Progression through the RNA polymerase II CTD cycle

c-Myc deregulation induces mRNA capping enzyme dependency

c-Myc is a potent driver of many human cancers. Since strategies for directly targeting c-Myc protein have had limited success, upstream regulators and downstream effectors of c-Myc are being investigated as alternatives for therapeutic intervention. c-Myc regulates transcription and formation of the mRNA cap, which is important for transcript maturation and translation. However, the direct mechanism by which c-Myc upregulates mRNA capping is unclear. mRNA cap formation initiates with the linkage of inverted guanosine via a triphosphate bridge to the first transcribed nucleotide, catalysed by mRNA capping enzyme (CE/RNGTT). Here we report that c-Myc increases the recruitment of catalytically active CE to RNA polymerase II and to its target genes. c-Myc-induced target gene expression, cell proliferation and cell transformation is highly dependent on CE, but only when c-Myc is deregulated. Cells retaining normal control of c-Myc expression are insensitive to repression of CE. c-Myc expression is also dependent on CE. Therefore, inhibiting CE provides an attractive route for selective therapeutic targeting of cancer cells which have acquired deregulated c-Myc.

Nutrient-Driven O-GlcNAcylation at Promoters Impacts Genome-Wide RNA Pol II Distribution

Nutrient-driven O-GlcNAcylation has been linked to epigenetic regulation of gene expression in metazoans. In C. elegans, O-GlcNAc marks the promoters of over 800 developmental, metabolic, and stress-related genes; these O-GlcNAc marked genes show a strong 5', promoter-proximal bias in the distribution of RNA Polymerase II (Pol II). In response to starvation or feeding, the steady state distribution of O-GlcNAc at promoters remain nearly constant presumably due to dynamic cycling mediated by the transferase OGT-1 and the O-GlcNAcase OGA-1. However, in viable mutants lacking either of these enzymes of O-GlcNAc metabolism, the nutrient-responsive GlcNAcylation of promoters is dramatically altered. Blocked O-GlcNAc cycling leads to a striking nutrient-dependent accumulation of O-GlcNAc on RNA Pol II. O-GlcNAc cycling mutants also show an exaggerated, nutrient-responsive redistribution of promoter-proximal RNA Pol II isoforms and extensive transcriptional deregulation. Our findings suggest a complex interplay between the O-GlcNAc modification at promoters, the kinase-dependent "CTD-code," and co-factors regulating RNA Pol II dynamics. Nutrient-responsive O-GlcNAc cycling may buffer the transcriptional apparatus from dramatic swings in nutrient availability by modulating promoter activity to meet metabolic and developmental needs.

Novel layers of RNA polymerase III control affecting tRNA gene transcription in eukaryotes

RNA polymerase III (Pol III) transcribes a limited set of short genes in eukaryotes producing abundant small RNAs, mostly tRNA. The originally defined yeast Pol III transcriptome appears to be expanding owing to the application of new methods. Also, several factors required for assembly and nuclear import of Pol III complex have been identified recently. Models of Pol III based on cryo-electron microscopy reconstructions of distinct Pol III conformations reveal unique features distinguishing Pol III from other polymerases. Novel concepts concerning Pol III functioning involve recruitment of general Pol III-specific transcription factors and distinctive mechanisms of transcription initiation, elongation and termination. Despite the short length of Pol III transcription units, mapping of transcriptionally active Pol III with nucleotide resolution has revealed strikingly uneven polymerase distribution along all genes. This may be related, at least in part, to the transcription factors bound at the internal promoter regions. Pol III uses also a specific negative regulator, Maf1, which binds to polymerase under stress conditions; however, a subset of Pol III genes is not controlled by Maf1. Among other RNA polymerases, Pol III machinery represents unique features related to a short transcript length and high transcription efficiency.

