, which can cause the stalling of replication forks. In this review, we provide a synopsis of past and recent findings on the functions of Sen1 in yeast and of its human homologue Senataxin (SETX).

Post-transcriptional regulation shapes the transcriptome of quiescent budding yeast

To facilitate long-term survival, cells must exit the cell cycle and enter quiescence, a reversible non-replicative state. Budding yeast cells reprogram their gene expression during quiescence entry to silence transcription, but how the nascent transcriptome changes in quiescence is unknown. By investigating the nascent transcriptome, we identified over a thousand noncoding RNAs in quiescent and G1 yeast cells, and found noncoding transcription represented a larger portion of the quiescent transcriptome than in G1. Additionally, both mRNA and ncRNA are subject to increased post-transcriptional regulation in quiescence compared to G1. We found that, in quiescence, the nuclear exosome-NNS pathway suppresses over one thousand mRNAs, in addition to canonical noncoding RNAs. RNA sequencing through quiescent entry revealed two distinct time points at which the nuclear exosome controls the abundance of mRNAs involved in protein production, cellular organization, and metabolism, thereby facilitating efficient quiescence entry. Our work identified a previously unknown key biological role for the nuclear exosome-NNS pathway in mRNA regulation and uncovered a novel layer of gene-expression control in quiescence.

Functional analysis of a random-sequence chromosome reveals a high level and the molecular nature of transcriptional noise in yeast cells

We measure transcriptional noise in yeast by analyzing chromatin structure and transcription of an 18-kb region of DNA whose sequence was randomly generated. Nucleosomes fully occupy random-sequence DNA, but nucleosome-depleted regions (NDRs) are much less frequent, and there are fewer well-positioned nucleosomes and shorter nucleosome arrays. Steady-state levels of random-sequence RNAs are comparable to yeast mRNAs, although transcription and decay rates are higher. Transcriptional initiation from random-sequence DNA occurs at numerous sites, indicating very low intrinsic specificity of the RNA Pol II machinery. In contrast, poly(A) profiles of random-sequence RNAs are roughly comparable to those of yeast mRNAs, suggesting limited evolutionary restraints on poly(A) site choice. Random-sequence RNAs show higher cell-to-cell variability than yeast mRNAs, suggesting that functional elements limit variability. These observations indicate that transcriptional noise occurs at high levels in yeast, and they provide insight into how chromatin and transcription patterns arise from the evolved yeast genome.

Beyond rRNA: nucleolar transcription generates a complex network of RNAs with multiple roles in maintaining cellular homeostasis

Nucleoli are the major cellular compartments for the synthesis of rRNA and assembly of ribosomes, the macromolecular complexes responsible for protein synthesis. Given the abundance of ribosomes, there is a huge demand for rRNA, which indeed constitutes ∼80% of the mass of RNA in the cell. Thus, nucleoli are characterized by extensive transcription of multiple rDNA loci by the dedicated polymerase, RNA polymerase (Pol) I. However, in addition to producing rRNAs, there is considerable additional transcription in nucleoli by RNA Pol II as well as Pol I, producing multiple noncoding (nc) and, in one instance, coding RNAs. In this review, we discuss important features of these transcripts, which often appear species-specific and reflect transcription antisense to pre-rRNA by Pol II and within the intergenic spacer regions on both strands by both Pol I and Pol II. We discuss how expression of these RNAs is regulated, their propensity to form cotranscriptional R loops, and how they modulate rRNA transcription, nucleolar structure, and cellular homeostasis more generally.

The interaction between the Spt6-tSH2 domain and Rpb1 affects multiple functions of RNA Polymerase II

The conserved transcription elongation factor Spt6 makes several contacts with the RNA Polymerase II (RNAPII) complex, including a high-affinity interaction between the Spt6 tandem SH2 domain (Spt6-tSH2) and phosphorylated residues of the Rpb1 subunit in the linker between the catalytic core and the C-terminal domain (CTD) heptad repeats. This interaction contributes to generic localization of Spt6, but we show here that it also has gene-specific roles. Disrupting the interface affected transcription start site selection at a subset of genes whose expression is regulated by this choice, and this was accompanied by changes in a distinct pattern of Spt6 accumulation at these sites. Splicing efficiency was also diminished, as was apparent progression through introns that encode snoRNAs. Chromatin-mediated repression was impaired, and a distinct role in maintaining +1 nucleosomes was identified, especially at ribosomal protein genes. The Spt6-tSH2:Rpb1 interface therefore has both genome-wide functions and local roles at subsets of genes where dynamic decisions regarding initiation, transcript processing, or termination are made. We propose that the interaction modulates the availability or activity of the core elongation and histone chaperone functions of Spt6, contributing to coordination between RNAPII and its accessory factors as varying local conditions call for dynamic responses.

Functional interaction between the RNA exosome and the sirtuin deacetylase Hst3 maintains transcriptional homeostasis

In this study, Bryll et al. found that inactivation of the RNA exosome leads to global reduction of nascent mRNA transcripts, and that this defect is accentuated by loss of deposition of histone variant H2A.Z. They identify the mRNA for the sirtuin deacetylase Hst3 as a key target for the RNA exosome that mediates communication between RNA degradation and transcription machineries.

Eukaryotic cells maintain an optimal level of mRNAs through unknown mechanisms that balance RNA synthesis and degradation. We found that inactivation of the RNA exosome leads to global reduction of nascent mRNA transcripts, and that this defect is accentuated by loss of deposition of histone variant H2A.Z. We identify the mRNA for the sirtuin deacetylase Hst3 as a key target for the RNA exosome that mediates communication between RNA degradation and transcription machineries. These findings reveal how the RNA exosome and H2A.Z function together to control a deacetylase, ensuring proper levels of transcription in response to changes in RNA degradation.

