Effects of Transcription Elongation Rate and Xrn2 Exonuclease Activity on RNA Polymerase II Termination Suggest Widespread Kinetic Competition

Protein phosphatase PP1 regulation of RNA polymerase II transcription termination and allelic exclusion of VSG genes in trypanosomes

The genomes of Leishmania and trypanosomes are organized into polycistronic transcription units flanked by a modified DNA base J involved in promoting RNA polymerase II (Pol II) termination. We recently characterized a Leishmania complex containing a J-binding protein, PP1 protein phosphatase 1, and PP1 regulatory protein (PNUTS) that controls transcription termination potentially via dephosphorylation of Pol II by PP1. While T. brucei contains eight PP1 isoforms, none purified with the PNUTS complex, complicating the analysis of PP1 function in termination. We now demonstrate that the PP1-binding motif of TbPNUTS is required for function in termination in vivo and that TbPP1-1 modulates Pol II termination in T. brucei and dephosphorylation of the large subunit of Pol II. PP1-1 knock-down results in increased cellular levels of phosphorylated RPB1 accompanied by readthrough transcription and aberrant transcription of the chromosome by Pol II, including Pol I transcribed loci that are typically silent, such as telomeric VSG expression sites involved in antigenic variation. These results provide important insights into the mechanism underlying Pol II transcription termination in primitive eukaryotes that rely on polycistronic transcription and maintain allelic exclusion of VSG genes.

Chromatin regulates alternative polyadenylation via the RNA polymerase II elongation rate

The RNA polymerase II (Pol II) elongation rate influences poly(A) site selection, with slow and fast Pol II derivatives causing upstream and downstream shifts, respectively, in poly(A) site utilization. In yeast, depletion of either of the histone chaperones FACT or Spt6 causes an upstream shift of poly(A) site use that strongly resembles the poly(A) profiles of slow Pol II mutant strains. Like slow Pol II mutant strains, FACT- and Spt6-depleted cells exhibit Pol II processivity defects, indicating that both Spt6 and FACT stimulate the Pol II elongation rate. Poly(A) profiles of some genes show atypical downstream shifts; this subset of genes overlaps well for FACT- or Spt6-depleted strains but is different from the atypical genes in Pol II speed mutant strains. In contrast, depletion of histone H3 or H4 causes a downstream shift of poly(A) sites for most genes, indicating that nucleosomes inhibit the Pol II elongation rate in vivo. Thus, chromatin-based control of the Pol II elongation rate is a potential mechanism, distinct from direct effects on the cleavage/polyadenylation machinery, to regulate alternative polyadenylation in response to genetic or environmental changes.

KAP1 negatively regulates RNA polymerase II elongation kinetics to activate signal-induced transcription

Signal-induced transcriptional programs regulate critical biological processes through the precise spatiotemporal activation of Immediate Early Genes (IEGs); however, the mechanisms of transcription induction remain poorly understood. By combining an acute depletion system with high resolution genomics approaches to interrogate synchronized, temporal transcription, we reveal that KAP1/TRIM28 is a first responder that fulfills the temporal and heightened transcriptional demand of IEGs. Unexpectedly, acute KAP1 loss triggers an increase in RNA polymerase II elongation kinetics during early stimulation time points. This elongation defect derails the normal progression through the transcriptional cycle during late stimulation time points, ultimately leading to decreased recruitment of the transcription apparatus for re-initiation thereby dampening IEGs transcriptional output. Collectively, KAP1 plays a counterintuitive role by negatively regulating transcription elongation to support full activation across multiple transcription cycles of genes critical for cell physiology and organismal functions.

A concerted increase in readthrough and intron retention drives transposon expression during aging and senescence

Aging and senescence are characterized by pervasive transcriptional dysfunction, including increased expression of transposons and introns. Our aim was to elucidate mechanisms behind this increased expression. Most transposons are found within genes and introns, with a large minority being close to genes. This raises the possibility that transcriptional readthrough and intron retention are responsible for age-related changes in transposon expression rather than expression of autonomous transposons. To test this, we compiled public RNA-seq datasets from aged human fibroblasts, replicative and drug-induced senescence in human cells, and RNA-seq from aging mice and senescent mouse cells. Indeed, our reanalysis revealed a correlation between transposons expression, intron retention, and transcriptional readthrough across samples and within samples. Both intron retention and readthrough increased with aging or cellular senescence and these transcriptional defects were more pronounced in human samples as compared to those of mice. In support of a causal connection between readthrough and transposon expression, analysis of models showing induced transcriptional readthrough confirmed that they also show elevated transposon expression. Taken together, our data suggest that elevated transposon reads during aging seen in various RNA-seq dataset are concomitant with multiple transcriptional defects. Intron retention and transcriptional readthrough are the most likely explanation for the expression of transposable elements that lack a functional promoter.

Structural basis of exoribonuclease-mediated mRNA transcription termination

Efficient termination is required for robust gene transcription. Eukaryotic organisms use a conserved exoribonuclease-mediated mechanism to terminate the mRNA transcription by RNA polymerase II (Pol II)1-5. Here we report two cryogenic electron microscopy structures of Saccharomyces cerevisiae Pol II pre-termination transcription complexes bound to the 5'-to-3' exoribonuclease Rat1 and its partner Rai1. Our structures show that Rat1 displaces the elongation factor Spt5 to dock at the Pol II stalk domain. Rat1 shields the RNA exit channel of Pol II, guides the nascent RNA towards its active centre and stacks three nucleotides at the 5' terminus of the nascent RNA. The structures further show that Rat1 rotates towards Pol II as it shortens RNA. Our results provide the structural mechanism for the Rat1-mediated termination of mRNA transcription by Pol II in yeast and the exoribonuclease-mediated termination of mRNA transcription in other eukaryotes.

(Alternative) transcription start sites as regulators of RNA processing

Alternative transcription start site usage (ATSS) is a widespread regulatory strategy that enables genes to choose between multiple genomic loci for initiating transcription. This mechanism is tightly controlled during development and is often altered in disease states. In this review, we examine the growing evidence highlighting a role for transcription start sites (TSSs) in the regulation of mRNA isoform selection during and after transcription. We discuss how the choice of transcription initiation sites influences RNA processing and the importance of this crosstalk for cell identity and organism function. We also speculate on possible mechanisms underlying the integration of transcriptional and post-transcriptional processes.

The association of the RSC remodeler complex with chromatin is influenced by the prefoldin-like Bud27 and determines nucleosome positioning and polyadenylation sites usage in Saccharomyces cerevisiae

The tripartite interaction between the chromatin remodeler complex RSC, RNA polymerase subunit Rpb5 and prefoldin-like Bud27 is necessary for proper RNA pol II elongation. Indeed lack of Bud27 alters this association and affects transcription elongation. This work investigates the consequences of lack of Bud27 on the chromatin association of RSC and RNA pol II, and on nucleosome positioning. Our results demonstrate that RSC binds chromatin in gene bodies and lack of Bud27 alters this association, mainly around polyA sites. This alteration impacts chromatin organization and leads to the accumulation of RNA pol II molecules around polyA sites, likely due to pausing or arrest. Our data suggest that RSC is necessary to maintain chromatin organization around those sites, and any alteration of this organization results in the widespread use of alternative polyA sites. Finally, we also find a similar molecular phenotype that occurs upon TOR inhibition with rapamycin, which suggests that alternative polyadenylation observed upon TOR inhibition is likely Bud27-dependent.