MicroScale Thermophoresis as a Tool to Study Protein-peptide Interactions in the Context of Large Eukaryotic Protein Complexes

Protein-peptide interactions are part of many physiological processes, for example, epigenetics where peptide regions of histone complexes are crucial for regulation of chromatin structure. Short peptides are often also used as alternatives to small molecule drugs to target protein complexes. Studying the interactions between proteins and peptides is thus an important task in systems biology, cell biology, biochemistry, and drug design. However, this task is often hampered by the drawbacks of classical biophysical methods for analysis of molecular interactions like surface plasmon resonance (SPR) or isothermal titration calorimetry (ITC), which require immobilization of the interaction partners or very high sample concentrations. MicroScale Thermophoresis (MST) is an innovative method that offers the possibility to determine the important parameters of a molecular interaction, such as dissociation constant, stoichiometry, and thermodynamics. Moreover, it does so in a rapid and precise manner, with free choice of buffers or biological liquids, no need for sample immobilization, and very low sample consumption. Here we describe two MST assays in detail, which analyze (i) the interactions between certain peptide stretches of the eukaryotic RNA polymerase II and a protein subunit of the eukaryotic transcription elongation complex and (ii) interactions between N-terminal histone tail peptides and epigenetic reader proteins. These experiments show that MST is able to characterize protein-peptide interactions that are triggered by only minor changes in the peptide, for example, only one phosphorylation at a specific serine residue.

An essential domain of an early-diverged RNA polymerase II functions to accurately decode a primitive chromatin landscape

A unique feature of RNA polymerase II (RNA pol II) is its long C-terminal extension, called the carboxy-terminal domain (CTD). The well-studied eukaryotes possess a tandemly repeated 7-amino-acid sequence, called the canonical CTD, which orchestrates various steps in mRNA synthesis. Many eukaryotes possess a CTD devoid of repeats, appropriately called a non-canonical CTD, which performs completely unknown functions. Trypanosoma brucei, the etiologic agent of African Sleeping Sickness, deploys an RNA pol II that contains a non-canonical CTD to accomplish an unusual transcriptional program; all protein-coding genes are transcribed as part of a polygenic precursor mRNA (pre-mRNA) that is initiated within a several-kilobase-long region, called the transcription start site (TSS), which is upstream of the first protein-coding gene in the polygenic array. In this report, we show that the non-canonical CTD of T. brucei RNA pol II is important for normal protein-coding gene expression, likely directing RNA pol II to the TSSs within the genome. Our work reveals the presence of a primordial CTD code within eukarya and indicates that proper recognition of the chromatin landscape is a central function of this RNA pol II-distinguishing domain.

Microprocessor Recruitment to Elongating RNA Polymerase II Is Required for Differential Expression of MicroRNAs

The cellular abundance of mature microRNAs (miRNAs) is dictated by the efficiency of nuclear processing of primary miRNA transcripts (pri-miRNAs) into pre-miRNA intermediates. The Microprocessor complex of Drosha and DGCR8 carries this out, but it has been unclear what controls Microprocessor's differential processing of various pri-miRNAs. Here, we show that Drosophila DGCR8 (Pasha) directly associates with the C-terminal domain of the RNA polymerase II elongation complex when it is phosphorylated by the Cdk9 kinase (pTEFb). When association is blocked by loss of Cdk9 activity, a global change in pri-miRNA processing is detected. Processing of pri-miRNAs with a UGU sequence motif in their apical junction domain increases, while processing of pri-miRNAs lacking this motif decreases. Therefore, phosphorylation of RNA polymerase II recruits Microprocessor for co-transcriptional processing of non-UGU pri-miRNAs that would otherwise be poorly processed. In contrast, UGU-positive pri-miRNAs are robustly processed by Microprocessor independent of RNA polymerase association.

Transcriptional Regulation of Pattern-Triggered Immunity in Plants

Perception of microbe-associated molecular patterns (MAMPs) by cell-surface-resident pattern recognition receptors (PRRs) induces rapid, robust, and selective transcriptional reprogramming, which is central for launching effective pattern-triggered immunity (PTI) in plants. Signal relay from PRR complexes to the nuclear transcriptional machinery via intracellular kinase cascades rapidly activates primary immune response genes. The coordinated action of gene-specific transcription factors and the general transcriptional machinery contribute to the selectivity of immune gene activation. In addition, PRR complexes and signaling components are often transcriptionally upregulated upon MAMP perception to ensure the robustness and sustainability of PTI outputs. In this review, we discuss recent advances in deciphering the signaling pathways and regulatory mechanisms that coordinately lead to timely and accurate MAMP-induced gene expression in plants.

