Stable pausing by RNA polymerase II provides an opportunity to target and integrate regulatory signals

Defective transcription elongation in a subset of cancers confers immunotherapy resistance

The nature and role of global transcriptional deregulations in cancers are not fully understood. We report that a large proportion of cancers have widespread defects in mRNA transcription elongation (TE). Cancers with TE defects (TEdeff) display spurious transcription and defective mRNA processing of genes characterized by long genomic length, poised promoters and inducible expression. Signaling pathways regulated by such genes, such as pro-inflammatory response pathways, are consistently suppressed in TEdeff tumors. Remarkably, TEdeff correlates with the poor response and outcome in immunotherapy, but not chemo- or targeted therapy, -treated renal cell carcinoma and metastatic melanoma patients. Forced pharmacologic or genetic induction of TEdeff in tumor cells impairs pro-inflammatory response signaling, and imposes resistance to the innate and adaptive anti-tumor immune responses and checkpoint inhibitor therapy in vivo. Therefore, defective TE is a previously unknown mechanism of tumor immune resistance, and should be assessed in cancer patients undergoing immunotherapy.

Eukaryotic core promoters and the functional basis of transcription initiation

RNA polymerase II (Pol II) core promoters are specialized DNA sequences at transcription start sites of protein-coding and non-coding genes that support the assembly of the transcription machinery and transcription initiation. They enable the highly regulated transcription of genes by selectively integrating regulatory cues from distal enhancers and their associated regulatory proteins. In this Review, we discuss the defining properties of gene core promoters, including their sequence features, chromatin architecture and transcription initiation patterns. We provide an overview of molecular mechanisms underlying the function and regulation of core promoters and their emerging functional diversity, which defines distinct transcription programmes. On the basis of the established properties of gene core promoters, we discuss transcription start sites within enhancers and integrate recent results obtained from dedicated functional assays to propose a functional model of transcription initiation. This model can explain the nature and function of transcription initiation at gene starts and at enhancers and can explain the different roles of core promoters, of Pol II and its associated factors and of the activating cues provided by enhancers and the transcription factors and cofactors they recruit.

Dynamic turnover of paused Pol II complexes at human promoters

Paused RNA polymerase II (Pol II) that piles up near most human promoters is the target of mechanisms that control entry into productive elongation. Whether paused Pol II is a stable or dynamic target remains unresolved. We report that most 5' paused Pol II throughout the genome is turned over within 2 min. This process is revealed under hypertonic conditions that prevent Pol II recruitment to promoters. This turnover requires cell viability but is not prevented by inhibiting transcription elongation, suggesting that it is mediated at the level of termination. When initiation was prevented by triptolide during recovery from high salt, a novel preinitiated state of Pol II lacking the pausing factor Spt5 accumulated at transcription start sites. We propose that Pol II occupancy near 5' ends is governed by a cycle of ongoing assembly of preinitiated complexes that transition to pause sites followed by eviction from the DNA template. This model suggests that mechanisms regulating the transition to productive elongation at pause sites operate on a dynamic population of Pol II that is turning over at rates far higher than previously suspected. We suggest that a plausible alternative to elongation control via escape from a stable pause is by escape from premature termination.

Alternative DNA secondary structure formation affects RNA polymerase II promoter-proximal pausing in human

BACKGROUND

Alternative DNA secondary structures can arise from single-stranded DNA when duplex DNA is unwound during DNA processes such as transcription, resulting in the regulation or perturbation of these processes. We identify sites of high propensity to form stable DNA secondary structure across the human genome using Mfold and ViennaRNA programs with parameters for analyzing DNA.

RESULTS

The promoter-proximal regions of genes with paused transcription are significantly and energetically more favorable to form DNA secondary structure than non-paused genes or genes without RNA polymerase II (Pol II) binding. Using Pol II ChIP-seq, GRO-seq, NET-seq, and mNET-seq data, we arrive at a robust set of criteria for Pol II pausing, independent of annotation, and find that a highly stable secondary structure is likely to form about 10-50 nucleotides upstream of a Pol II pausing site. Structure probing data confirm the existence of DNA secondary structures enriched at the promoter-proximal regions of paused genes in human cells. Using an in vitro transcription assay, we demonstrate that Pol II pausing at HSPA1B, a human heat shock gene, is affected by manipulating DNA secondary structure upstream of the pausing site.

CONCLUSIONS

Our results indicate alternative DNA secondary structure formation as a mechanism for how GC-rich sequences regulate RNA Pol II promoter-proximal pausing genome-wide.

Intact Arabidopsis RPB1 functions in stem cell niches maintenance and cell cycling control

Plant meristem activity depends on accurate execution of transcriptional networks required for establishing optimum functioning of stem cell niches. An Arabidopsis mutant card1-1 (constitutive auxin response with DR5:GFP) that encodes a truncated RPB1 (RNA Polymerase II's largest subunit) with shortened C-terminal domain (CTD) was identified. Phosphorylation of the CTD repeats of RPB1 is coupled to transcription in eukaryotes. Here we uncover that the truncated CTD of RPB1 disturbed cell cycling and enlarged the size of shoot and root meristem. The defects in patterning of root stem cell niche in card1-1 indicates that intact CTD of RPB1 is necessary for fine-tuning the specific expression of genes responsible for cell-fate determination. The gene-edited plants with different CTD length of RPB1, created by CRISPR-CAS9 technology, confirmed that both the full length and the DK-rich tail of RPB1's CTD play roles in the accurate transcription of CYCB1;1 encoding a cell-cycle marker protein in root meristem and hence participate in maintaining root meristem size. Our experiment proves that the intact RPB1 CTD is necessary for stem cell niche maintenance, which is mediated by transcriptional regulation of cell cycling genes.