Genome-wide kinetic profiling of pre-mRNA 3' end cleavage

Cleavage and polyadenylation is necessary for the formation of mature mRNA molecules. The rate at which this process occurs can determine the temporal availability of mRNA for subsequent function throughout the cell and is likely tightly regulated. Despite advances in high-throughput approaches for global kinetic profiling of RNA maturation, genome-wide 3' end cleavage rates have never been measured. Here, we describe a novel approach to estimate the rates of cleavage, using metabolic labeling of nascent RNA, high-throughput sequencing, and mathematical modeling. Using in silico simulations of nascent RNA-seq data, we show that our approach can accurately and precisely estimate cleavage half-lives for both constitutive and alternative sites. We find that 3' end cleavage is fast on average, with half-lives under a minute, but highly variable across individual sites. Rapid cleavage is promoted by the presence of canonical sequence elements and an increased density of polyadenylation signals near a cleavage site. Finally, we find that cleavage rates are associated with the localization of RNA polymerase II at the end of a gene, and faster cleavage leads to quicker degradation of downstream readthrough RNA. Our findings shed light on the features important for efficient 3' end cleavage and the regulation of transcription termination.

Molecular Basis of XRN2-Deficient Cancer Cell Sensitivity to Poly(ADP-ribose) Polymerase Inhibition

R-loops (RNA-DNA hybrids with displaced single-stranded DNA) have emerged as a potent source of DNA damage and genomic instability. The termination of defective RNA polymerase II (RNAPII) is one of the major sources of R-loop formation. 5'-3'-exoribonuclease 2 (XRN2) promotes genome-wide efficient RNAPII termination, and XRN2-deficient cells exhibit increased DNA damage emanating from elevated R-loops. Recently, we showed that DNA damage instigated by XRN2 depletion in human fibroblast cells resulted in enhanced poly(ADP-ribose) polymerase 1 (PARP1) activity. Additionally, we established a synthetic lethal relationship between XRN2 and PARP1. However, the underlying cellular stress response promoting this synthetic lethality remains elusive. Here, we delineate the molecular consequences leading to the synthetic lethality of XRN2-deficient cancer cells induced by PARP inhibition. We found that XRN2-deficient lung and breast cancer cells display sensitivity to two clinically relevant PARP inhibitors, Rucaparib and Olaparib. At a mechanistic level, PARP inhibition combined with XRN2 deficiency exacerbates R-loop and DNA double-strand break formation in cancer cells. Consistent with our previous findings using several different siRNAs, we also show that XRN2 deficiency in cancer cells hyperactivates PARP1. Furthermore, we observed enhanced replication stress in XRN2-deficient cancer cells treated with PARP inhibitors. Finally, the enhanced stress response instigated by compromised PARP1 catalytic function in XRN2-deficient cells activates caspase-3 to initiate cell death. Collectively, these findings provide mechanistic insights into the sensitivity of XRN2-deficient cancer cells to PARP inhibition and strengthen the underlying translational implications for targeted therapy.

Co-transcriptional folding of the glmS ribozyme enables a rapid response to metabolite

The glmS ribozyme riboswitch, located in the 5' untranslated region of the Bacillus subtilis glmS messenger RNA (mRNA), regulates cell wall biosynthesis through ligand-induced self-cleavage and decay of the glmS mRNA. Although self-cleavage of the refolded glmS ribozyme has been studied extensively, it is not known how early the ribozyme folds and self-cleaves during transcription. Here, we combine single-molecule fluorescence with kinetic modeling to show that self-cleavage can occur during transcription before the ribozyme is fully synthesized. Moreover, co-transcriptional folding of the RNA at a physiological elongation rate allows the ribozyme catalytic core to react without the downstream peripheral stability domain. Dimethyl sulfate footprinting further revealed how slow sequential folding favors formation of the native core structure through fraying of misfolded helices and nucleation of a native pseudoknot. Ribozyme self-cleavage at an early stage of transcription may benefit glmS regulation in B. subtilis, as it exposes the mRNA to exoribonuclease before translation of the open reading frame can begin. Our results emphasize the importance of co-transcriptional folding of RNA tertiary structure for cis-regulation of mRNA stability.

Distinct roles of two SEC scaffold proteins, AFF1 and AFF4, in regulating RNA polymerase II transcription elongation

The super elongation complex (SEC) containing positive transcription elongation factor b plays a critical role in regulating transcription elongation. AFF1 and AFF4, two members of the AF4/FMR2 family, act as central scaffold proteins of SEC and are associated with various human diseases. However, their precise roles in transcriptional control remain unclear. Here, we investigate differences in the genomic distribution patterns of AFF1 and AFF4 around transcription start sites (TSSs). AFF1 mainly binds upstream of the TSS, while AFF4 is enriched downstream of the TSS. Notably, disruption of AFF4 results in slow elongation and early termination in a subset of AFF4-bound active genes, whereas AFF1 deletion leads to fast elongation and transcriptional readthrough in the same subset of genes. Additionally, AFF1 knockdown increases AFF4 levels at chromatin, and vice versa. In summary, these findings demonstrate that AFF1 and AFF4 function antagonistically to regulate RNA polymerase II transcription.

Elongation rate of RNA polymerase II affects pausing patterns across 3' UTRs

Yeast mRNAs are polyadenylated at multiple sites in their 3' untranslated regions (3' UTRs), and poly(A) site usage is regulated by the rate of transcriptional elongation by RNA polymerase II (Pol II). Slow Pol II derivatives favor upstream poly(A) sites, and fast Pol II derivatives favor downstream poly(A) sites. Transcriptional elongation and polyadenylation are linked at the nucleotide level, presumably reflecting Pol II dwell time at each residue that influences the level of polyadenylation. Here, we investigate the effect of Pol II elongation rate on pausing patterns and the relationship between Pol II pause sites and poly(A) sites within 3' UTRs. Mutations that affect Pol II elongation rate alter sequence preferences at pause sites within 3' UTRs, and pausing preferences differ between 3' UTRs and coding regions. In addition, sequences immediately flanking the pause sites show preferences that are largely independent of Pol II speed. In wild-type cells, poly(A) sites are preferentially located < 50 nucleotides upstream from Pol II pause sites, but this spatial relationship is diminished in cells harboring Pol II speed mutants. Based on a random forest classifier, Pol II pause sites are modestly predicted by the distance to poly(A) sites but are better predicted by the chromatin landscape in Pol II speed derivatives. Transcriptional regulatory proteins can influence the relationship between Pol II pausing and polyadenylation but in a manner distinct from Pol II elongation rate derivatives. These results indicate a complex relationship between Pol II pausing and polyadenylation.