Differential expression by chromatin modifications of alcohol dehydrogenase 1 of Chorispora bungeana in cold stress

Epigenetic modifications regulate plant genes to cope with a variety of environmental stresses. Chorispora bungeana is an alpine subnival plant with strong tolerance to multiple abiotic stresses, especially cold stress. In this study, we characterized the alcohol dehydrogenase 1 gene from Chorispora bungeana, CbADH1, that is up-regulated in cold conditions. Overexpression of CbADH1 in Arabidopsis thaliana improved cold tolerance, as indicated by a decreased lethal temperature (LT50). Chromatin immunoprecipitation assays showed that histone H3 is removed from the promoter region and the middle-coding region of the gene. H3K9 acetylation and H3K4 trimethylation increased throughout the gene and in the proximal promoter region, respectively. Moreover, increased Ser5P and Ser2P polymerase II accumulation further indicated changes in the transcription initiation and elongation of CbADH1 were due to the cold stress. Taken together, our results suggested that CbADH1 is highly expressed during cold stress, and is regulated by epigenetic modifications. This study expands our understanding of the regulation of gene expression by epigenetic modifications in response to environmental cues.

Sub1 and RNAPII, until termination does them part

Sub1 was initially identified as a coactivator factor with a role during transcription initiation. However, over the last years, many evidences showed that it influences processes downstream during mRNA biogenesis, such as elongation, termination, and RNAPII phosphorylation. The recent discover that Sub1 directly interacts with the RNAPII stalk adds new insights into how it achieves all these tasks.

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

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

Interplay between Metabolism and Epigenetics: A Nuclear Adaptation to Environmental Changes

The physiological identity of every cell is maintained by highly specific transcriptional networks that establish a coherent molecular program that is in tune with nutritional conditions. The regulation of cell-specific transcriptional networks is accomplished by an epigenetic program via chromatin-modifying enzymes, whose activity is directly dependent on metabolites such as acetyl-coenzyme A, S-adenosylmethionine, and NAD+, among others. Therefore, these nuclear activities are directly influenced by the nutritional status of the cell. In addition to nutritional availability, this highly collaborative program between epigenetic dynamics and metabolism is further interconnected with other environmental cues provided by the day-night cycles imposed by circadian rhythms. Herein, we review molecular pathways and their metabolites associated with epigenetic adaptations modulated by histone- and DNA-modifying enzymes and their responsiveness to the environment in the context of health and disease.

Sub1 contacts the RNA polymerase II stalk to modulate mRNA synthesis

Biogenesis of messenger RNA is critically influenced by the phosphorylation state of the carboxy-terminal domain (CTD) in the largest RNA polymerase II (RNAPII) subunit. Several kinases and phosphatases are required to maintain proper CTD phosphorylation levels and, additionally, several other proteins modulate them, including Rpb4/7 and Sub1. The Rpb4/7 heterodimer, constituting the RNAPII stalk, promote phosphatase functions and Sub1 globally influences CTD phosphorylation, though its mechanism remains mostly unknown. Here, we show that Sub1 physically interacts with the RNAPII stalk domain, Rpb4/7, likely through its C-terminal region, and associates with Fcp1. While Rpb4 is not required for Sub1 interaction with RNAPII complex, a fully functional heterodimer is required for Sub1 association to promoters. We also demonstrate that a complete CTD is necessary for proper association of Sub1 to chromatin and to the RNAPII. Finally, genetic data show a functional relationship between Sub1 and the RNAPII clamp domain. Altogether, our results indicate that Sub1, Rpb4/7 and Fcp1 interaction modulates CTD phosphorylation. In addition, Sub1 interaction with Rpb4/7 can also modulate transcription start site selection and transcription elongation rate likely by influencing the clamp function.

The emerging picture of CDK9/P-TEFb: more than 20 years of advances since PITALRE

CDK9 is a prominent member of the transcriptional CDKs subfamily, a group of kinases whose function is to control the primary steps of mRNA synthesis and processing by eukaryotic RNA polymerase II. As a cyclin-dependent kinase, CDK9 activation in vivo depends upon its association with T-type cyclins to assemble the positive transcription elongation factor (P-TEFb). Although CDK9/P-TEFb phosphorylates the C-terminal domain of RNAP II in the same positions targeted by CDK7 (TFIIH) and CDK8 (Mediator), the former does not participate in the transcription initiation, but rather plays a unique role by driving the polymerase to productive elongation. In addition to RNAP II CTD, the negative transcription elongation factors DSIF and NELF also represent major CDK9 substrates, whose phosphorylation is required to overcome the proximal pause of the polymerase. CDK9 is recruited to specific genes through proteins that interact with both P-TEFb and distinct elements in DNA, RNA or chromatin, where it modulates the activity of individual RNAP II transcription complexes. The regulation of CDK9 function is an intricate network that includes post-translational modifications (phosphorylation/dephosphorylation and acetylation/deacetylation of key residues) as well as the association of P-TEFb with various proteins that can stimulate or inhibit its kinase activity. Several cases of CDK9 deregulation have been linked to important human diseases, including various types of cancer and also AIDS (due to its essential role in HIV replication). Not only HIV, but also many other human viruses have been shown to depend strongly on CDK9 activity to be transcribed within host cells. This review summarizes the main advances made on CDK9/P-TEFb field in more than 20 years, introducing the structural, functional and genetic aspects that have been elucidated ever since.