The HIV-1 Tat protein recruits a ubiquitin ligase to reorganize the 7SK snRNP for transcriptional activation

The HIV-1 Tat protein hijacks P-TEFb kinase to activate paused RNA polymerase II (RNAP II) at the viral promoter. Tat binds additional host factors, but it is unclear how they regulate RNAP II elongation. Here, we identify the cytoplasmic ubiquitin ligase UBE2O as critical for Tat transcriptional activity. Tat hijacks UBE2O to ubiquitinate the P-TEFb kinase inhibitor HEXIM1 of the 7SK snRNP, a fraction of which also resides in the cytoplasm bound to P-TEFb. HEXIM1 ubiquitination sequesters it in the cytoplasm and releases P-TEFb from the inhibitory 7SK complex. Free P-TEFb then becomes enriched in chromatin, a process that is also stimulated by treating cells with a CDK9 inhibitor. Finally, we demonstrate that UBE2O is critical for P-TEFb recruitment to the HIV-1 promoter. Together, the data support a unique model of elongation control where non-degradative ubiquitination of nuclear and cytoplasmic 7SK snRNP pools increases P-TEFb levels for transcriptional activation.

Classification of Transcription Boundary-Associated RNAs (TBARs) in Animals and Plants

There is increasing evidence suggesting the contribution of non-coding RNAs (ncRNAs) to the phenotypic and physiological complexity of organisms. A novel ncRNA species has been identified near the transcription boundaries of protein-coding genes in eukaryotes, bacteria, and archaea. This review provides a detailed description of these transcription boundary-associated RNAs (TBARs), including their classification. Based on their genomic distribution, TBARs are divided into two major groups: promoter-associated RNAs (PARs) and terminus-associated RNAs (TARs). Depending on the sequence length, each group is further classified into long RNA species (>200 nt) and small RNA species (<200 nt). According to these rules of TBAR classification, divergent ncRNAs with confusing nomenclatures, such as promoter upstream transcripts (PROMPTs), upstream antisense RNAs (uaRNAs), stable unannotated transcripts (SUTs), cryptic unstable transcripts (CUTs), upstream non-coding transcripts (UNTs), transcription start site-associated RNAs (TSSaRNAs), transcription initiation RNAs (tiRNAs), and transcription termination site-associated RNAs (TTSaRNAs), were assigned to specific classes. Although the biogenesis pathways of PARs and TARs have not yet been clearly elucidated, previous studies indicate that some of the PARs have originated either through divergent transcription or via RNA polymerase pausing. Intriguing findings regarding the functional implications of the TBARs such as the long-range “gene looping” model, which explains their role in the transcriptional regulation of protein-coding genes, are also discussed. Altogether, this review provides a comprehensive overview of the current research status of TBARs, which will promote further investigations in this research area.

MyD88-dependent dendritic and epithelial cell crosstalk orchestrates immune responses to allergens

Sensitization to inhaled allergens is dependent on activation of conventional dendritic cells (cDCs) and on the adaptor molecule, MyD88. However, many cell types in the lung express Myd88, and it is unclear how signaling in these different cell types reprograms cDCs and leads to allergic inflammation of the airway. By combining ATAC-seq with RNA profiling, we found that MyD88 signaling in cDCs maintained open chromatin at select loci even at steady state, allowing genes to be rapidly induced during allergic sensitization. A distinct set of genes related to metabolism was indirectly controlled in cDCs through MyD88 signaling in airway epithelial cells (ECs). In mouse models of asthma, Myd88 expression in ECs was critical for eosinophilic inflammation, whereas Myd88 expression in cDCs was required for Th17 cell differentiation and consequent airway neutrophilia. Thus, both cell-intrinsic and cell-extrinsic MyD88 signaling controls gene expression in cDCs and orchestrates immune responses to inhaled allergens.

Viral targeting of TFIIB impairs de novo polymerase II recruitment and affects antiviral immunity

Viruses have evolved a plethora of mechanisms to target host antiviral responses. Here, we propose a yet uncharacterized mechanism of immune regulation by the orthomyxovirus Thogoto virus (THOV) ML protein through engaging general transcription factor TFIIB. ML generates a TFIIB depleted nuclear environment by re-localizing it into the cytoplasm. Although a broad effect on gene expression would be anticipated, ML expression, delivery of an ML-derived functional domain or experimental depletion of TFIIB only leads to altered expression of a limited number of genes. Our data indicate that TFIIB is critically important for the de novo recruitment of Pol II to promoter start sites and that TFIIB may not be required for regulated gene expression from paused promoters. Since many immune genes require de novo recruitment of Pol II, targeting of TFIIB by THOV represents a neat mechanism to affect immune responses while keeping other cellular transcriptional activities intact. Thus, interference with TFIIB activity may be a favourable site for therapeutic intervention to control undesirable inflammation.

Viruses target the innate immune system at critical vulnerability points. Here we show that the orthomyxovirus Thogoto virus impairs activity of general transcription factor IIB (TFIIB). Surprisingly, impairment of TFIIB function does not result in a general inhibition of transcription but in a rather specific impairment of selective genes. Transcriptome and functional analyses intersected with published CHIP-Seq datasets suggest that affected genes require de novo recruitment of the polymerase complex. Since the innate immune system heavily relies on genes that require de novo recruitment of the polymerase complex, targeting of TFIIB represents a neat mechanism to broadly affect antiviral immunity. Conversely, therapeutic targeting of TFIIB may represent a mechanism to limit pathological side effects caused by overshooting immune reactions.