Nrd1p identifies aberrant and natural exosomal target messages during the nuclear mRNA surveillance in Saccharomyces cerevisiae

Nuclear degradation of aberrant mRNAs in Saccharomyces cerevisiae is accomplished by the nuclear exosome and its cofactors TRAMP/CTEXT. Evidence from this investigation establishes a universal role of the Nrd1p-Nab3p-Sen1p (NNS) complex in the nuclear decay of all categories of aberrant mRNAs. In agreement with this, both nrd1-1 and nrd1-2 mutations impaired the decay of all classes of aberrant messages. This phenotype is similar to that displayed by GAL::RRP41 and rrp6-Δ mutant yeast strains. Remarkably, however, nrd1ΔCID mutation (lacking the C-terminal domain required for interaction of Nrd1p with RNAPII) only diminished the decay of aberrant messages with defects occurring during the early stage of mRNP biogenesis, without affecting other messages with defects generated later in the process. Co-transcriptional recruitment of Nrd1p on the aberrant mRNAs was vital for their concomitant decay. Strikingly, this recruitment on to mRNAs defective in the early phases of biogenesis is solely dependent upon RNAPII. In contrast, Nrd1p recruitment onto export-defective transcripts with defects occurring in the later stage of biogenesis is independent of RNAPII and dependent on the CF1A component, Pcf11p, which explains the observed characteristic phenotype of nrd1ΔCID mutation. Consistently, pcf11-2 mutation displayed a selective impairment in the degradation of only the export-defective messages.

Altered rRNA processing disrupts nuclear RNA homeostasis via competition for the poly(A)-binding protein Nab2

RNA-binding proteins (RBPs) are key mediators of RNA metabolism. Whereas some RBPs exhibit narrow transcript specificity, others function broadly across both coding and non-coding RNAs. Here, in Saccharomyces cerevisiae, we demonstrate that changes in RBP availability caused by disruptions to distinct cellular processes promote a common global breakdown in RNA metabolism and nuclear RNA homeostasis. Our data shows that stabilization of aberrant ribosomal RNA (rRNA) precursors in an enp1-1 mutant causes phenotypes similar to RNA exosome mutants due to nucleolar sequestration of the poly(A)-binding protein (PABP) Nab2. Decreased nuclear PABP availability is accompanied by genome-wide changes in RNA metabolism, including increased pervasive transcripts levels and snoRNA processing defects. These phenotypes are mitigated by overexpression of PABPs, inhibition of rDNA transcription, or alterations in TRAMP activity. Our results highlight the need for cells to maintain poly(A)-RNA levels in balance with PABPs and other RBPs with mutable substrate specificity across nucleoplasmic and nucleolar RNA processes.

A crucial RNA-binding lysine residue in the Nab3 RRM domain undergoes SET1 and SET3-responsive methylation

The Nrd1-Nab3-Sen1 (NNS) complex integrates molecular cues to direct termination of noncoding transcription in budding yeast. NNS is positively regulated by histone methylation as well as through Nrd1 binding to the initiating form of RNA PolII. These cues collaborate with Nrd1 and Nab3 binding to target RNA sequences in nascent transcripts through their RRM RNA recognition motifs. In this study, we identify nine lysine residues distributed amongst Nrd1, Nab3 and Sen1 that are methylated, suggesting novel molecular inputs for NNS regulation. We identify mono-methylation of one these residues (Nab3-K363me1) as being partly dependent on the H3K4 methyltransferase, Set1, a known regulator of NNS function. Moreover, the accumulation of Nab3-K363me1 is essentially abolished in strains lacking SET3, a SET domain containing protein that is positively regulated by H3K4 methylation. Nab3-K363 resides within its RRM and physically contacts target RNA. Mutation of Nab3-K363 to arginine (Nab3-K363R) decreases RNA binding of the Nab3 RRM in vitro and causes transcription termination defects and slow growth. These findings identify SET3 as a potential contextual regulator of Nab3 function through its role in methylation of Nab3-K363. Consistent with this hypothesis, we report that SET3 exhibits genetic activation of NAB3 that is observed in a sensitized context.

RNA Polymerase II CTD phosphatase Rtr1 fine-tunes transcription termination

RNA Polymerase II (RNAPII) transcription termination is regulated by the phosphorylation status of the C-terminal domain (CTD). The phosphatase Rtr1 has been shown to regulate serine 5 phosphorylation on the CTD; however, its role in the regulation of RNAPII termination has not been explored. As a consequence of RTR1 deletion, interactions within the termination machinery and between the termination machinery and RNAPII were altered as quantified by Disruption-Compensation (DisCo) network analysis. Of note, interactions between RNAPII and the cleavage factor IA (CF1A) subunit Pcf11 were reduced in rtr1Δ, whereas interactions with the CTD and RNA-binding termination factor Nrd1 were increased. Globally, rtr1Δ leads to decreases in numerous noncoding RNAs that are linked to the Nrd1, Nab3 and Sen1 (NNS) -dependent RNAPII termination pathway. Genome-wide analysis of RNAPII and Nrd1 occupancy suggests that loss of RTR1 leads to increased termination at noncoding genes. Additionally, premature RNAPII termination increases globally at protein-coding genes with a decrease in RNAPII occupancy occurring just after the peak of Nrd1 recruitment during early elongation. The effects of rtr1Δ on RNA expression levels were lost following deletion of the exosome subunit Rrp6, which works with the NNS complex to rapidly degrade a number of noncoding RNAs following termination. Overall, these data suggest that Rtr1 restricts the NNS-dependent termination pathway in WT cells to prevent premature termination of mRNAs and ncRNAs. Rtr1 facilitates low-level elongation of noncoding transcripts that impact RNAPII interference thereby shaping the transcriptome.

Many cellular RNAs including those that encode for proteins are produced by the enzyme RNA Polymerase II. In this work, we have defined a new role for the phosphatase Rtr1 in the regulation of RNA Polymerase II progression from the start of transcription to the 3’ end of the gene where the nascent RNA from protein-coding genes is typically cleaved and polyadenylated. Deletion of the gene that encodes RTR1 leads to changes in the interactions between RNA polymerase II and the termination machinery. Rtr1 loss also causes early termination of RNA Polymerase II at many of its target gene types, including protein coding genes and noncoding RNAs. Evidence suggests that the premature termination observed in RTR1 knockout cells occurs through the termination factor and RNA binding protein Nrd1 and its binding partner Nab3. Deletion of RRP6, a known component of the Nrd1-Nab3 termination coupled RNA degradation pathway, is epistatic to RTR1 suggesting that Rrp6 is required to terminate and/or degrade many of the noncoding RNAs that have increased turnover in RTR1 deletion cells. These findings suggest that Rtr1 normally promotes elongation of RNA Polymerase II transcripts through prevention of Nrd1-directed termination.