Variant Gene Expression and Antigenic Variation by Malaria Parasites

Malaria is a significant threat throughout the developing world. Among the most fascinating aspects of the protozoan parasites responsible for this disease are the methods they employ to avoid the immune system and perpetuate chronic infections. Key among these is antigenic variation: By systematically altering antigens that are displayed to the host's immune system, the parasite renders the adaptive immune response ineffective. For Plasmodium falciparum, the species responsible for the most severe form of human malaria, this process involves a complicated molecular mechanism that results in continuously changing patterns of variant-antigen-encoding gene expression. Although many features of this process remain obscure, significant progress has been made in recent years to decipher various molecular aspects of the regulatory cascade that causes chronic infection.

The interplay between transcription and mRNA degradation in Saccharomyces cerevisiae

The cellular transcriptome is shaped by both the rates of mRNA synthesis in the nucleus and mRNA degradation in the cytoplasm under a specified condition. The last decade witnessed an exciting development in the field of post-transcriptional regulation of gene expression which underscored a strong functional coupling between the transcription and mRNA degradation. The functional integration is principally mediated by a group of specialized promoters and transcription factors that govern the stability of their cognate transcripts by “marking” them with a specific factor termed “coordinator.” The “mark” carried by the message is later decoded in the cytoplasm which involves the stimulation of one or more mRNA-decay factors, either directly by the “coordinator” itself or in an indirect manner. Activation of the decay factor(s), in turn, leads to the alteration of the stability of the marked message in a selective fashion. Thus, the integration between mRNA synthesis and decay plays a potentially significant role to shape appropriate gene expression profiles during cell cycle progression, cell division, cellular differentiation and proliferation, stress, immune and inflammatory responses, and may enhance the rate of biological evolution.

Kaposi's Sarcoma-Associated Herpesvirus Hijacks RNA Polymerase II To Create a Viral Transcriptional Factory

Locally concentrated nuclear factors ensure efficient binding to DNA templates, facilitating RNA polymerase II recruitment and frequent reutilization of stable preinitiation complexes. We have uncovered a mechanism for effective viral transcription by focal assembly of RNA polymerase II around Kaposi's sarcoma-associated herpesvirus (KSHV) genomes in the host cell nucleus. Using immunofluorescence labeling of latent nuclear antigen (LANA) protein, together with fluorescence in situ RNA hybridization (RNA-FISH) of the intron region of immediate early transcripts, we visualized active transcription of viral genomes in naturally infected cells. At the single-cell level, we found that not all episomes were uniformly transcribed following reactivation stimuli. However, those episomes that were being transcribed would spontaneously aggregate to form transcriptional “factories,” which recruited a significant fraction of cellular RNA polymerase II. Focal assembly of “viral transcriptional factories” decreased the pool of cellular RNA polymerase II available for cellular gene transcription, which consequently impaired cellular gene expression globally, with the exception of selected ones. The viral transcriptional factories localized with replicating viral genomic DNAs. The observed colocalization of viral transcriptional factories with replicating viral genomic DNA suggests that KSHV assembles an “all-in-one” factory for both gene transcription and DNA replication. We propose that the assembly of RNA polymerase II around viral episomes in the nucleus may be a previously unexplored aspect of KSHV gene regulation by confiscation of a limited supply of RNA polymerase II in infected cells.

IMPORTANCE B cells infected with Kaposi's sarcoma-associated herpesvirus (KSHV) harbor multiple copies of the KSHV genome in the form of episomes. Three-dimensional imaging of viral gene expression in the nucleus allows us to study interactions and changes in the physical distribution of these episomes following stimulation. The results showed heterogeneity in the responses of individual KSHV episomes to stimuli within a single reactivating cell; those episomes that did respond to stimulation, aggregated within large domains that appear to function as viral transcription factories. A significant portion of cellular RNA polymerase II was trapped in these factories and served to transcribe viral genomes, which coincided with an overall decrease in cellular gene expression. Our findings uncover a strategy of KSHV gene regulation through focal assembly of KSHV episomes and a molecular mechanism of late gene expression.