Live-cell analysis of endogenous GFP-RPB1 uncovers rapid turnover of initiating and promoter-paused RNA Polymerase II

Transcription by RNA Polymerase II (Pol II) is a highly dynamic process that is tightly regulated at each step of the transcription cycle. We generated GFP-RPB1 knockin cells and developed photobleaching of endogenous Pol II combined with computational modeling to study the in vivo dynamics of Pol II in real time. This approach allowed us to dissect promoter-paused Pol II from initiating and elongating Pol II and showed that initiation and promoter proximal pausing are surprisingly dynamic events, due to premature termination of Pol II. Our study provides new insights into Pol II dynamics and suggests that the iterative release and reinitiation of promoter-bound Pol II is an important component of transcriptional regulation.

Initiation and promoter-proximal pausing are key regulatory steps of RNA Polymerase II (Pol II) transcription. To study the in vivo dynamics of endogenous Pol II during these steps, we generated fully functional GFP-RPB1 knockin cells. GFP-RPB1 photobleaching combined with computational modeling revealed four kinetically distinct Pol II fractions and showed that on average 7% of Pol II are freely diffusing, while 10% are chromatin-bound for 2.4 seconds during initiation, and 23% are promoter-paused for only 42 seconds. This unexpectedly high turnover of Pol II at promoters is most likely caused by premature termination of initiating and promoter-paused Pol II and is in sharp contrast to the 23 minutes that elongating Pol II resides on chromatin. Our live-cell–imaging approach provides insights into Pol II dynamics and suggests that the continuous release and reinitiation of promoter-bound Pol II is an important component of transcriptional regulation.

Differential Occupancy of Two GA-Binding Proteins Promotes Targeting of the Drosophila Dosage Compensation Complex to the Male X Chromosome

Little is known about how variation in sequence composition alters transcription factor occupancy to precisely recruit large transcription complexes. A key model for understanding how transcription complexes are targeted is the Drosophila dosage compensation system in which the male-specific lethal (MSL) transcription complex specifically identifies and regulates the male X chromosome. The chromatin-linked adaptor for MSL proteins (CLAMP) zinc-finger protein targets MSL to the X chromosome but also binds to GA-rich sequence elements throughout the genome. Furthermore, the GAGA-associated factor (GAF) transcription factor also recognizes GA-rich sequences but does not associate with the MSL complex. Here, we demonstrate that MSL complex recruitment sites are optimal CLAMP targets. Specificity for CLAMP binding versus GAF binding is driven by variability in sequence composition within similar GA-rich motifs. Therefore, variation within seemingly similar cis elements drives the context-specific targeting of a large transcription complex.

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.

Transcription start site profiling uncovers divergent transcription and enhancer-associated RNAs in Drosophila melanogaster

Background

High-resolution transcription start site (TSS) mapping in D. melanogaster embryos and cell lines has revealed a rich and detailed landscape of both cis- and trans-regulatory elements and factors. However, TSS profiling has not been investigated in an orthogonal in vivo setting. Here, we present a comprehensive dataset that links TSS dynamics with nucleosome occupancy and gene expression in the wandering third instar larva, a developmental stage characterized by large-scale shifts in transcriptional programs in preparation for metamorphosis.

Results

The data recapitulate major regulatory classes of TSSs, based on peak width, promoter-proximal polymerase pausing, and cis-regulatory element density. We confirm the paucity of divergent transcription units in D. melanogaster, but also identify notable exceptions. Furthermore, we identify thousands of novel initiation events occurring at unannotated TSSs that can be classified into functional categories by their local density of histone modifications. Interestingly, a sub-class of these unannotated TSSs overlaps with functionally validated enhancer elements, consistent with a regulatory role for “enhancer RNAs” (eRNAs) in defining developmental transcription programs.

Conclusions

High-depth TSS mapping is a powerful strategy for identifying and characterizing low-abundance and/or low-stability RNAs. Global analysis of transcription initiation patterns in a developing organism reveals a vast number of novel initiation events that identify potential eRNAs as well as other non-coding transcripts critical for animal development.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4510-7) contains supplementary material, which is available to authorized users.

Multi-step regulation of transcription kinetics explains the non-linear relation between RNA polymerase II density and mRNA expression in dosage compensation

In heterogametic organisms, expression of unequal number of X chromosomes in males and females is balanced by a process called dosage compensation. In Drosophila and mammals, dosage compensation involves nearly two-fold up-regulation of the X chromosome mediated by dosage compensation complex (DCC). Experimental studies on the role of DCC on RNA polymerase II (Pol II) transcription in mammals disclosed a non-linear relationship between Pol II densities at different transcription steps and mRNA expression. An ∼20-30% increase in Pol II densities corresponds to a rough 200% increase in mRNA expression and two-fold up-regulation. Here, using a simple kinetic model of Pol II transcription calibrated by in vivo measured rate constants of different transcription steps in mammalian cells, we demonstrate how this non-linearity can be explained by multi-step transcriptional regulation. Moreover, we show how multi-step enhancement of Pol II transcription can increase mRNA production while leaving Pol II densities unaffected. Our theoretical analysis not only recapitulates experimentally observed Pol II densities upon two-fold up-regulation but also points to erroneous interpretations of Pol II profiles from chromatin immunoprecipitation sequencing (ChIP-seq) or global run-on assays.

Assessing sufficiency and necessity of enhancer activities for gene expression and the mechanisms of transcription activation

Enhancers are important genomic regulatory elements directing cell type-specific transcription. They assume a key role during development and disease, and their identification and functional characterization have long been the focus of scientific interest. The advent of next-generation sequencing and clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-based genome editing has revolutionized the means by which we study enhancer biology. In this review, we cover recent developments in the prediction of enhancers based on chromatin characteristics and their identification by functional reporter assays and endogenous DNA perturbations. We discuss that the two latter approaches provide different and complementary insights, especially in assessing enhancer sufficiency and necessity for transcription activation. Furthermore, we discuss recent insights into mechanistic aspects of enhancer function, including findings about cofactor requirements and the role of post-translational histone modifications such as monomethylation of histone H3 Lys4 (H3K4me1). Finally, we survey how these approaches advance our understanding of transcription regulation with respect to promoter specificity and transcriptional bursting and provide an outlook covering open questions and promising developments.