The innate immune factor RPRD2/REAF and its role in the Lv2 restriction of HIV

Intracellular innate immunity involves co-evolved antiviral restriction factors that specifically inhibit infecting viruses. Studying these restrictions has increased our understanding of viral replication, host-pathogen interactions, and pathogenesis, and represent potential targets for novel antiviral therapies. Lentiviral restriction 2 (Lv2) was identified as an unmapped early-phase restriction of HIV-2 and later shown to also restrict HIV-1 and simian immunodeficiency virus. The viral determinants of Lv2 susceptibility have been mapped to the envelope and capsid proteins in both HIV-1 and HIV-2, and also viral protein R (Vpr) in HIV-1, and appears dependent on cellular entry mechanism. A genome-wide screen identified several likely contributing host factors including members of the polymerase-associated factor 1 (PAF1) and human silencing hub (HUSH) complexes, and the newly characterized regulation of nuclear pre-mRNA domain containing 2 (RPRD2). Subsequently, RPRD2 (or RNA-associated early-stage antiviral factor) has been shown to be upregulated upon T cell activation, is highly expressed in myeloid cells, binds viral reverse transcripts, and potently restricts HIV-1 infection. RPRD2 is also bound by HIV-1 Vpr and targeted for degradation by the proteasome upon reverse transcription, suggesting RPRD2 impedes reverse transcription and Vpr targeting overcomes this block. RPRD2 is mainly localized to the nucleus and binds RNA, DNA, and DNA:RNA hybrids. More recently, RPRD2 has been shown to negatively regulate genome-wide transcription and interact with the HUSH and PAF1 complexes which repress HIV transcription and are implicated in maintenance of HIV latency. In this review, we examine Lv2 restriction and the antiviral role of RPRD2 and consider potential mechanism(s) of action.

Protein Phosphatase PP1 Regulation of Pol II Phosphorylation is Linked to Transcription Termination and Allelic Exclusion of VSG Genes and TERRA in Trypanosomes

The genomes of Leishmania and trypanosomes are organized into polycistronic transcription units flanked by a modified DNA base J involved in promoting RNA polymerase II (Pol II) termination. We recently characterized a Leishmania complex containing a J-binding protein, PP1 protein phosphatase 1, and PP1 regulatory protein (PNUTS) that controls transcription termination potentially via dephosphorylation of Pol II by PP1. While T. brucei contains eight PP1 isoforms, none purified with the PNUTS complex, suggesting a unique PP1-independent mechanism of termination. We now demonstrate that the PP1-binding motif of TbPNUTS is required for function in termination in vivo and that TbPP1-1 modulates Pol II termination in T. brucei involving dephosphorylation of the C-terminal domain of the large subunit of Pol II. PP1-1 knock-down results in increased cellular levels of phosphorylated large subunit of Pol II accompanied by readthrough transcription and pervasive transcription of the entire genome by Pol II, including Pol I transcribed loci that are typically silent, such as telomeric VSG expression sites involved in antigenic variation and production of TERRA RNA. These results provide important insights into the mechanism underlying Pol II transcription termination in primitive eukaryotes that rely on polycistronic transcription and maintain allelic exclusion of VSG genes.

Merging short and stranded long reads improves transcript assembly

Long-read RNA sequencing has arisen as a counterpart to short-read sequencing, with the potential to capture full-length isoforms, albeit at the cost of lower depth. Yet this potential is not fully realized due to inherent limitations of current long-read assembly methods and underdeveloped approaches to integrate short-read data. Here, we critically compare the existing methods and develop a new integrative approach to characterize a particularly challenging pool of low-abundance long noncoding RNA (lncRNA) transcripts from short- and long-read sequencing in two distinct cell lines. Our analysis reveals severe limitations in each of the sequencing platforms. For short-read assemblies, coverage declines at transcript termini resulting in ambiguous ends, and uneven low coverage results in segmentation of a single transcript into multiple transcripts. Conversely, long-read sequencing libraries lack depth and strand-of-origin information in cDNA-based methods, culminating in erroneous assembly and quantitation of transcripts. We also discover a cDNA synthesis artifact in long-read datasets that markedly impacts the identity and quantitation of assembled transcripts. Towards remediating these problems, we develop a computational pipeline to "strand" long-read cDNA libraries that rectifies inaccurate mapping and assembly of long-read transcripts. Leveraging the strengths of each platform and our computational stranding, we also present and benchmark a hybrid assembly approach that drastically increases the sensitivity and accuracy of full-length transcript assembly on the correct strand and improves detection of biological features of the transcriptome. When applied to a challenging set of under-annotated and cell-type variable lncRNA, our method resolves the segmentation problem of short-read sequencing and the depth problem of long-read sequencing, resulting in the assembly of coherent transcripts with precise 5' and 3' ends. Our workflow can be applied to existing datasets for superior demarcation of transcript ends and refined isoform structure, which can enable better differential gene expression analyses and molecular manipulations of transcripts.

Structural basis of transcription: RNA Polymerase II substrate binding and metal coordination at 3.0 Å using a free-electron laser

Catalysis and translocation of multi-subunit DNA-directed RNA polymerases underlie all cellular mRNA synthesis. RNA polymerase II (Pol II) synthesizes eukaryotic pre-mRNAs from a DNA template strand buried in its active site. Structural details of catalysis at near atomic resolution and precise arrangement of key active site components have been elusive. Here we present the free electron laser (FEL) structure of a matched ATP-bound Pol II, revealing the full active site interaction network at the highest resolution to date, including the trigger loop (TL) in the closed conformation, bonafide occupancy of both site A and B Mg2+, and a putative third (site C) Mg2+ analogous to that described for some DNA polymerases but not observed previously for cellular RNA polymerases. Molecular dynamics (MD) simulations of the structure indicate that the third Mg2+ is coordinated and stabilized at its observed position. TL residues provide half of the substrate binding pocket while multiple TL/bridge helix (BH) interactions induce conformational changes that could propel translocation upon substrate hydrolysis. Consistent with TL/BH communication, a FEL structure and MD simulations of the hyperactive Rpb1 T834P bridge helix mutant reveals rearrangement of some active site interactions supporting potential plasticity in active site function and long-distance effects on both the width of the central channel and TL conformation, likely underlying its increased elongation rate at the expense of fidelity.

DNA-directed termination of RNA polymerase II transcription

RNA polymerase II (RNAPII) transcription involves initiation from a promoter, transcriptional elongation through the gene, and termination in the terminator region. In bacteria, terminators often contain specific DNA elements provoking polymerase dissociation, but RNAPII transcription termination is thought to be driven entirely by protein co-factors. We used biochemical reconstitution, single-molecule studies, and genome-wide analysis in yeast to study RNAPII termination. Transcription into natural terminators by pure RNAPII results in spontaneous termination at specific sequences containing T-tracts. Single-molecule analysis indicates that termination involves pausing without backtracking. The "torpedo" Rat1-Rai1 exonuclease (XRN2 in humans) greatly stimulates spontaneous termination but is ineffectual on other paused RNAPIIs. By contrast, elongation factor Spt4-Spt5 (DSIF) suppresses termination. Genome-wide analysis further indicates that termination occurs by transcript cleavage at the poly(A) site exposing a new 5' RNA-end that allows Rat1-Rai1 loading, which then catches up with destabilized RNAPII at specific termination sites to end transcription.