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.

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

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

Absence of Gim proteins, but not GimC complex, alters stress-induced transcription

Saccharomyces cerevisiae GimC (mammalian Prefoldin) is a hexameric (Gim1-6) cytoplasmic complex involved in the folding pathway of actin/tubulin. In contrast to a shared role in GimC complex, we show that absence of individual Gim proteins results in distinct stress responses. No concomitant alteration in F-actin integrity was observed. Transcription of stress responsive genes is altered in gim2Δ, gim3Δ and gim6Δ mutants: TRX2 gene is induced in these mutants but with a profile diverging from type cells, whereas CTT1 and HSP26 fail to be induced. Remaining gimΔ mutants display stress transcript abundance comparable to wild type cells. No alteration in the nuclear localization of the transcriptional activators for TRX2 (Yap1) and CTT1/HSP26 (Msn2) was observed in gim2Δ. In accordance with TRX2 induction, RNA polymerase II occupancy at TRX2 discriminates the wild type from gim2Δ and gim6Δ. In contrast, RNA polymerase II occupancy at CTT1 is similar in wild type and gim2Δ, but higher in gim6Δ. The absence of active RNA polymerase II at CTT1 in gim2Δ, but not in wild type and gim1Δ, explains the respective CTT1 transcript outputs. Altogether our results put forward the need of Gim2, Gim3 and Gim6 in oxidative and osmotic stress activated transcription; others Gim proteins are dispensable. Consequently, the participation of Gim proteins in activated-transcription is independent from the GimC complex.

Modulation of gene expression dynamics by co-transcriptional histone methylations

Co-transcriptional methylations of histone H3 at lysines 4 and 36, highly conserved methyl marks from yeast to humans, have profound roles in regulation of histone acetylation. These modifications function to recruit and/or activate distinct histone acetyltransferases (HATs) or histone deacetylases (HDACs). Whereas H3K4me3 increases acetylation at promoters via multiple HATs, H3K4me2 targets Set3 HDAC to deacetylate histones in 5' transcribed regions. In 3' regions of genes, H3K36me2/3 facilitates deacetylation by Rpd3S HDAC and slows elongation. Despite their important functions in deacetylation, no strong effects on global gene expression have been seen under optimized or laboratory growth conditions. Instead, H3K4me2-Set3 HDAC and Set2-Rpd3S pathways primarily delay the kinetics of messenger RNA (mRNA) and long noncoding RNA (lncRNA) induction upon environmental changes. A majority of mRNA genes regulated by these pathways have an overlapping lncRNA transcription either from an upstream or an antisense promoter. Surprisingly, the distance between mRNA and lncRNA promoters seems to specify the repressive effects of the two pathways. Given that co-transcriptional methylations and acetylation have been linked to many cancers, studying their functions in a dynamic condition or during cancer progression will be much more important and help identify novel genes associated with cancers.

Multiplex Substrate Profiling by Mass Spectrometry for Kinases as a Method for Revealing Quantitative Substrate Motifs

The more than 500 protein kinases comprising the human kinome catalyze hundreds of thousands of phosphorylation events to regulate a diversity of cellular functions; however, the extended substrate specificity is still unknown for many of these kinases. We report here a method for quantitatively describing kinase substrate specificity using an unbiased peptide library-based approach with direct measurement of phosphorylation by tandem liquid chromatography-tandem mass spectrometry (LC-MS/MS) peptide sequencing (multiplex substrate profiling by mass spectrometry, MSP-MS). This method can be deployed with as low as 10 nM enzyme to determine activity against S/T/Y-containing peptides; additionally, label-free quantitation is used to ascertain catalytic efficiency values for individual peptide substrates in the multiplex assay. Using this approach we developed quantitative motifs for a selection of kinases from each branch of the kinome, with and without known substrates, highlighting the applicability of the method. The sensitivity of this approach is evidenced by its ability to detect phosphorylation events from nanogram quantities of immunoprecipitated material, which allows for wider applicability of this method. To increase the information content of the quantitative kinase motifs, a sublibrary approach was used to expand the testable sequence space within a peptide library of approximately 100 members for CDK1, CDK7, and CDK9. Kinetic analysis of the HIV-1 Tat (transactivator of transcription)-positive transcription elongation factor b (P-TEFb) interaction allowed for localization of the P-TEFb phosphorylation site as well as characterization of the stimulatory effect of Tat on P-TEFb catalytic efficiency.