Widespread transcriptional pausing and elongation control at enhancers

In this study, Henriques et al. demonstrate that transcription is a nearly universal feature of enhancers in Drosophila and mammalian cells and that nascent RNA sequencing strategies are optimal for identification of both enhancers and superenhancers. Their findings provide insights into the unique characteristics of superenhancers, which stimulate high-level gene expression through rapid pause release; interestingly, this property renders associated genes resistant to loss of factors that stabilize paused RNAPII.

Regulation by gene-distal enhancers is critical for cell type-specific and condition-specific patterns of gene expression. Thus, to understand the basis of gene activity in a given cell type or tissue, we must identify the precise locations of enhancers and functionally characterize their behaviors. Here, we demonstrate that transcription is a nearly universal feature of enhancers in Drosophila and mammalian cells and that nascent RNA sequencing strategies are optimal for identification of both enhancers and superenhancers. We dissect the mechanisms governing enhancer transcription and discover remarkable similarities to transcription at protein-coding genes. We show that RNA polymerase II (RNAPII) undergoes regulated pausing and release at enhancers. However, as compared with mRNA genes, RNAPII at enhancers is less stable and more prone to early termination. Furthermore, we found that the level of histone H3 Lys4 (H3K4) methylation at enhancers corresponds to transcriptional activity such that highly active enhancers display H3K4 trimethylation rather than the H3K4 monomethylation considered a hallmark of enhancers. Finally, our work provides insights into the unique characteristics of superenhancers, which stimulate high-level gene expression through rapid pause release; interestingly, this property renders associated genes resistant to the loss of factors that stabilize paused RNAPII.

The Importance of Controlling Transcription Elongation at Coding and Noncoding RNA Loci

Here we discuss current paradigms for how transcription initiation and elongation control are achieved in mammalian cells, and how they differ at protein-coding mRNA genes versus noncoding RNA (ncRNA) loci. We present a model for the function of ncRNAs wherein the act of transcription is regulatory, rather than the ncRNA products themselves. We further describe how the establishment of transcriptionally engaged, but paused, RNA polymerase II impacts chromatin structure around divergent transcription start sites, and how this can influence transcription factor binding and mRNA gene activity in the region.

Global inhibition of transcription causes an increase in histone H2A.Z incorporation within gene bodies

H2A.Z histone variant is an important regulator of gene transcription, which is enriched at regulatory regions but is also found within gene bodies. Recent evidence suggests that active recruitment of H2A.Z within gene bodies is required to induce gene repression. In contrast to this view, we show that global inhibition of transcription results in H2A.Z accumulation at gene transcription start sites, as well as within gene bodies. Our results indicate that accumulation of H2A.Z within repressed genes can also be a consequence of the repression of gene transcription rather than an active mechanism required to establish the repression.

The kinetics of pre-mRNA splicing in the Drosophila genome and the influence of gene architecture

Production of most eukaryotic mRNAs requires splicing of introns from pre-mRNA. The splicing reaction requires definition of splice sites, which are initially recognized in either intron-spanning (‘intron definition’) or exon-spanning (‘exon definition’) pairs. To understand how exon and intron length and splice site recognition mode impact splicing, we measured splicing rates genome-wide in Drosophila, using metabolic labeling/RNA sequencing and new mathematical models to estimate rates. We found that the modal intron length range of 60–70 nt represents a local maximum of splicing rates, but that much longer exon-defined introns are spliced even faster and more accurately. We observed unexpectedly low variation in splicing rates across introns in the same gene, suggesting the presence of gene-level influences, and we identified multiple gene level variables associated with splicing rate. Together our data suggest that developmental and stress response genes may have preferentially evolved exon definition in order to enhance the rate or accuracy of splicing.

RNA elongation by respiratory syncytial virus polymerase is calibrated by conserved region V

The large polymerase subunit (L) of non-segmented negative strand RNA viruses transcribes viral mRNAs and replicates the viral genome. Studies with VSV have shown that conserved region V (CRV) of the L protein is part of the capping domain. However, CRV folds over and protrudes into the polymerization domain, suggesting that it might also have a role in RNA synthesis. In this study, the role of respiratory syncytial virus (RSV) CRV was evaluated using single amino acid substitutions and a small molecule inhibitor called BI-D. Effects were analyzed using cell-based minigenome and in vitro biochemical assays. Several amino acid substitutions inhibited production of capped, full-length mRNA and instead resulted in accumulation of short transcripts of approximately 40 nucleotides in length, confirming that RSV CRV has a role in capping. In addition, all six variants tested were either partially or completely defective in RNA replication. This was due to an inability of the polymerase to efficiently elongate the RNA within the promoter region. BI-D also inhibited transcription and replication. In this case, polymerase elongation activity within the promoter region was enhanced, such that the small RNA transcribed from the promoter was not released and instead was elongated past the first gene start signal. This was accompanied by a decrease in mRNA initiation at the first gene start signal and accumulation of aberrant RNAs of varying length. Thus, in addition to its function in mRNA capping, conserved region V modulates the elongation properties of the polymerase to enable productive transcription and replication to occur.