High-sensitive nascent transcript sequencing reveals BRD4-specific control of widespread enhancer and target gene transcription

Gene transcription by RNA polymerase II (Pol II) is under control of promoters and distal regulatory elements known as enhancers. Enhancers are themselves transcribed by Pol II correlating with their activity. How enhancer transcription is regulated and coordinated with transcription at target genes has remained unclear. Here, we developed a high-sensitive native elongating transcript sequencing approach, called HiS-NET-seq, to provide an extended high-resolution view on transcription, especially at lowly transcribed regions such as enhancers. HiS-NET-seq uncovers new transcribed enhancers in human cells. A multi-omics analysis shows that genome-wide enhancer transcription depends on the BET family protein BRD4. Specifically, BRD4 co-localizes to enhancer and promoter-proximal gene regions, and is required for elongation activation at enhancers and their genes. BRD4 keeps a set of enhancers and genes in proximity through long-range contacts. From these studies BRD4 emerges as a general regulator of enhancer transcription that may link transcription at enhancers and genes.

Structural basis of archaeal FttA-dependent transcription termination

The ribonuclease FttA mediates factor-dependent transcription termination in archaea 1-3 . Here, we report the structure of a Thermococcus kodakarensis transcription pre-termination complex comprising FttA, Spt4, Spt5, and a transcription elongation complex (TEC). The structure shows that FttA interacts with the TEC in a manner that enables RNA to proceed directly from the TEC RNA-exit channel to the FttA catalytic center and that enables endonucleolytic cleavage of RNA by FttA, followed by 5'→3' exonucleolytic cleavage of RNA by FttA and concomitant 5'→3' translocation of FttA on RNA, to apply mechanical force to the TEC and trigger termination. The structure further reveals that Spt5 bridges FttA and the TEC, explaining how Spt5 stimulates FttA-dependent termination. The results reveal functional analogy between bacterial and archaeal factor-dependent termination, reveal functional homology between archaeal and eukaryotic factor-dependent termination, and reveal fundamental mechanistic similarities in factor-dependent termination in the three domains of life: bacterial, archaeal, and eukaryotic.

One sentence summary

Cryo-EM reveals the structure of the archaeal FttA pre-termination complex.

Elevated pre-mRNA 3' end processing activity in cancer cells renders vulnerability to inhibition of cleavage and polyadenylation

Cleavage and polyadenylation (CPA) is responsible for 3' end processing of eukaryotic poly(A)+ RNAs and preludes transcriptional termination. JTE-607, which targets CPSF-73, is the first known CPA inhibitor (CPAi) in mammalian cells. Here we show that JTE-607 perturbs gene expression through both transcriptional readthrough and alternative polyadenylation (APA). Sensitive genes are associated with features similar to those previously identified for PCF11 knockdown, underscoring a unified transcriptomic signature of CPAi. The degree of inhibition of an APA site by JTE-607 correlates with its usage level and, consistently, cells with elevated CPA activities, such as those with induced overexpression of FIP1, display greater transcriptomic disturbances when treated with JTE-607. Moreover, JTE-607 causes S phase crisis and is hence synergistic with inhibitors of DNA damage repair pathways. Together, our data reveal CPA activity and proliferation rate as determinants of CPAi-mediated cell death, raising the possibility of using CPAi as an adjunct therapy to suppress certain cancers.

Knockout of protein phosphatase 1 in Leishmania major reveals its role during RNA polymerase II transcription termination

The genomes of kinetoplastids are organized into polycistronic transcription units that are flanked by a modified DNA base (base J, beta-D-glucosyl-hydroxymethyluracil). Previous work established a role of base J in promoting RNA polymerase II (Pol II) termination in Leishmania major and Trypanosoma brucei. We recently identified a PJW/PP1 complex in Leishmania containing a J-binding protein (JBP3), PP1 phosphatase 1, PP1 interactive-regulatory protein (PNUTS) and Wdr82. Analyses suggested the complex regulates transcription termination by recruitment to termination sites via JBP3-base J interactions and dephosphorylation of proteins, including Pol II, by PP1. However, we never addressed the role of PP1, the sole catalytic component, in Pol II transcription termination. We now demonstrate that deletion of the PP1 component of the PJW/PP1 complex in L. major, PP1-8e, leads to readthrough transcription at the 3'-end of polycistronic gene arrays. We show PP1-8e has in vitro phosphatase activity that is lost upon mutation of a key catalytic residue and associates with PNUTS via the conserved RVxF motif. Additionally, purified PJW complex with associated PP1-8e, but not complex lacking PP1-8e, led to dephosphorylation of Pol II, suggesting a direct role of PNUTS/PP1 holoenzymes in regulating transcription termination via dephosphorylating Pol II in the nucleus.

A restrictor complex of ZC3H4, WDR82, and ARS2 integrates with PNUTS to control unproductive transcription

The transcriptional termination of unstable non-coding RNAs (ncRNAs) is poorly understood compared to coding transcripts. We recently identified ZC3H4-WDR82 ("restrictor") as restricting human ncRNA transcription, but how it does this is unknown. Here, we show that ZC3H4 additionally associates with ARS2 and the nuclear exosome targeting complex. The domains of ZC3H4 that contact ARS2 and WDR82 are required for ncRNA restriction, suggesting their presence in a functional complex. Consistently, ZC3H4, WDR82, and ARS2 co-transcriptionally control an overlapping population of ncRNAs. ZC3H4 is proximal to the negative elongation factor, PNUTS, which we show enables restrictor function and is required to terminate the transcription of all major RNA polymerase II transcript classes. In contrast to short ncRNAs, longer protein-coding transcription is supported by U1 snRNA, which shields transcripts from restrictor and PNUTS at hundreds of genes. These data provide important insights into the mechanism and control of transcription by restrictor and PNUTS.

The conserved RNA-binding protein Seb1 promotes cotranscriptional ribosomal RNA processing by controlling RNA polymerase I progression

Transcription by RNA polymerase I (RNAPI) represents most of the transcriptional activity in eukaryotic cells and is associated with the production of mature ribosomal RNA (rRNA). As several rRNA maturation steps are coupled to RNAPI transcription, the rate of RNAPI elongation directly influences processing of nascent pre-rRNA, and changes in RNAPI transcription rate can result in alternative rRNA processing pathways in response to growth conditions and stress. However, factors and mechanisms that control RNAPI progression by influencing transcription elongation rate remain poorly understood. We show here that the conserved fission yeast RNA-binding protein Seb1 associates with the RNAPI transcription machinery and promotes RNAPI pausing states along the rDNA. The overall faster progression of RNAPI at the rDNA in Seb1-deficient cells impaired cotranscriptional pre-rRNA processing and the production of mature rRNAs. Given that Seb1 also influences pre-mRNA processing by modulating RNAPII progression, our findings unveil Seb1 as a pause-promoting factor for RNA polymerases I and II to control cotranscriptional RNA processing.