Termination of non-coding transcription in yeast relies on both an RNA Pol II CTD interaction domain and a CTD-mimicking region in Sen1

Pervasive transcription is a widespread phenomenon leading to the production of a plethora of non‐coding RNAs (ncRNAs) without apparent function. Pervasive transcription poses a threat to proper gene expression that needs to be controlled. In yeast, the highly conserved helicase Sen1 restricts pervasive transcription by inducing termination of non‐coding transcription. However, the mechanisms underlying the specific function of Sen1 at ncRNAs are poorly understood. Here, we identify a motif in an intrinsically disordered region of Sen1 that mimics the phosphorylated carboxy‐terminal domain (CTD) of RNA polymerase II, and structurally characterize its recognition by the CTD‐interacting domain of Nrd1, an RNA‐binding protein that binds specific sequences in ncRNAs. In addition, we show that Sen1‐dependent termination strictly requires CTD recognition by the N‐terminal domain of Sen1. We provide evidence that the Sen1‐CTD interaction does not promote initial Sen1 recruitment, but rather enhances Sen1 capacity to induce the release of paused RNAPII from the DNA. Our results shed light on the network of protein–protein interactions that control termination of non‐coding transcription by Sen1.

Sen1 Is Recruited to Replication Forks via Ctf4 and Mrc1 and Promotes Genome Stability

DNA replication and RNA transcription compete for the same substrate during S phase. Cells have evolved several mechanisms to minimize such conflicts. Here, we identify the mechanism by which the transcription termination helicase Sen1 associates with replisomes. We show that the N terminus of Sen1 is both sufficient and necessary for replisome association and that it binds to the replisome via the components Ctf4 and Mrc1. We generated a separation of function mutant, sen1-3, which abolishes replisome binding without affecting transcription termination. We observe that the sen1-3 mutants show increased genome instability and recombination levels. Moreover, sen1-3 is synthetically defective with mutations in genes involved in RNA metabolism and the S phase checkpoint. RNH1 overexpression suppresses defects in the former, but not the latter. These findings illustrate how Sen1 plays a key function at replication forks during DNA replication to promote fork progression and chromosome stability.

Purification and In Vitro Analysis of the Exosome Cofactors Nrd1-Nab3 and Trf4-Air2

In many eukaryotic organisms from yeast to human, the exosome plays an important role in the control of pervasive transcription and in non-coding RNA (ncRNA) processing and quality control by trimming precursor RNAs and degrading aberrant transcripts. In Saccharomyces cerevisiae this function is enabled by the interaction of the exosome with several cofactors: the Nrd1-Nab3 heterodimer and the Trf4-Air2-Mtr4 (TRAMP4) complex. Nrd1 and Nab3 are RNA binding proteins that recognize specific motifs enriched in the target ncRNAs, whereas TRAMP4 adds polyA tails at the 3' end of transcripts and stimulates RNA degradation by the exosome. This chapter provides protocols for the purification of recombinant forms of these exosome cofactors and for the in vitro assessment of their activity.

Purification of Endogenous Tagged TRAMP4/5 and Exosome Complexes from Yeast and In Vitro Polyadenylation-Exosome Activation Assays

The RNA exosome processes a wide variety of RNA and mediates RNA maturation, quality control and decay. In marked contrast to its high processivity in vivo, the purified exosome exhibits only weak activity on RNA substrates in vitro. Its activity is regulated by several auxiliary proteins, and protein complexes. In budding yeast, the activity of exosome is enhanced by the polyadenylation complex referred to as TRAMP. TRAMP oligoadenylates precursors and aberrant forms of RNAs to promote their trimming or complete degradation by exosomes. This chapter provides protocols for the purification of TRAMP and exosome complexes from yeast and the in vitro evaluation of exosome activation by the TRAMP complex. The protocols can be used for different purposes, such as the assessment of the role of individual subunits, protein domains or particular mutations in TRAMP-exosome RNA processing in vitro.

Antisense transcriptional interference mediates condition-specific gene repression in budding yeast

Pervasive transcription generates many unstable non-coding transcripts in budding yeast. The transcription of such noncoding RNAs, in particular antisense RNAs (asRNAs), has been shown in a few examples to repress the expression of the associated mRNAs. Yet, such mechanism is not known to commonly contribute to the regulation of a given class of genes. Using a mutant context that stabilized pervasive transcripts, we observed that the least expressed mRNAs during the exponential phase were associated with high levels of asRNAs. These asRNAs also overlapped their corresponding gene promoters with a much higher frequency than average. Interrupting antisense transcription of a subset of genes corresponding to quiescence-enriched mRNAs restored their expression. The underlying mechanism acts in cis and involves several chromatin modifiers. Our results convey that transcription interference represses up to 30% of the 590 least expressed genes, which includes 163 genes with quiescence-enriched mRNAs. We also found that pervasive transcripts constitute a higher fraction of the transcriptome in quiescence relative to the exponential phase, consistent with gene expression itself playing an important role to suppress pervasive transcription. Accordingly, the HIS1 asRNA, normally only present in quiescence, is expressed in exponential phase upon HIS1 mRNA transcription interruption.