Oxidative stress rapidly stabilizes promoter-proximal paused Pol II across the human genome

Oxidative stress has pervasive effects on cells but how they respond transcriptionally upon the initial insult is incompletely understood. We developed a nuclear walk-on assay that semi-globally quantifies nascent transcripts in promoter-proximal paused RNA polymerase II (Pol II). Using this assay in conjunction with ChIP-Seq, in vitro transcription, and a chromatin retention assay, we show that within a minute, hydrogen peroxide causes accumulation of Pol II near promoters and enhancers that can best be explained by a rapid decrease in termination. Some of the accumulated polymerases slowly move or ‘creep’ downstream. This second effect is correlated with and probably results from loss of NELF association and function. Notably, both effects were independent of DNA damage and ADP-ribosylation. Our results demonstrate the unexpected speed at which a global transcriptional response can occur. The findings provide strong support for the residence time of paused Pol II elongation complexes being much shorter than estimated from previous studies.

The Battle of RNA Synthesis: Virus versus Host

Transcription control is the foundation of gene regulation. Whereas a cell is fully equipped for this task, viruses often depend on the host to supply tools for their transcription program. Over the course of evolution and adaptation, viruses have found diverse ways to optimally exploit cellular host processes such as transcription to their own benefit. Just as cells are increasingly understood to employ nascent RNAs in transcription regulation, recent discoveries are revealing how viruses use nascent RNAs to benefit their own gene expression. In this review, we first outline the two different transcription programs used by viruses, i.e., transcription (DNA-dependent) and RNA-dependent RNA synthesis. Subsequently, we use the distinct stages (initiation, elongation, termination) to describe the latest insights into nascent RNA-mediated regulation in the context of each relevant stage.

CDK9-dependent RNA polymerase II pausing controls transcription initiation

Gene transcription can be activated by decreasing the duration of RNA polymerase II pausing in the promoter-proximal region, but how this is achieved remains unclear. Here we use a 'multi-omics' approach to demonstrate that the duration of polymerase pausing generally limits the productive frequency of transcription initiation in human cells ('pause-initiation limit'). We further engineer a human cell line to allow for specific and rapid inhibition of the P-TEFb kinase CDK9, which is implicated in polymerase pause release. CDK9 activity decreases the pause duration but also increases the productive initiation frequency. This shows that CDK9 stimulates release of paused polymerase and activates transcription by increasing the number of transcribing polymerases and thus the amount of mRNA synthesized per time. CDK9 activity is also associated with long-range chromatin interactions, suggesting that enhancers can influence the pause-initiation limit to regulate transcription.

Nascent RNA sequencing reveals mechanisms of gene regulation in the human malaria parasite Plasmodium falciparum

Gene expression in Plasmodium falciparum is tightly regulated to ensure successful propagation of the parasite throughout its complex life cycle. The earliest transcriptomics studies in P. falciparum suggested a cascade of transcriptional activity over the course of the 48-hour intraerythrocytic developmental cycle (IDC); however, the just-in-time transcriptional model has recently been challenged by findings that show the importance of post-transcriptional regulation. To further explore the role of transcriptional regulation, we performed the first genome-wide nascent RNA profiling in P. falciparum. Our findings indicate that the majority of genes are transcribed simultaneously during the trophozoite stage of the IDC and that only a small subset of genes is subject to differential transcriptional timing. RNA polymerase II is engaged with promoter regions prior to this transcriptional burst, suggesting that Pol II pausing plays a dominant role in gene regulation. In addition, we found that the overall transcriptional program during gametocyte differentiation is surprisingly similar to the IDC, with the exception of relatively small subsets of genes. Results from this study suggest that further characterization of the molecular players that regulate stage-specific gene expression and Pol II pausing will contribute to our continuous search for novel antimalarial drug targets.

Genome-wide Single-Molecule Footprinting Reveals High RNA Polymerase II Turnover at Paused Promoters

Transcription initiation entails chromatin opening followed by pre-initiation complex formation and RNA polymerase II recruitment. Subsequent polymerase elongation requires additional signals, resulting in increased residence time downstream of the start site, a phenomenon referred to as pausing. Here, we harnessed single-molecule footprinting to quantify distinct steps of initiation in vivo throughout the Drosophila genome. This identifies the impact of promoter structure on initiation dynamics in relation to nucleosomal occupancy. Additionally, perturbation of transcriptional initiation reveals an unexpectedly high turnover of polymerases at paused promoters-an observation confirmed at the level of nascent RNAs. These observations argue that absence of elongation is largely caused by premature termination rather than by stable polymerase stalling. In support of this non-processive model, we observe that induction of the paused heat shock promoter depends on continuous initiation. Our study provides a framework to quantify protein binding at single-molecule resolution and refines concepts of transcriptional pausing.

Nuclear Proximity of Mtr4 to RNA Exosome Restricts DNA Mutational Asymmetry

The distribution of sense and antisense strand DNA mutations on transcribed duplex DNA contributes to the development of immune and neural systems along with the progression of cancer. Because developmentally matured B cells undergo biologically programmed strand-specific DNA mutagenesis at focal DNA/RNA hybrid structures, they make a convenient system to investigate strand-specific mutagenesis mechanisms. We demonstrate that the sense and antisense strand DNA mutagenesis at the immunoglobulin heavy chain locus and some other regions of the B cell genome depends upon localized RNA processing protein complex formation in the nucleus. Both the physical proximity and coupled activities of RNA helicase Mtr4 (and senataxin) with the noncoding RNA processing function of RNA exosome determine the strand-specific distribution of DNA mutations. Our study suggests that strand-specific DNA mutagenesis-associated mechanisms will play major roles in other undiscovered aspects of organismic development.