Auxin-inducible degron system: an efficient protein degradation tool to study protein function

Targeted protein degradation, with its rapid protein depletion kinetics, allows the measurement of acute changes in the cell. The auxin-inducible degron (AID) system, rapidly degrades AID-tagged proteins only in the presence of auxin. The AID system being inducible makes the study of essential genes and dynamic processes like cell differentiation, cell cycle and genome organization feasible. The AID degradation system has been adapted to yeast, protozoans, C. elegans, Drosophila, zebrafish, mouse and mammalian cell lines. Using the AID system, researchers have unveiled novel functions for essential proteins at developmental stages that were previously difficult to investigate due to early lethality. This comprehensive review discusses the development, advancements, applications and drawbacks of the AID system and compares it with other available protein degradation systems.

XRN2 suppresses aberrant entry of tRNA trailers into argonaute in humans and Arabidopsis

MicroRNAs (miRNAs) are a well-characterized class of small RNAs (sRNAs) that regulate gene expression post-transcriptionally. miRNAs function within a complex milieu of other sRNAs of similar size and abundance, with the best characterized being tRNA fragments or tRFs. The mechanism by which the RNA-induced silencing complex (RISC) selects for specific sRNAs over others is not entirely understood in human cells. Several highly expressed tRNA trailers (tRF-1s) are strikingly similar to microRNAs in length but are generally excluded from the microRNA effector pathway. This exclusion provides a paradigm for identifying mechanisms of RISC selectivity. Here, we show that 5' to 3' exoribonuclease XRN2 contributes to human RISC selectivity. Although highly abundant, tRF-1s are highly unstable and degraded by XRN2 which blocks tRF-1 accumulation in RISC. We also find that XRN mediated degradation of tRF-1s and subsequent exclusion from RISC is conserved in plants. Our findings reveal a conserved mechanism that prevents aberrant entry of a class of highly produced sRNAs into Ago2.

Plant terminators: the unsung heroes of gene expression

To be properly expressed, genes need to be accompanied by a terminator, a region downstream of the coding sequence that contains the information necessary for the maturation of the mRNA 3' end. The main event in this process is the addition of a poly(A) tail at the 3' end of the new transcript, a critical step in mRNA biology that has important consequences for the expression of genes. Here, we review the mechanism leading to cleavage and polyadenylation of newly transcribed mRNAs and how this process can affect the final levels of gene expression. We give special attention to an aspect often overlooked, the effect that different terminators can have on the expression of genes. We also discuss some exciting findings connecting the choice of terminator to the biogenesis of small RNAs, which are a central part of one of the most important mechanisms of regulation of gene expression in plants.

The super elongation complex drives transcriptional addiction in MYCN-amplified neuroblastoma

MYCN amplification in neuroblastoma leads to aberrant expression of MYCN oncoprotein, which binds active genes promoting transcriptional amplification. Yet, how MYCN coordinates transcription elongation to meet productive transcriptional amplification and which elongation machinery represents MYCN-driven vulnerability remain to be identified. We conducted a targeted screen of transcription elongation factors and identified the super elongation complex (SEC) as a unique vulnerability in MYCN-amplified neuroblastomas. MYCN directly binds EAF1 and recruits SEC to enhance processive transcription elongation. Depletion of EAF1 or AFF1/AFF4, another core subunit of SEC, leads to a global reduction in transcription elongation and elicits selective apoptosis of MYCN-amplified neuroblastoma cells. A combination screen reveals SEC inhibition synergistically potentiates the therapeutic efficacies of FDA-approved BCL-2 antagonist ABT-199, in part due to suppression of MCL1 expression, both in MYCN-amplified neuroblastoma cells and in patient-derived xenografts. These findings identify disruption of the MYCN-SEC regulatory axis as a promising therapeutic strategy in neuroblastoma.

Knowing when to stop: Transcription termination on protein-coding genes by eukaryotic RNAPII

Gene expression is controlled in a dynamic and regulated manner to allow for the consistent and steady expression of some proteins as well as the rapidly changing production of other proteins. Transcription initiation has been a major focus of study because it is highly regulated. However, termination of transcription also plays an important role in controlling gene expression. Transcription termination on protein-coding genes is intimately linked with 3' end cleavage and polyadenylation of transcripts, and it generally results in the production of a mature mRNA that is exported from the nucleus. Termination on many non-coding genes can also result in the production of a mature transcript. Termination is dynamically regulated-premature termination and transcription readthrough occur in response to a number of cellular signals, and these can have varied consequences on gene expression. Here, we review eukaryotic transcription termination by RNA polymerase II (RNAPII), focusing on protein-coding genes.

Mechanisms of eukaryotic transcription termination at a glance

Transcription termination is the final step of a transcription cycle, which induces the release of the transcript at the termination site and allows the recycling of the polymerase for the next round of transcription. Timely transcription termination is critical for avoiding interferences between neighbouring transcription units as well as conflicts between transcribing RNA polymerases (RNAPs) and other DNA-associated processes, such as replication or DNA repair. Understanding the mechanisms by which the very stable transcription elongation complex is dismantled is essential for appreciating how physiological gene expression is maintained and also how concurrent processes that occur synchronously on the DNA are coordinated. Although the strategies employed by the different classes of eukaryotic RNAPs are traditionally considered to be different, novel findings point to interesting commonalities. In this Cell Science at a Glance and the accompanying poster, we review the current understanding about the mechanisms of transcription termination by the three eukaryotic RNAPs.

Emerging roles of BET proteins in transcription and co-transcriptional RNA processing

Transcription by RNA polymerase II (Pol II) gives rise to all nuclear protein-coding and a large set of non-coding RNAs, and is strictly regulated and coordinated with RNA processing. Bromodomain and extraterminal (BET) family proteins including BRD2, BRD3, and BRD4 have been implicated in the regulation of Pol II transcription in mammalian cells. However, only recent technological advances have allowed the analysis of direct functions of individual BET proteins with high precision in cells. These studies shed new light on the molecular mechanisms of transcription control by BET proteins challenging previous longstanding views. The most studied BET protein, BRD4, emerges as a master regulator of transcription elongation with roles also in coupling nascent transcription with RNA processing. In contrast, BRD2 is globally required for the formation of transcriptional boundaries to restrict enhancer activity to nearby genes. Although these recent findings suggest non-redundant functions of BRD4 and BRD2 in Pol II transcription, more research is needed for further clarification. Little is known about the roles of BRD3. Here, we illuminate experimental work that has initially linked BET proteins to Pol II transcription in mammalian cells, outline main methodological breakthroughs that have strongly advanced the understanding of BET protein functions, and discuss emerging roles of individual BET proteins in transcription and transcription-coupled RNA processing. Finally, we propose an updated model for the function of BRD4 in transcription and co-transcriptional RNA maturation. This article is categorized under: RNA Processing > 3' End Processing RNA Processing > Splicing Regulation/Alternative Splicing.