SCAF4 and SCAF8, mRNA Anti-Terminator Proteins

Accurate regulation of mRNA termination is required for correct gene expression. Here, we describe a role for SCAF4 and SCAF8 as anti-terminators, suppressing the use of early, alternative polyadenylation (polyA) sites. The SCAF4/8 proteins bind the hyper-phosphorylated RNAPII C-terminal repeat domain (CTD) phosphorylated on both Ser2 and Ser5 and are detected at early, alternative polyA sites. Concomitant knockout of human SCAF4 and SCAF8 results in altered polyA selection and subsequent early termination, leading to expression of truncated mRNAs and proteins lacking functional domains and is cell lethal. While SCAF4 and SCAF8 work redundantly to suppress early mRNA termination, they also have independent, non-essential functions. SCAF8 is an RNAPII elongation factor, whereas SCAF4 is required for correct termination at canonical, distal transcription termination sites in the presence of SCAF8. Together, SCAF4 and SCAF8 coordinate the transition between elongation and termination, ensuring correct polyA site selection and RNAPII transcriptional termination in human cells.

Yeast Sirtuin Family Members Maintain Transcription Homeostasis to Ensure Genome Stability

The mammalian sirtuin, SIRT6, is a key tumor suppressor that maintains genome stability and regulates transcription, though how SIRT6 family members control genome stability is unclear. Here, we use multiple genome-wide approaches to demonstrate that the yeast SIRT6 homologs, Hst3 and Hst4, prevent genome instability by tuning levels of both coding and noncoding transcription. While nascent RNAs are elevated in the absence of Hst3 and Hst4, a global impact on steady-state mRNAs is masked by the nuclear exosome, indicating that sirtuins and the exosome provide two levels of regulation to maintain transcription homeostasis. We find that, in the absence of Hst3 and Hst4, increased transcription is associated with excessive DNA-RNA hybrids (R-loops) that appear to lead to new DNA double-strand breaks. Importantly, dissolution of R-loops suppresses the genome instability phenotypes of hst3 hst4 mutants, suggesting that the sirtuins maintain genome stability by acting as a rheostat to prevent promiscuous transcription.

Writing a wrong: Coupled RNA polymerase II transcription and RNA quality control

Processing and maturation of precursor RNA species is coupled to RNA polymerase II transcription. Co-transcriptional RNA processing helps to ensure efficient and proper capping, splicing, and 3' end processing of different RNA species to help ensure quality control of the transcriptome. Many improperly processed transcripts are not exported from the nucleus, are restricted to the site of transcription, and are in some cases degraded, which helps to limit any possibility of aberrant RNA causing harm to cellular health. These critical quality control pathways are regulated by the highly dynamic protein-protein interaction network at the site of transcription. Recent work has further revealed the extent to which the processes of transcription and RNA processing and quality control are integrated, and how critically their coupling relies upon the dynamic protein interactions that take place co-transcriptionally. This review focuses specifically on the intricate balance between 3' end processing and RNA decay during transcription termination. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Processing > 3' End Processing RNA Processing > Splicing Mechanisms RNA Processing > Capping and 5' End Modifications.

RNA Polymerase II CTD Tyrosine 1 Is Required for Efficient Termination by the Nrd1-Nab3-Sen1 Pathway

In Saccharomyces cerevisiae, transcription termination at protein-coding genes is coupled to the cleavage of the nascent transcript, whereas most non-coding RNA transcription relies on a cleavage-independent termination pathway involving Nrd1, Nab3, and Sen1 (NNS). Termination involves RNA polymerase II CTD phosphorylation, but a systematic analysis of the contribution of individual residues would improve our understanding of the role of the CTD in this process. Here we investigated the effect of mutating phosphorylation sites in the CTD on termination. We observed widespread termination defects at protein-coding genes in mutants for Ser2 or Thr4 but rare defects in Tyr1 mutants for this genes class. Instead, mutating Tyr1 led to widespread termination defects at non-coding genes terminating via NNS. Finally, we showed that Tyr1 is important for pausing in the 5' end of genes and that slowing down transcription suppresses termination defects. Our work highlights the importance of Tyr1-mediated pausing in NNS-dependent termination.

The Nuclear RNA Exosome and Its Cofactors

The RNA exosome is a highly conserved ribonuclease endowed with 3'-5' exonuclease and endonuclease activities. The multisubunit complex resides in both the nucleus and the cytoplasm, with varying compositions and activities between the two compartments. While the cytoplasmic exosome functions mostly in mRNA quality control pathways, the nuclear RNA exosome partakes in the 3'-end processing and complete decay of a wide variety of substrates, including virtually all types of noncoding (nc) RNAs. To handle these diverse tasks, the nuclear exosome engages with dedicated cofactors, some of which serve as activators by stimulating decay through oligoA addition and/or RNA helicase activities or, as adaptors, by recruiting RNA substrates through their RNA-binding capacities. Most nuclear exosome cofactors contain the essential RNA helicase Mtr4 (MTR4 in humans). However, apart from Mtr4, nuclear exosome cofactors have undergone significant evolutionary divergence. Here, we summarize biochemical and functional knowledge about the nuclear exosome and exemplify its cofactor variety by discussing the best understood model organisms-the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and human cells.

Noncoding transcription influences the replication initiation program through chromatin regulation

In eukaryotic organisms, replication initiation follows a temporal program. Among the parameters that regulate this program in Saccharomyces cerevisiae, chromatin structure has been at the center of attention without considering the contribution of transcription. Here, we revisit the replication initiation program in the light of widespread genomic noncoding transcription. We find that noncoding RNA transcription termination in the vicinity of autonomously replicating sequences (ARSs) shields replication initiation from transcriptional readthrough. Consistently, high natural nascent transcription correlates with low ARS efficiency and late replication timing. High readthrough transcription is also linked to increased nucleosome occupancy and high levels of H3K36me3. Moreover, forcing ARS readthrough transcription promotes these chromatin features. Finally, replication initiation defects induced by increased transcriptional readthrough are partially rescued in the absence of H3K36 methylation. Altogether, these observations indicate that natural noncoding transcription into ARSs influences replication initiation through chromatin regulation.