Paused RNA polymerase II inhibits new transcriptional initiation

RNA polymerase II (Pol II) pauses downstream of the transcription initiation site before beginning productive elongation. This pause is a key component of metazoan gene expression regulation. Some promoters have a strong disposition for Pol II pausing and often mediate faster, more synchronous changes in expression. This requires multiple rounds of transcription and thus cannot rely solely on pause release. However, it is unclear how pausing affects the initiation of new transcripts during consecutive rounds of transcription. Using our recently developed ChIP-nexus method, we find that Pol II pausing inhibits new initiation. We propose that paused Pol II helps prevent new initiation between transcription bursts, which may reduce noise.

RNA Pol II Dynamics Modulate Co-transcriptional Chromatin Modification, CTD Phosphorylation, and Transcriptional Direction

Eukaryotic genes are marked by conserved post-translational modifications on the RNA pol II C-terminal domain (CTD) and the chromatin template. How the 5'-3' profiles of these marks are established is poorly understood. Using pol II mutants in human cells, we found that slow transcription repositioned specific co-transcriptionally deposited chromatin modifications; histone H3 lysine 36 trimethyl (H3K36me3) shifted within genes toward 5' ends, and histone H3 lysine 4 dimethyl (H3K4me2) extended farther upstream of start sites. Slow transcription also evoked a hyperphosphorylation of CTD Ser2 residues at 5' ends of genes that is conserved in yeast. We propose a "dwell time in the target zone" model to explain the effects of transcriptional dynamics on the establishment of co-transcriptionally deposited protein modifications. Promoter-proximal Ser2 phosphorylation is associated with a longer pol II dwell time at start sites and reduced transcriptional polarity because of strongly enhanced divergent antisense transcription at promoters. These results demonstrate that pol II dynamics help govern the decision between sense and divergent antisense transcription.

Mll3 and Mll4 Facilitate Enhancer RNA Synthesis and Transcription from Promoters Independently of H3K4 Monomethylation

Monomethylation of histone H3 at lysine 4 (H3K4me1) and acetylation of histone H3 at lysine 27 (H3K27ac) are correlated with transcriptionally engaged enhancer elements, but the functional impact of these modifications on enhancer activity is not well understood. Here we used CRISPR/Cas9 genome editing to separate catalytic activity-dependent and independent functions of Mll3 (Kmt2c) and Mll4 (Kmt2d, Mll2), the major enhancer H3K4 monomethyltransferases. Loss of H3K4me1 from enhancers in Mll3/4 catalytically deficient cells causes partial reduction of H3K27ac, but has surprisingly minor effects on transcription from either enhancers or promoters. In contrast, loss of Mll3/4 proteins leads to strong depletion of enhancer Pol II occupancy and eRNA synthesis, concomitant with downregulation of target genes. Interestingly, downregulated genes exhibit reduced polymerase levels in gene bodies, but not at promoters, suggestive of pause-release defects. Altogether, our results suggest that enhancer H3K4me1 provides only a minor contribution to the long-range coactivator function of Mll3/4.

Histone gene replacement reveals a post-transcriptional role for H3K36 in maintaining metazoan transcriptome fidelity

Histone H3 lysine 36 methylation (H3K36me) is thought to participate in a host of co-transcriptional regulatory events. To study the function of this residue independent from the enzymes that modify it, we used a ‘histone replacement’ system in Drosophila to generate a non-modifiable H3K36 lysine-to-arginine (H3K36R) mutant. We observed global dysregulation of mRNA levels in H3K36R animals that correlates with the incidence of H3K36me3. Similar to previous studies, we found that mutation of H3K36 also resulted in H4 hyperacetylation. However, neither cryptic transcription initiation, nor alternative pre-mRNA splicing, contributed to the observed changes in expression, in contrast with previously reported roles for H3K36me. Interestingly, knockdown of the RNA surveillance nuclease, Xrn1, and members of the CCR4-Not deadenylase complex, restored mRNA levels for a class of downregulated, H3K36me3-rich genes. We propose a post-transcriptional role for modification of replication-dependent H3K36 in the control of metazoan gene expression.

DOI:http://dx.doi.org/10.7554/eLife.23249.001

In a single human cell there is enough DNA to stretch over a meter if laid end to end. To fit this DNA inside the cell – which is less than 20 micrometers in diameter – the DNA is tightly wrapped around millions of proteins known as histones, which look like “beads” along a “string” of DNA. These histones can prevent other proteins from binding to DNA and activating specific genes. Therefore, cells use enzymes to chemically modify histones to allow particular stretches of DNA to be unwrapped at specific times.

Proteins are made up of building blocks called amino acids. A specific amino acid on histones known as H3K36 is modified in certain sections of DNA that suggest it affects the activities of many genes. However, the precise role of this amino acid remains unclear. Previous studies have tried to investigate this by removing the enzymes that modify it, but these enzymes can also modify many other proteins, making it difficult to know what exactly causes the changes in gene activity.

Fruit flies are often used in experiments as models of how genetic processes work in humans and other animals. Like us, fruit flies also package their DNA using histones. To investigate the role of H3K36, Meers et al. generated a mutant fruit fly that has a version of the amino acid that cannot be chemically modified by the normal enzymes. Unexpectedly, the experiments suggest that some changes in gene activity that have been previously reported to be caused by modifying H3K36 might actually be due to other factors. Meers et al. found that H3K36 modifications may instead “mark” certain genes to be more active than they otherwise would be.

These findings provide a starting point for understanding exactly how H3K36 regulates gene activity. The next challenge is to refine our understanding of how H3K36 modification affects genes in cancer and other diseases, which may aid the development of new therapies to treat these conditions.