Elongation factor-specific capture of RNA polymerase II complexes

Transcription of protein-coding genes is regulated by dynamic association of co-factors with RNA polymerase II (RNAPII). The function of these factors and their relationship with RNAPII is often poorly understood. Here, we present an approach for elongation-factor-specific mNET capture (ELCAP) of RNAPII complexes for sequencing and mass spectrometry analysis aimed at investigating the function of such RNAPII regulatory proteins. As proof of principle, we apply ELCAP to the RNAPII-associated proteins SCAF4 and SCAF8, which share an essential role as mRNA anti-terminators but have individual roles at the 3' end of genes. Mass spectrometry analysis shows that both SCAF4 and SCAF8 are part of RNAPII elongation complexes containing 3' end processing factors but depleted of splicing components. Importantly, the ELCAP sequencing (ELCAP-seq) profiles of SCAF4- and SCAF8-RNAPII complexes nicely reflect their function as mRNA-anti-terminators and their competing functions at the end of genes, where they prevent or promote transcriptional readthrough.

THOC5 complexes with DDX5, DDX17, and CDK12 to regulate R loop structures and transcription elongation rate

THOC5, a member of the THO complex, is essential for the 3'processing of some inducible genes, the export of a subset of mRNAs and stem cell survival. Here we show that THOC5 depletion results in altered 3'cleavage of >50% of mRNAs and changes in RNA polymerase II binding across genes. THOC5 is recruited close to high-density polymerase II sites, suggesting that THOC5 is involved in transcriptional elongation. Indeed, measurement of elongation rates in vivo demonstrated decreased rates in THOC5-depleted cells. Furthermore, THOC5 is preferentially recruited to its target genes in slow polymerase II cells compared with fast polymerase II cells. Importantly chromatin-associated THOC5 interacts with CDK12 (a modulator of transcription elongation) and RNA helicases DDX5, DDX17, and THOC6 only in slow polymerase II cells. The CDK12/THOC5 interaction promotes CDK12 recruitment to R-loops in a THOC6-dependent manner. These data demonstrate a novel function of THOC5 in transcription elongation.

A cohesin traffic pattern genetically linked to gene regulation

Cohesin-mediated loop extrusion has been shown to be blocked at specific cis-elements, including CTCF sites, producing patterns of loops and domain boundaries along chromosomes. Here we explore such cis-elements, and their role in gene regulation. We find that transcription termination sites of active genes form cohesin- and RNA polymerase II-dependent domain boundaries that do not accumulate cohesin. At these sites, cohesin is first stalled and then rapidly unloaded. Start sites of transcriptionally active genes form cohesin-bound boundaries, as shown before, but are cohesin-independent. Together with cohesin loading, possibly at enhancers, these sites create a pattern of cohesin traffic that guides enhancer-promoter interactions. Disrupting this traffic pattern, by removing CTCF, renders cells sensitive to knockout of genes involved in transcription initiation, such as the SAGA complexes, and RNA processing such DEAD/H-Box RNA helicases. Without CTCF, these factors are less efficiently recruited to active promoters.

The transcriptional terminator XRN2 and the RNA-binding protein Sam68 link alternative polyadenylation to cell cycle progression in prostate cancer

Alternative polyadenylation (APA) yields transcripts differing in their 3'-end, and its regulation is altered in cancer, including prostate cancer. Here we have uncovered a mechanism of APA regulation impinging on the interaction between the exonuclease XRN2 and the RNA-binding protein Sam68, whose increased expression in prostate cancer is promoted by the transcription factor MYC. Genome-wide transcriptome profiling revealed a widespread impact of the Sam68/XRN2 complex on APA. XRN2 promotes recruitment of Sam68 to its target transcripts, where it competes with the cleavage and polyadenylation specificity factor for binding to strong polyadenylation signals at distal ends of genes, thus promoting usage of suboptimal proximal polyadenylation signals. This mechanism leads to 3' untranslated region shortening and translation of transcripts encoding proteins involved in G1/S progression and proliferation. Thus, our findings indicate that the APA program driven by Sam68/XRN2 promotes cell cycle progression and may represent an actionable target for therapeutic intervention.

Xrn2 substrate mapping identifies torpedo loading sites and extensive premature termination of RNA pol II transcription

The exonuclease torpedo Xrn2 loads onto nascent RNA 5'-PO₄ ends and chases down pol II to promote termination downstream from polyA sites. We report that Xrn2 is recruited to preinitiation complexes and "travels" to 3' ends of genes. Mapping of 5'-PO₄ ends in nascent RNA identified Xrn2 loading sites stabilized by an active site mutant, Xrn2(D235A). Xrn2 loading sites are approximately two to 20 bases downstream from where CPSF73 cleaves at polyA sites and histone 3' ends. We propose that processing of all mRNA 3' ends comprises cleavage and limited 5'-3' trimming by CPSF73, followed by handoff to Xrn2. A similar handoff occurs at tRNA 3' ends, where cotranscriptional RNase Z cleavage generates novel Xrn2 substrates. Exonuclease-dead Xrn2 increased transcription in 3' flanking regions by inhibiting polyA site-dependent termination. Surprisingly, the mutant Xrn2 also rescued transcription in promoter-proximal regions to the same extent as in 3' flanking regions. eNET-seq revealed Xrn2-mediated degradation of sense and antisense nascent RNA within a few bases of the TSS, where 5'-PO₄ ends may be generated by decapping or endonucleolytic cleavage. These results suggest that a major fraction of pol II complexes terminates prematurely close to the start site under normal conditions by an Xrn2-mediated torpedo mechanism.

BRD4: a general regulator of transcription elongation

Transcription elongation by RNA polymerase II (Pol II) has emerged as a regulatory hub in gene expression. A key control point occurs during early transcription elongation when Pol II pauses in the promoter-proximal region at the majority of genes in mammalian cells and at a large set of genes in Drosophila. An increasing number of trans-acting factors have been linked to promoter-proximal pausing. Some factors help to establish the pause, whereas others are required for the release of Pol II into productive elongation. A dysfunction of this elongation control point leads to aberrant gene expression and can contribute to disease development. The BET bromodomain protein BRD4 has been implicated in elongation control. However, only recently direct BRD4-specific functions in Pol II transcription elongation have been uncovered. This mainly became possible with technological advances that allow selective and rapid ablation of BRD4 in cells along with the availability of approaches that capture the immediate consequences on nascent transcription. This review sheds light on the experimental breakthroughs that led to the emerging view of BRD4 as a general regulator of transcription elongation.

It's a DoG-eat-DoG world-altered transcriptional mechanisms drive downstream-of-gene (DoG) transcript production

The past decade has revolutionized our understanding of regulatory noncoding RNAs (ncRNAs). Among the most recently identified ncRNAs are downstream-of-gene (DoG)-containing transcripts that are produced by widespread transcriptional readthrough. The discovery of DoGs has set the stage for future studies to address many unanswered questions regarding the mechanisms that promote readthrough transcription, RNA processing, and the cellular functions of the unique transcripts. In this review, we summarize current findings regarding the biogenesis, function, and mechanisms regulating this exciting new class of RNA molecules.