H3K4 Methylation Dependent and Independent Chromatin Regulation by JHD2 and SET1 in Budding Yeast

Set1 and Jhd2 regulate the methylation state of histone H3 lysine-4 (H3K4me) through their opposing methyltransferase and demethylase activities in the budding yeast Saccharomyces cerevisiae. H3K4me associates with actively transcribed genes and, like both SET1 and JHD2 themselves, is known to regulate gene expression diversely. It remains unclear, however, if Set1 and Jhd2 act solely through H3K4me. Relevantly, Set1 methylates lysine residues in the kinetochore protein Dam1 while genetic studies of the S. pombe SET1 ortholog suggest the existence of non-H3K4 Set1 targets relevant to gene regulation. We interrogated genetic interactions of JHD2 and SET1 with essential genes involved in varied aspects of the transcription cycle. Our findings implicate JHD2 in genetic inhibition of the histone chaperone complexes Spt16-Pob3 (FACT) and Spt6-Spn1. This targeted screen also revealed that JHD2 inhibits the Nrd1-Nab3-Sen1 (NNS) transcription termination complex. We find that while Jhd2’s impact on these transcription regulatory complexes likely acts via H3K4me, Set1 governs the roles of FACT and NNS through opposing H3K4-dependent and -independent functions. We also identify diametrically opposing consequences for mutation of H3K4 to alanine or arginine, illuminating that caution must be taken in interpreting histone mutation studies. Unlike FACT and NNS, detailed genetic studies suggest an H3K4me-centric mode of Spt6-Spn1 regulation by JHD2 and SET1. Chromatin immunoprecipitation and transcript quantification experiments show that Jhd2 opposes the positioning of a Spt6-deposited nucleosome near the transcription start site of SER3, a Spt6-Spn1 regulated gene, leading to hyper-induction of SER3. In addition to confirming and extending an emerging role for Jhd2 in the control of nucleosome occupancy near transcription start sites, our findings suggest some of the chromatin regulatory functions of Set1 are independent of H3K4 methylation.

Bidirectional promoters exhibit characteristic chromatin modification signature associated with transcription elongation in both sense and antisense directions

Background

In contrast to unidirectional promoters wherein antisense transcription results in short transcripts which are rapidly degraded, bidirectional promoters produce mature transcripts in both sense and antisense orientation. To understand the molecular mechanism of how productive bidirectional transcription is regulated, we focused on delineating the chromatin signature of bidirectional promoters.

Results

We report generation and utility of a reporter system that enables simultaneous scoring of transcriptional activity in opposite directions. Testing of putative bidirectional promoters in this system demonstrates no measurable bias towards any one direction of transcription. We analyzed the NUP26L-PIH1D3 bidirectional gene pair during Retinoic acid mediated differentiation of embryonic carcinoma cells. In their native context, we observed that the chromatin landscape at and around the transcription regulatory region between the pair of bidirectional genes is modulated in concordance with transcriptional activity of each gene in the pair. We then extended this analysis to 974 bidirectional gene pairs in two different cell lines, H1 human embryonic stem cells and CD4 positive T cells using publicly available ChIP-Seq and RNA-Seq data. Bidirectional gene pairs were classified based on the intergenic distance separating the two TSS of the transcripts analyzed as well as the relative expression of each transcript in a bidirectional gene pair. We report that for the entire range of intergenic distance separating bidirectional genes, the expression profile of such genes (symmetric or asymmetric) matches the histone modification profile of marks associated with active transcription initiation and elongation.

Conclusions

We demonstrate unique distribution of histone modification marks that correlate robustly with the transcription status of genes regulated by bidirectional promoters. These findings strongly imply that occurrence of these marks might signal the transcription machinery to drive maturation of antisense transcription from the bidirectional promoters.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4697-7) contains supplementary material, which is available to authorized users.

Long Noncoding RNAs in Yeast Cells and Differentiated Subpopulations of Yeast Colonies and Biofilms

We summarize current knowledge regarding regulatory functions of long noncoding RNAs (lncRNAs) in yeast, with emphasis on lncRNAs identified recently in yeast colonies and biofilms. Potential regulatory functions of these lncRNAs in differentiated cells of domesticated colonies adapted to plentiful conditions versus yeast colony biofilms are discussed. We show that specific cell types differ in their complements of lncRNA, that this complement changes over time in differentiating upper cells, and that these lncRNAs target diverse functional categories of genes in different cell subpopulations and specific colony types.

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.

Surveillance-ready transcription: nuclear RNA decay as a default fate

Eukaryotic cells synthesize enormous quantities of RNA from diverse classes, most of which are subject to extensive processing. These processes are inherently error-prone, and cells have evolved robust quality control mechanisms to selectively remove aberrant transcripts. These surveillance pathways monitor all aspects of nuclear RNA biogenesis, and in addition remove nonfunctional transcripts arising from spurious transcription and a host of non-protein-coding RNAs (ncRNAs). Surprisingly, this is largely accomplished with only a handful of RNA decay enzymes. It has, therefore, been unclear how these factors efficiently distinguish between functional RNAs and huge numbers of diverse transcripts that must be degraded. Here we describe how bona fide transcripts are specifically protected, particularly by 5' and 3' modifications. Conversely, a plethora of factors associated with the nascent transcripts all act to recruit the RNA quality control, surveillance and degradation machinery. We conclude that initiating RNAPII is 'surveillance ready', with degradation being a default fate for all transcripts that lack specific protective features. We further postulate that this promiscuity is a key feature that allowed the proliferation of vast numbers of ncRNAs in eukaryotes, including humans.

High-resolution transcription maps reveal the widespread impact of roadblock termination in yeast

Transcription termination delimits transcription units but also plays important roles in limiting pervasive transcription. We have previously shown that transcription termination occurs when elongating RNA polymerase II (RNAPII) collides with the DNA-bound general transcription factor Reb1. We demonstrate here that many different DNA-binding proteins can induce termination by a similar roadblock (RB) mechanism. We generated high-resolution transcription maps by the direct detection of RNAPII upon nuclear depletion of two essential RB factors or when the canonical termination pathways for coding and non-coding RNAs are defective. We show that RB termination occurs genomewide and functions independently of (and redundantly with) the main transcription termination pathways. We provide evidence that transcriptional readthrough at canonical terminators is a significant source of pervasive transcription, which is controlled to a large extent by RB termination. Finally, we demonstrate the occurrence of RB termination around centromeres and tRNA genes, which we suggest shields these regions from RNAPII to preserve their functional integrity.