DOI:http://dx.doi.org/10.7554/eLife.23249.002

On-site remodeling at chromatin: How multiprotein complexes are rebuilt during DNA repair and transcriptional activation

In this review, we discuss a novel on-site remodeling function that is mediated by the H2A-ubiquitin binding protein ZRF1. ZRF1 facilitates the remodeling of multiprotein complexes at chromatin and lies at the heart of signaling processes that occur at DNA damage sites and during transcriptional activation. In nucleotide excision repair ZRF1 remodels E3 ubiquitin ligase complexes at the damage site. During embryonic stem cell differentiation, it contributes to retinoic acid-mediated gene activation by altering the subunit composition of the Mediator complex. We postulate that ZRF1 operates in conjunction with cellular remodeling machines and suggest that on-site remodeling might be a hallmark of many chromatin-associated signaling pathways. We discuss yet unexplored functions of ZRF1-mediated remodeling in replication and double strand break repair. In conclusion, we postulate that on-site remodeling of multiprotein complexes is essential for the timing of chromatin signaling processes.

A Systematic Approach to Identify Candidate Transcription Factors that Control Cell Identity

Hundreds of transcription factors (TFs) are expressed in each cell type, but cell identity can be induced through the activity of just a small number of core TFs. Systematic identification of these core TFs for a wide variety of cell types is currently lacking and would establish a foundation for understanding the transcriptional control of cell identity in development, disease, and cell-based therapy. Here, we describe a computational approach that generates an atlas of candidate core TFs for a broad spectrum of human cells. The potential impact of the atlas was demonstrated via cellular reprogramming efforts where candidate core TFs proved capable of converting human fibroblasts to retinal pigment epithelial-like cells. These results suggest that candidate core TFs from the atlas will prove a useful starting point for studying transcriptional control of cell identity and reprogramming in many human cell types.

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Core transcription factors (TFs) are predicted for >200 cell types/tissues

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Predicted TFs for retinal pigment epithelial (RPE) cells can reprogram fibroblasts

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These reprogrammed RPE-like cells are functionally similar to primary RPE

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The sets of predicted factors may facilitate studies of control of cell identity

Linking Core Promoter Classes to Circadian Transcription

Circadian rhythms in transcription are generated by rhythmic abundances and DNA binding activities of transcription factors. Propagation of rhythms to transcriptional initiation involves the core promoter, its chromatin state, and the basal transcription machinery. Here, I characterize core promoters and chromatin states of genes transcribed in a circadian manner in mouse liver and in Drosophila. It is shown that the core promoter is a critical determinant of circadian mRNA expression in both species. A distinct core promoter class, strong circadian promoters (SCPs), is identified in mouse liver but not Drosophila. SCPs are defined by specific core promoter features, and are shown to drive circadian transcriptional activities with both high averages and high amplitudes. Data analysis and mathematical modeling further provided evidence for rhythmic regulation of both polymerase II recruitment and pause release at SCPs. The analysis provides a comprehensive and systematic view of core promoters and their link to circadian mRNA expression in mouse and Drosophila, and thus reveals a crucial role for the core promoter in regulated, dynamic transcription.

Transcription factors GAF and HSF act at distinct regulatory steps to modulate stress-induced gene activation

The coordinated regulation of gene expression at the transcriptional level is fundamental to development and homeostasis. Inducible systems are invaluable when studying transcription because the regulatory process can be triggered instantaneously, allowing the tracking of ordered mechanistic events. Here, we use precision run-on sequencing (PRO-seq) to examine the genome-wide heat shock (HS) response in Drosophila and the function of two key transcription factors on the immediate transcription activation or repression of all genes regulated by HS. We identify the primary HS response genes and the rate-limiting steps in the transcription cycle that GAGA-associated factor (GAF) and HS factor (HSF) regulate. We demonstrate that GAF acts upstream of promoter-proximally paused RNA polymerase II (Pol II) formation (likely at the step of chromatin opening) and that GAF-facilitated Pol II pausing is critical for HS activation. In contrast, HSF is dispensable for establishing or maintaining Pol II pausing but is critical for the release of paused Pol II into the gene body at a subset of highly activated genes. Additionally, HSF has no detectable role in the rapid HS repression of thousands of genes.

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.

A subset of replication-dependent histone mRNAs are expressed as polyadenylated RNAs in terminally differentiated tissues

Histone proteins are synthesized in large amounts during S-phase to package the newly replicated DNA, and are among the most stable proteins in the cell. The replication-dependent (RD)-histone mRNAs expressed during S-phase end in a conserved stem-loop rather than a polyA tail. In addition, there are replication-independent (RI)-histone genes that encode histone variants as polyadenylated mRNAs. Most variants have specific functions in chromatin, but H3.3 also serves as a replacement histone for damaged histones in long-lived terminally differentiated cells. There are no reported replacement histone genes for histones H2A, H2B or H4. We report that a subset of RD-histone genes are expressed in terminally differentiated tissues as polyadenylated mRNAs, likely serving as replacement histone genes in long-lived non-dividing cells. Expression of two genes, HIST2H2AA3 and HIST1H2BC, is conserved in mammals. They are expressed as polyadenylated mRNAs in fibroblasts differentiated in vitro, but not in serum starved fibroblasts, suggesting that their expression is part of the terminal differentiation program. There are two histone H4 genes and an H3 gene that encode mRNAs that are polyadenylated and expressed at 5- to 10-fold lower levels than the mRNAs from H2A and H2B genes, which may be replacement genes for the H3.1 and H4 proteins.