CAPTURE of the Human U2 snRNA Genes Expands the Repertoire of Associated Factors

In order to identify factors involved in transcription of human snRNA genes and 3' end processing of the transcripts, we have carried out CRISPR affinity purification in situ of regulatory elements (CAPTURE), which is deadCas9-mediated pull-down, of the tandemly repeated U2 snRNA genes in human cells. CAPTURE enriched many factors expected to be associated with these human snRNA genes including RNA polymerase II (pol II), Cyclin-Dependent Kinase 7 (CDK7), Negative Elongation Factor (NELF), Suppressor of Ty 5 (SPT5), Mediator 23 (MED23) and several subunits of the Integrator Complex. Suppressor of Ty 6 (SPT6); Cyclin K, the partner of Cyclin-Dependent Kinase 12 (CDK12) and Cyclin-Dependent Kinase 13 (CDK13); and SWI/SNF chromatin remodelling complex-associated SWI/SNF-related, Matrix-associated, Regulator of Chromatin (SMRC) factors were also enriched. Several polyadenylation factors, including Cleavage and Polyadenylation Specificity Factor 1 (CPSF1), Cleavage Stimulation Factors 1 and 2 (CSTF1,and CSTF2) were enriched by U2 gene CAPTURE. We have already shown by chromatin immunoprecipitation (ChIP) that CSTF2-and Pcf11 and Ssu72, which are also polyadenylation factors-are associated with the human U1 and U2 genes. ChIP-seq and ChIP-qPCR confirm the association of SPT6, Cyclin K, and CDK12 with the U2 genes. In addition, knockdown of SPT6 causes loss of subunit 3 of the Integrator Complex (INTS3) from the U2 genes, indicating a functional role in snRNA gene expression. CAPTURE has therefore expanded the repertoire of transcription and RNA processing factors associated with these genes and helped to identify a functional role for SPT6.

Who let the DoGs out? - biogenesis of stress-induced readthrough transcripts

Readthrough transcription caused by inefficient 3'-end cleavage of nascent mRNAs has emerged as a hallmark of the mammalian cellular stress response and results in the production of long noncoding RNAs known as downstream-of-gene (DoG)-containing transcripts. DoGs arise from around 10% of human protein-coding genes and are retained in the nucleus. They are produced minutes after cell exposure to stress and can be detected hours after stress removal. However, their biogenesis and the role(s) that DoGs or their production play in the cellular stress response are incompletely understood. We discuss findings that implicate host and viral proteins in the mechanisms underlying DoG production, as well as the transcriptional landscapes that accompany DoG induction under different stress conditions.

Regulation of the Alternative Neural Transcriptome by ELAV/Hu RNA Binding Proteins

The process of alternative polyadenylation (APA) generates multiple 3' UTR isoforms for a given locus, which can alter regulatory capacity and on occasion change coding potential. APA was initially characterized for a few genes, but in the past decade, has been found to be the rule for metazoan genes. While numerous differences in APA profiles have been catalogued across genetic conditions, perturbations, and diseases, our knowledge of APA mechanisms and biology is far from complete. In this review, we highlight recent findings regarding the role of the conserved ELAV/Hu family of RNA binding proteins (RBPs) in generating the broad landscape of lengthened 3' UTRs that is characteristic of neurons. We relate this to their established roles in alternative splicing, and summarize ongoing directions that will further elucidate the molecular strategies for neural APA, the in vivo functions of ELAV/Hu RBPs, and the phenotypic consequences of these regulatory paradigms in neurons.

Heat shock induces premature transcript termination and reconfigures the human transcriptome

The heat shock (HS) response involves rapid induction of HS genes, whereas transcriptional repression is established more slowly at most other genes. Previous data suggested that such repression results from inhibition of RNA polymerase II (RNAPII) pause release, but here, we show that HS strongly affects other phases of the transcription cycle. Intriguingly, while elongation rates increase upon HS, processivity markedly decreases, so that RNAPII frequently fails to reach the end of genes. Indeed, HS results in widespread premature transcript termination at cryptic, intronic polyadenylation (IPA) sites near gene 5'-ends, likely via inhibition of U1 telescripting. This results in dramatic reconfiguration of the human transcriptome with production of new, previously unannotated, short mRNAs that accumulate in the nucleus. Together, these results shed new light on the basic transcription mechanisms induced by growth at elevated temperature and show that a genome-wide shift toward usage of IPA sites can occur under physiological conditions.

Alternative polyadenylation by sequential activation of distal and proximal PolyA sites

Analogous to alternative splicing, alternative polyadenylation (APA) has long been thought to occur independently at proximal and distal polyA sites. Using fractionation-seq, we unexpectedly identified several hundred APA genes in human cells whose distal polyA isoforms are retained in chromatin/nuclear matrix and whose proximal polyA isoforms are released into the cytoplasm. Global metabolic PAS-seq and Nanopore long-read RNA-sequencing provide further evidence that the strong distal polyA sites are processed first and the resulting transcripts are subsequently anchored in chromatin/nuclear matrix to serve as precursors for further processing at proximal polyA sites. Inserting an autocleavable ribozyme between the proximal and distal polyA sites, coupled with a Cleave-seq approach that we describe here, confirms that the distal polyA isoform is indeed the precursor to the proximal polyA isoform. Therefore, unlike alternative splicing, APA sites are recognized independently, and in many cases, in a sequential manner. This provides a versatile strategy to regulate gene expression in mammalian cells.

Landscape of transcription termination in Arabidopsis revealed by single-molecule nascent RNA sequencing

BACKGROUND

The dynamic process of transcription termination produces transient RNA intermediates that are difficult to distinguish from each other via short-read sequencing methods.

RESULTS

Here, we use single-molecule nascent RNA sequencing to characterize the various forms of transient RNAs during termination at genome-wide scale in wildtype Arabidopsis and in atxrn3, fpa, and met1 mutants. Our data reveal a wide range of termination windows among genes, ranging from ~ 50 nt to over 1000 nt. We also observe efficient termination before downstream tRNA genes, suggesting that chromatin structure around the promoter region of tRNA genes may block pol II elongation. 5' Cleaved readthrough transcription in atxrn3 with delayed termination can run into downstream genes to produce normally spliced and polyadenylated mRNAs in the absence of their own transcription initiation. Consistent with previous reports, we also observe long chimeric transcripts with cryptic splicing in fpa mutant; but loss of CG DNA methylation has no obvious impact on termination in the met1 mutant.

CONCLUSIONS

Our method is applicable to establish a comprehensive termination landscape in a broad range of species.

Integrator enforces the fidelity of transcriptional termination at protein-coding genes

Integrator regulates the 3′-end processing and termination of multiple classes of noncoding RNAs. Depletion of INTS11, the catalytic subunit of Integrator, or ectopic expression of its catalytic dead enzyme impairs the 3′-end processing and termination of a set of protein-coding transcripts termed Integrator-regulated termination (IRT) genes. This defect is manifested by increased RNA polymerase II (RNAPII) readthrough and occupancy of serine-2 phosphorylated RNAPII, de novo trimethylation of lysine-36 on histone H3, and a compensatory elevation of the cleavage and polyadenylation (CPA) complex beyond the canonical polyadenylation sites. 3′ RNA sequencing reveals that proximal polyadenylation site usage relies on the endonuclease activity of INTS11. The DNA sequence encompassing the transcription end sites of IRT genes features downstream polyadenylation motifs and an enrichment of GC content that permits the formation of secondary structures within the 3′UTR. Together, this study identifies a subset of protein-coding transcripts whose 3′ end processing requires the Integrator complex.