MPK1/SLT2 Links Multiple Stress Responses with Gene Expression in Budding Yeast by Phosphorylating Tyr1 of the RNAP II CTD

The RNA polymerase II largest subunit C-terminal domain consists of repeated YSPTSPS heptapeptides. The role of tyrosine-1 (Tyr1) remains incompletely understood, as, for example, mutating all Tyr1 residues to Phe (Y1F) is lethal in vertebrates but a related mutant has only a mild phenotype in S. pombe. Here we show that Y1F substitution in budding yeast resulted in a strong slow-growth phenotype. The Y1F strain was also hypersensitive to several different cellular stresses that involve MAP kinase signaling. These phenotypes were all linked to transcriptional changes, and we also identified genetic and biochemical interactions between Tyr1 and both transcription initiation and termination factors. Further studies uncovered defects related to MAP kinase I (Slt2) pathways, and we provide evidence that Slt2 phosphorylates Tyr1 in vitro and in vivo. Our study has thus identified Slt2 as a Tyr1 kinase, and in doing so provided links between stress response activation and Tyr1 phosphorylation.

ARS2 is a general suppressor of pervasive transcription

Termination of transcription is important for establishing gene punctuation marks. It is also critical for suppressing many of the pervasive transcription events occurring throughout eukaryotic genomes and coupling their RNA products to efficient decay. In human cells, the ARS2 protein has been implicated in such function as its depletion causes transcriptional read-through of selected gene terminators and because it physically interacts with the ribonucleolytic nuclear RNA exosome. Here, we study the role of ARS2 on transcription and RNA metabolism genome wide. We show that ARS2 depletion negatively impacts levels of promoter-proximal RNA polymerase II at protein-coding (pc) genes. Moreover, our results reveal a general role of ARS2 in transcription termination-coupled RNA turnover at short transcription units like snRNA-, replication-dependent histone-, promoter upstream transcript- and enhancer RNA-loci. Depletion of the ARS2 interaction partner ZC3H18 mimics the ARS2 depletion, although to a milder extent, whereas depletion of the exosome core subunit RRP40 only impacts RNA abundance post-transcriptionally. Interestingly, ARS2 is also involved in transcription termination events within first introns of pc genes. Our work therefore establishes ARS2 as a general suppressor of pervasive transcription with the potential to regulate pc gene expression.

Spurious transcription and its impact on cell function

Most RNA polymerases can initiate transcription from diverse DNA template sequences with relatively few outright sequence restraints. Recent reports have demonstrated that failure to subdue the promiscuity of RNA polymerase in vivo can severely impede cell function. This phenomenon appears common to all cell types with undesirable effects ranging from growth inhibition in prokaryotes to cancer in higher organisms. Here we discuss similarities and differences in strategies employed by cells to minimise spurious transcription across life's domains.

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.

Biochemical characterization of the helicase Sen1 provides new insights into the mechanisms of non-coding transcription termination

Pervasive transcription is widespread and needs to be controlled in order to avoid interference with gene expression. In Saccharomyces cerevisiae, the highly conserved helicase Sen1 plays a key role in restricting pervasive transcription by eliciting early termination of non-coding transcription. However, many aspects of the mechanism of termination remain unclear. In this study we characterize the biochemical activities of Sen1 and their role in termination. First, we demonstrate that the helicase domain (HD) is sufficient to dissociate the elongation complex (EC) in vitro. Both full-length Sen1 and its HD can translocate along single-stranded RNA and DNA in the 5΄ to 3΄ direction. Surprisingly, however, we show that Sen1 is a relatively poorly processive enzyme, implying that it must be recruited in close proximity to the RNA polymerase II (RNAPII) for efficient termination. We present evidence that Sen1 can promote forward translocation of stalled polymerases by acting on the nascent transcript. In addition, we find that dissociation of the EC by Sen1 is favoured by the reannealing of the DNA upstream of RNAPII. Taken together, our results provide new clues to understand the mechanism of Sen1-dependent transcription termination and a rationale for the kinetic competition between elongation and termination.

Yeast RNA-Binding Protein Nab3 Regulates Genes Involved in Nitrogen Metabolism

Termination of Saccharomyces cerevisiae RNA polymerase II (Pol II) transcripts occurs through two alternative pathways. Termination of mRNAs is coupled to cleavage and polyadenylation while noncoding transcripts are terminated through the Nrd1-Nab3-Sen1 (NNS) pathway in a process that is linked to RNA degradation by the nuclear exosome. Some mRNA transcripts are also attenuated through premature termination directed by the NNS complex. In this paper we present the results of nuclear depletion of the NNS component Nab3. As expected, many noncoding RNAs fail to terminate properly. In addition, we observe that nitrogen catabolite-repressed genes are upregulated by Nab3 depletion.

The Nrd1-like protein Seb1 coordinates cotranscriptional 3' end processing and polyadenylation site selection

Termination of RNA polymerase II (RNAPII) transcription is associated with RNA 3' end formation. For coding genes, termination is initiated by the cleavage/polyadenylation machinery. In contrast, a majority of noncoding transcription events in Saccharomyces cerevisiae does not rely on RNA cleavage for termination but instead terminates via a pathway that requires the Nrd1-Nab3-Sen1 (NNS) complex. Here we show that the Schizosaccharomyces pombe ortholog of Nrd1, Seb1, does not function in NNS-like termination but promotes polyadenylation site selection of coding and noncoding genes. We found that Seb1 associates with 3' end processing factors, is enriched at the 3' end of genes, and binds RNA motifs downstream from cleavage sites. Importantly, a deficiency in Seb1 resulted in widespread changes in 3' untranslated region (UTR) length as a consequence of increased alternative polyadenylation. Given that Seb1 levels affected the recruitment of conserved 3' end processing factors, our findings indicate that the conserved RNA-binding protein Seb1 cotranscriptionally controls alternative polyadenylation.