Multiple P-TEFbs cooperatively regulate the release of promoter-proximally paused RNA polymerase II

The association of DSIF and NELF with initiated RNA Polymerase II (Pol II) is the general mechanism for inducing promoter-proximal pausing of Pol II. However, it remains largely unclear how the paused Pol II is released in response to stimulation. Here, we show that the release of the paused Pol II is cooperatively regulated by multiple P-TEFbs which are recruited by bromodomain-containing protein Brd4 and super elongation complex (SEC) via different recruitment mechanisms. Upon stimulation, Brd4 recruits P-TEFb to Spt5/DSIF via a recruitment pathway consisting of Med1, Med23 and Tat-SF1, whereas SEC recruits P-TEFb to NELF-A and NELF-E via Paf1c and Med26, respectively. P-TEFb-mediated phosphorylation of Spt5, NELF-A and NELF-E results in the dissociation of NELF from Pol II, thereby transiting transcription from pausing to elongation. Additionally, we demonstrate that P-TEFb-mediated Ser2 phosphorylation of Pol II is dispensable for pause release. Therefore, our studies reveal a co-regulatory mechanism of Brd4 and SEC in modulating the transcriptional pause release by recruiting multiple P-TEFbs via a Mediator- and Paf1c-coordinated recruitment network.

Architecture and RNA binding of the human negative elongation factor

Transcription regulation in metazoans often involves promoter-proximal pausing of RNA polymerase (Pol) II, which requires the 4-subunit negative elongation factor (NELF). Here we discern the functional architecture of human NELF through X-ray crystallography, protein crosslinking, biochemical assays, and RNA crosslinking in cells. We identify a NELF core subcomplex formed by conserved regions in subunits NELF-A and NELF-C, and resolve its crystal structure. The NELF-AC subcomplex binds single-stranded nucleic acids in vitro, and NELF-C associates with RNA in vivo. A positively charged face of NELF-AC is involved in RNA binding, whereas the opposite face of the NELF-AC subcomplex binds NELF-B. NELF-B is predicted to form a HEAT repeat fold, also binds RNA in vivo, and anchors the subunit NELF-E, which is confirmed to bind RNA in vivo. These results reveal the three-dimensional architecture and three RNA-binding faces of NELF.

DOI:http://dx.doi.org/10.7554/eLife.14981.001

Chromatin structure and pre-mRNA processing work together

Chromatin is the natural context for transcription elongation. However, the elongating RNA polymerase II (RNAPII) is forced to pause by the positioned nucleosomes present in gene bodies. Here, we briefly discuss the current results suggesting that those pauses could serve as a mechanism to coordinate transcription elongation with pre-mRNA processing. Further, histone post-translational modifications have been found to regulate the recruitment of factors involved in pre-mRNA processing. This view highlights the important regulatory role of the chromatin context in the whole process of the mature mRNA synthesis.

Transcription elongation control by the 7SK snRNP complex: Releasing the pause

The ability for the eukaryotic cell to transcriptionally respond to various stimuli is critical for the overall homeostasis of the cell, and in turn, the organism. The human RNA polymerase II complex (Pol II), which is responsible for the transcription of protein-encoding genes and non-coding RNAs, is paused at promoter-proximal regions to ensure their rapid activation. In response to stimulation, Pol II pause release is facilitated by the action of positive transcription elongation factors such as the P-TEFb kinase. However, the majority of P-TEFb is held in a catalytically inactivate state, assembled into the 7SK small nuclear ribonucleoprotein (snRNP) complex, and must be dislodged to become catalytically active. In this review, we discuss mechanisms of 7SK snRNP recruitment to promoter-proximal regions and P-TEFb disassembly from the inhibitory snRNP to regulate 'on site' kinase activation and Pol II pause release.

7SKiing on chromatin: Move globally, act locally

RNA polymerase II (Pol II) pausing at promoter-proximal regions is a highly regulated step in the transcription cycle. Pause release is facilitated by the P-TEFb kinase, which phosphorylates Pol II and negative elongation factors. Recent studies suggest that P-TEFb (as part of the inhibitory 7SK snRNP) is recruited to promoter-proximal regions through interaction with KAP1/TRIM28/TIF1β to facilitate 'on-site' kinase activation and transcription elongation. Here, I discuss features of this model and future challenges to further hone our understanding of transcriptional regulation including Pol II pausing and pause release.

Coupling of RNA Polymerase II Transcription Elongation with Pre-mRNA Splicing

Pre-mRNA maturation frequently occurs at the same time and place as transcription by RNA polymerase II. The co-transcriptionality of mRNA processing has permitted the evolution of mechanisms that functionally couple transcription elongation with diverse events that occur on the nascent RNA. This review summarizes the current understanding of the relationship between transcriptional elongation through a chromatin template and co-transcriptional splicing including alternative splicing decisions that affect the expression of most human genes.

Bioinformatic analyses of sense and antisense expression from terminal inverted repeat transposons in Drosophila somatic cells

Understanding regulation of transposon movement in somatic cells is important as mobile elements can cause detrimental genomic rearrangements. Generally, transposons move via one of 2 mechanisms; retrotransposons utilize an RNA intermediate, therefore copying themselves and amplifying throughout the genome, while terminal inverted repeat transposons (TIR Tns) excise DNA sequences from the genome and integrate into a new location. Our recently published work indicates that retrotransposons in Drosophila tissue culture cells are actively transcribed in the antisense direction. Our data support a model in which convergent transcription of retrotransposons from intra element transcription start sites results in complementary RNAs that hybridize to form substrates for Dicer-2, the endogenous small interfering (esi)RNA generating enzyme. Here, we extend our previous analysis to TIR Tns. In contrast to retrotransposons, our data show that antisense TIR Tn RNAs result from transcription of intronic TIR Tns oriented antisense to their host genes. Also, disproportionately less esiRNAs are generated from TIR transcripts than from retrotransposons and transcription of very few individual TIR Tns could be confirmed. Collectively, these data support a model in which TIR Tns are regulated at the level of Transposase production while retrotransposons are regulated with esiRNA post-transcriptional mechanisms in Drosophila somatic cells.