Oncogenic dysregulation of pre-mRNA processing by protein kinases: challenges and therapeutic opportunities

Alternative splicing and polyadenylation represent two major steps in pre-mRNA-processing, which ensure proper gene expression and diversification of human transcriptomes. Deregulation of these processes contributes to oncogenic programmes involved in the onset, progression and evolution of human cancers, which often result in the acquisition of resistance to existing therapies. On the other hand, cancer cells frequently increase their transcriptional rate and develop a transcriptional addiction, which imposes a high stress on the pre-mRNA-processing machinery and establishes a therapeutically exploitable vulnerability. A prominent role in fine-tuning pre-mRNA-processing mechanisms is played by three main families of protein kinases: serine arginine protein kinase (SRPK), CDC-like kinase (CLK) and cyclin-dependent kinase (CDK). These kinases phosphorylate the RNA polymerase, splicing factors and regulatory proteins involved in cleavage and polyadenylation of the nascent transcripts. The activity of SRPKs, CLKs and CDKs can be altered in cancer cells, and their inhibition was shown to exert anticancer effects. In this review, we describe key findings that have been reported on these topics and discuss challenges and opportunities of developing therapeutic approaches targeting splicing factor kinases.

β-CASP proteins removing RNA polymerase from DNA: when a torpedo is needed to shoot a sitting duck

During the first step of gene expression, RNA polymerase (RNAP) engages DNA to transcribe RNA, forming highly stable complexes. These complexes need to be dissociated at the end of transcription units or when RNAP stalls during elongation and becomes an obstacle (‘sitting duck’) to further transcription or replication. In this review, we first outline the mechanisms involved in these processes. Then, we explore in detail the torpedo mechanism whereby a 5′–3′ RNA exonuclease (torpedo) latches itself onto the 5′ end of RNA protruding from RNAP, degrades it and upon contact with RNAP, induces dissociation of the complex. This mechanism, originally described in Eukaryotes and executed by Xrn-type 5′–3′ exonucleases, was recently found in Bacteria and Archaea, mediated by β-CASP family exonucleases. We discuss the mechanistic aspects of this process across the three kingdoms of life and conclude that 5′–3′ exoribonucleases (β-CASP and Xrn families) involved in the ancient torpedo mechanism have emerged at least twice during evolution.

Landscape of functional interactions of human processive ribonucleases revealed by high-throughput siRNA screenings

Processive exoribonucleases are executors of RNA decay. In humans, their physical but not functional interactions were thoughtfully investigated. Here we have screened cells deficient in DIS3, XRN2, EXOSC10, DIS3L, and DIS3L2 with a custom siRNA library and determined their genetic interactions (GIs) with diverse pathways of RNA metabolism. We uncovered a complex network of positive interactions that buffer alterations in RNA degradation and reveal reciprocal cooperation with genes involved in transcription, RNA export, and splicing. Further, we evaluated the functional distinctness of nuclear DIS3 and cytoplasmic DIS3L using a library of all known genes associated with RNA metabolism. Our analysis revealed that DIS3 mutation suppresses RNA splicing deficiency, while DIS3L GIs disclose the interplay of cytoplasmic RNA degradation with nuclear RNA processing. Finally, genome-wide DIS3 GI map uncovered relations with genes not directly involved in RNA metabolism, like microtubule organization or regulation of telomerase activity.

Transcription and chromatin-based surveillance mechanism controls suppression of cryptic antisense transcription

Phosphorylation of the RNA polymerase II C-terminal domain Y₁S₂P₃T₄S₅P₆S₇ consensus sequence coordinates key events during transcription, and its deregulation leads to defects in transcription and RNA processing. Here, we report that the histone deacetylase activity of the fission yeast Hos2/Set3 complex plays an important role in suppressing cryptic initiation of antisense transcription when RNA polymerase II phosphorylation is dysregulated due to the loss of Ssu72 phosphatase. Interestingly, although single Hos2 and Set3 mutants have little effect, loss of Hos2 or Set3 combined with ssu72Δ results in a synergistic increase in antisense transcription globally and correlates with elevated sensitivity to genotoxic agents. We demonstrate a key role for the Ssu72/Hos2/Set3 mechanism in the suppression of cryptic antisense transcription at the 3' end of convergent genes that are most susceptible to these defects, ensuring the fidelity of gene expression within dense genomes of simple eukaryotes.

A BRD4-mediated elongation control point primes transcribing RNA polymerase II for 3'-processing and termination

Transcription elongation has emerged as a regulatory hub in gene expression of metazoans. A major control point occurs during early elongation before RNA polymerase II (Pol II) is released into productive elongation. Prior research has linked BRD4 with transcription elongation. Here, we use rapid BET protein and BRD4-selective degradation along with quantitative genome-wide approaches to investigate direct functions of BRD4 in Pol II transcription regulation. Notably, as an immediate consequence of acute BRD4 loss, promoter-proximal pause release is impaired, and transcriptionally engaged Pol II past this checkpoint undergoes readthrough transcription. An integrated proteome-wide analysis uncovers elongation and 3'-RNA processing factors as core BRD4 interactors. BRD4 ablation disrupts the recruitment of general 3'-RNA processing factors at the 5'-control region, which correlates with RNA cleavage and termination defects. These studies, performed in human cells, reveal a BRD4-mediated checkpoint and begin to establish a molecular link between 5'-elongation control and 3'-RNA processing.

RNA polymerase II speed: a key player in controlling and adapting transcriptome composition

RNA polymerase II (RNA Pol II) speed or elongation rate, i.e., the number of nucleotides synthesized per unit of time, is a major determinant of transcriptome composition. It controls co‐transcriptional processes such as splicing, polyadenylation, and transcription termination, thus regulating the production of alternative splice variants, circular RNAs, alternatively polyadenylated transcripts, or read‐through transcripts. RNA Pol II speed itself is regulated in response to intra‐ and extra‐cellular stimuli and can in turn affect the transcriptome composition in response to these stimuli. Evidence points to a potentially important role of transcriptome composition modification through RNA Pol II speed regulation for adaptation of cells to a changing environment, thus pointing to a function of RNA Pol II speed regulation in cellular physiology. Analyzing RNA Pol II speed dynamics may therefore be central to fully understand the regulation of physiological processes, such as the development of multicellular organisms. Recent findings also raise the possibility that RNA Pol II speed deregulation can be detrimental and participate in disease progression. Here, we review initial and current approaches to measure RNA Pol II speed, as well as providing an overview of the factors controlling speed and the co‐transcriptional processes which are affected. Finally, we discuss the role of RNA Pol II speed regulation in cell physiology.