Different phosphoisoforms of RNA polymerase II engage the Rtt103 termination factor in a structurally analogous manner

The carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) orchestrates dynamic recruitment of specific cellular machines during different stages of transcription. Signature phosphorylation patterns of Y₁S₂P₃T₄S₅P₆S₇ heptapeptide repeats of the CTD engage specific "readers." Whereas phospho-Ser5 and phospho-Ser2 marks are ubiquitous, phospho-Thr4 is reported to only impact specific genes. Here, we identify a role for phospho-Thr4 in transcription termination at noncoding small nucleolar RNA (snoRNA) genes. Quantitative proteomics reveals an interactome of known readers as well as protein complexes that were not known to rely on Thr4 for association with Pol II. The data indicate a key role for Thr4 in engaging the machinery used for transcription elongation and termination. We focus on Rtt103, a protein that binds phospho-Ser2 and phospho-Thr4 marks and facilitates transcription termination at protein-coding genes. To elucidate how Rtt103 engages two distinct CTD modifications that are differentially enriched at noncoding genes, we relied on NMR analysis of Rtt103 in complex with phospho-Thr4- or phospho-Ser2-bearing CTD peptides. The structural data reveal that Rtt103 interacts with phospho-Thr4 in a manner analogous to its interaction with phospho-Ser2-modified CTD. The same set of hydrogen bonds involving either the oxygen on phospho-Thr4 and the hydroxyl on Ser2, or the phosphate on Ser2 and the Thr4 hydroxyl, can be formed by rotation of an arginine side chain, leaving the intermolecular interface otherwise unperturbed. This economy of design enables Rtt103 to engage Pol II at distinct sets of genes with differentially enriched CTD marks.

Targeting the nuclear RNA exosome: Poly(A) binding proteins enter the stage

Centrally positioned in nuclear RNA metabolism, the exosome deals with virtually all transcript types. This 3'-5' exo- and endo-nucleolytic degradation machine is guided to its RNA targets by adaptor proteins that enable substrate recognition. Recently, the discovery of the 'Poly(A) tail exosome targeting (PAXT)' connection as an exosome adaptor to human nuclear polyadenylated transcripts has relighted the interest of poly(A) binding proteins (PABPs) in both RNA productive and destructive processes.

The code and beyond: transcription regulation by the RNA polymerase II carboxy-terminal domain

The carboxy-terminal domain (CTD) extends from the largest subunit of RNA polymerase II (Pol II) as a long, repetitive and largely unstructured polypeptide chain. Throughout the transcription process, the CTD is dynamically modified by post-translational modifications, many of which facilitate or hinder the recruitment of key regulatory factors of Pol II that collectively constitute the 'CTD code'. Recent studies have revealed how the physicochemical properties of the CTD promote phase separation in the presence of other low-complexity domains. Here, we discuss the intricacies of the CTD code and how the newly characterized physicochemical properties of the CTD expand the function of the CTD beyond the code.

Nuclear RNA Decay Pathways Aid Rapid Remodeling of Gene Expression in Yeast

In budding yeast, the nuclear RNA surveillance system is active on all pre-mRNA transcripts and modulated by nutrient availability. To test the role of nuclear surveillance in reprogramming gene expression, we identified transcriptome-wide binding sites for RNA polymerase II and the exosome cofactors Mtr4 (TRAMP complex) and Nab3 (NNS complex) by UV crosslinking immediately following glucose withdrawal (0, 4, and 8 min). In glucose, mRNA binding by Nab3 and Mtr4 was mainly restricted to promoter-proximal sites, reflecting early transcription termination. Following glucose withdrawal, many growth-related mRNAs showed reduced transcription but increased Nab3 binding, accompanied by downstream recruitment of Mtr4, and oligo(A) tailing. We conclude that transcription termination is followed by TRAMP-mediated RNA decay. Upregulated transcripts evaded increased surveillance factor binding following glucose withdrawal. Some upregulated genes showed use of alternative transcription starts to bypass strong NNS binding sites. We conclude that nuclear surveillance pathways regulate both positive and negative responses to glucose availability.

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Changes in nuclear surveillance factor binding very rapidly follow nutritional shift

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Downregulated genes frequently show strongly increased surveillance factor binding

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Upregulated genes are protected against elevated surveillance factor binding

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The behavior of functionally related genes indicates posttranscriptional coregulation

Short Synthetic Terminators for Assembly of Transcription Units in Vitro and Stable Chromosomal Integration in Yeast S. cerevisiae

Assembly of synthetic genetic circuits is central to synthetic biology. Yeast S. cerevisiae, in particular, has proven to be an ideal chassis for synthetic genome assemblies by exploiting its efficient homologous recombination. However, this property of efficient homologous recombination poses a problem for multigene assemblies in yeast, since repeated usage of standard parts, such as transcriptional terminators, can lead to rearrangements of the repeats in assembled DNA constructs in vivo. To address this issue in developing a library of orthogonal genetic components for yeast, we designed a set of short synthetic terminators based on a consensus sequence with random linkers to avoid repetitive sequences. We constructed a series of expression vectors with these synthetic terminators for efficient assembly of synthetic genes using Gateway recombination reactions. We also constructed two BAC (bacterial artificial chromosome) vectors for assembling multiple transcription units with the synthetic terminators in vitro and their integration in the yeast genome. The tandem array of synthetic genes integrated in the genome by this method is highly stable because there are few homologous segments in the synthetic constructs. Using this system of assembly and genomic integration of transcription units, we tested the synthetic terminators and their influence on the proximal transcription units. Although all the synthetic terminators have the common consensus with the identical length, they showed different activities and impacts on the neighboring transcription units.

RNA Exosome and Non-coding RNA-Coupled Mechanisms in AID-Mediated Genomic Alterations

The eukaryotic RNA exosome is a well-conserved protein complex with ribonuclease activity implicated in RNA metabolism. Various families of non-coding RNAs have been identified as substrates of the complex, underscoring its role as a non-coding RNA processing/degradation unit. However, the role of RNA exosome and its RNA processing activity on DNA mutagenesis/alteration events have not been investigated until recently. B lymphocytes use two DNA alteration mechanisms, class switch recombination (CSR) and somatic hypermutation (SHM), to re-engineer their antibody gene expressing loci until a tailored antibody gene for a specific antigen is satisfactorily generated. CSR and SHM require the essential activity of the DNA activation-induced cytidine deaminase (AID). Causing collateral damage to the B-cell genome during CSR and SHM, AID induces unwanted (and sometimes oncogenic) mutations at numerous non-immunoglobulin gene sequences. Recent studies have revealed that AID's DNA mutator activity is regulated by the RNA exosome complex, thus providing an example of a mechanism that relates DNA mutagenesis to RNA processing. Here, we review the emergent functions of RNA exosome during CSR, SHM, and other chromosomal alterations in B cells, and discuss implications relevant to mechanisms that maintain B-cell genomic integrity.

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.

Senataxin: Genome Guardian at the Interface of Transcription and Neurodegeneration

R-loops comprise an RNA/DNA hybrid and a displaced single-stranded DNA. They play crucial biological functions and are implicated in neurological diseases, including ataxias, amyotrophic lateral sclerosis, nucleotide expansion disorders (Friedreich ataxia and fragile X syndrome), and cancer. Currently, it is unclear which mechanisms cause R-loop structures to become pathogenic. The RNA/DNA helicase senataxin (SETX) is one of the best characterised R-loop-binding factors in vivo. Mutations in SETX are linked to two neurodegenerative disorders: ataxia with oculomotor apraxia type 2 (AOA2) and amyotrophic lateral sclerosis type 4 (ALS4). SETX is known to play a role in transcription, neurogenesis, and antiviral response. Here, we review the causes of R-loop dysregulation in neurodegenerative diseases and how these structures contribute to pathomechanisms. We will discuss the importance of SETX as a genome guardian in suppressing aberrant R-loop formation and analyse how SETX mutations can lead to neurodegeneration in AOA2/ALS4. Finally, we will discuss the implications for other R-loop-associated neurodegenerative diseases and point to future therapeutic approaches to treat these disorders.

Backmasking in the yeast genome: encoding overlapping information for protein-coding and RNA degradation

Backmasking is a recording technique used to hide a sound or message in a music track in reverse, meaning that it is only audible when the record is played backwards. Analogously, the compact yeast genome encodes for diverse sources of information such as overlapping coding and non-coding transcripts, and protein-binding sites on the two complementary DNA strands. Examples are the consensus binding site sequences of the RNA-binding proteins Nrd1 and Nab3 that target non-coding transcripts for degradation. Here, by examining the overlap of stable (SUTs, stable unannotated transcripts) and unstable (CUTs, cryptic unstable transcripts) transcripts with protein-coding genes, we show that the predicted Nrd1 and Nab3-binding site sequences occur at differing frequencies. They are always depleted in the sense direction of protein-coding genes, thus avoiding degradation of the transcript. However in the antisense direction, predicted binding sites occur at high frequencies in genes with overlapping unstable ncRNAs (CUTs), so limiting the availability of non-functional transcripts. In contrast they are depleted in genes with overlapping stable ncRNAs (SUTs), presumably to avoid degrading the non-coding transcript. The protein-coding genes maintain similar amino-acid contents, but they display distinct codon usages so that Nrd1 and Nab3-binding sites can arise at differing frequencies in antisense depending on the overlapping transcript type. Our study demonstrates how yeast has evolved to encode multiple layers of information-protein-coding genes in one strand and the relative chance of degrading antisense RNA in the other strand-in the same regions of a compact genome.

Evidence for Regulation of ECM3 Expression by Methylation of Histone H3 Lysine 4 and Intergenic Transcription in Saccharomyces cerevisiae

Transcription of nonprotein-coding DNA is widespread in eukaryotes and plays important regulatory roles for many genes, including genes that are misregulated in cancer cells. Its pervasiveness presents the potential for a wealth of diverse regulatory roles for noncoding transcription. We previously showed that the act of transcribing noncoding DNA (ncDNA) across the promoter of the protein-coding SER3 gene in Saccharomyces cerevisiae positions nucleosomes over the upstream activating sequences, leading to strong repression of SER3 transcription. To explore the possibility of other regulatory roles for ncDNA transcription, we selected six candidate S. cerevisiae genes that express ncRNAs over their promoters and analyzed the regulation of one of these genes, ECM3, in detail. Because noncoding transcription can lead to changes in the local chromatin landscape that impinge on the expression of nearby coding genes, we surveyed the effects of various chromatin regulators on the expression of ECM3. These analyses identified roles for the Paf1 complex in positively regulating ECM3 transcription through methylation of histone H3 at lysine 4 (K4) and for Paf1 in controlling the pattern of intergenic transcription at this locus. By deleting a putative promoter for the noncoding transcription unit that lies upstream of ECM3, we provide evidence for a positive correlation between intergenic transcription and ECM3 expression. Our results are consistent with a model in which cotranscriptional methylation of histone H3 K4, mediated by the Paf1 complex and noncoding transcription, leads to activation of ECM3 transcription.

Fail-safe transcription termination: Because one is never enough

Termination of RNA polymerase II (RNAPII) transcription is a fundamental step of gene expression that involves the release of the nascent transcript and dissociation of RNAPII from the DNA template. As transcription termination is intimately linked to RNA 3' end processing, termination pathways have a key decisive influence on the fate of the transcribed RNA. Quite remarkably, when reaching the 3' end of genes, a substantial fraction of RNAPII fail to terminate transcription, requiring the contribution of alternative or "fail-safe" mechanisms of termination to release the polymerase. This point of view covers redundant mechanisms of transcription termination and how they relate to conventional termination models. In particular, we expand on recent findings that propose a reverse torpedo model of termination, in which the 3'5' exonucleolytic activity of the RNA exosome targets transcription events associated with paused and backtracked RNAPII.