The Composition of the Arabidopsis RNA Polymerase II Transcript Elongation Complex Reveals the Interplay between Elongation and mRNA Processing Factors

Transcript elongation factors (TEFs) are a heterogeneous group of proteins that control the efficiency of transcript elongation of subsets of genes by RNA polymerase II (RNAPII) in the chromatin context. Using reciprocal tagging in combination with affinity purification and mass spectrometry, we demonstrate that in Arabidopsis thaliana, the TEFs SPT4/SPT5, SPT6, FACT, PAF1-C, and TFIIS copurified with each other and with elongating RNAPII, while P-TEFb was not among the interactors. Additionally, NAP1 histone chaperones, ATP-dependent chromatin remodeling factors, and some histone-modifying enzymes including Elongator were repeatedly found associated with TEFs. Analysis of double mutant plants defective in different combinations of TEFs revealed genetic interactions between genes encoding subunits of PAF1-C, FACT, and TFIIS, resulting in synergistic/epistatic effects on plant growth/development. Analysis of subnuclear localization, gene expression, and chromatin association did not provide evidence for an involvement of the TEFs in transcription by RNAPI (or RNAPIII). Proteomics analyses also revealed multiple interactions between the transcript elongation complex and factors involved in mRNA splicing and polyadenylation, including an association of PAF1-C with the polyadenylation factor CstF. Therefore, the RNAPII transcript elongation complex represents a platform for interactions among different TEFs, as well as for coordinating ongoing transcription with mRNA processing.

Cdc15 Phosphorylates the C-terminal Domain of RNA Polymerase II for Transcription during Mitosis

In eukaryotes, the basal transcription in interphase is orchestrated through the regulation by kinases (Kin28, Bur1, and Ctk1) and phosphatases (Ssu72, Rtr1, and Fcp1), which act through the post-translational modification of the C-terminal domain (CTD) of the largest subunit of RNA polymerase II. The CTD comprises the repeated Tyr-Ser-Pro-Thr-Ser-Pro-Ser motif with potential epigenetic modification sites. Despite the observation of transcription and periodic expression of genes during mitosis with entailing CTD phosphorylation and dephosphorylation, the associated CTD specific kinase(s) and its role in transcription remains unknown. Here we have identified Cdc15 as a potential kinase phosphorylating Ser-2 and Ser-5 of CTD for transcription during mitosis in the budding yeast. The phosphorylation of CTD by Cdc15 is independent of any prior Ser phosphorylation(s). The inactivation of Cdc15 causes reduction of global CTD phosphorylation during mitosis and affects the expression of genes whose transcript levels peak during mitosis. Cdc15 also influences the complete transcription of clb2 gene and phosphorylates Ser-5 at the promoter and Ser-2 toward the 3' end of the gene. The observation that Cdc15 could phosphorylate Ser-5, as well as Ser-2, during transcription in mitosis is in contrast to the phosphorylation marks put by the kinases in interphase (G₁, S, and G₂), where Cdck7/Kin28 phosphorylates Ser-5 at promoter and Bur1/Ctk1 phosphorylates Ser-2 at the 3' end of the genes.

Imaging Translational and Post-Translational Gene Regulatory Dynamics in Living Cells with Antibody-Based Probes

Antibody derivatives, such as antibody fragments (Fabs) and single-chain variable fragments (scFvs), are now being used to image traditionally hard-to-see protein subpopulations, including nascent polypeptides being translated and post-translationally modified proteins. This has allowed researchers to directly image and quantify, for the first time, translation initiation and elongation kinetics with single-transcript resolution and the temporal ordering and kinetics of post-translational histone and RNA polymerase II modifications. Here, we review these developments and discuss the strengths and weaknesses of live-cell imaging with antibody-based probes. Further development of these probes will increase their versatility and open new avenues of research for dissecting complex gene regulatory dynamics.

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

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

Subgenic Pol II interactomes identify region-specific transcription elongation regulators

Transcription, RNA processing, and chromatin‐related factors all interact with RNA polymerase II (Pol II) to ensure proper timing and coordination of transcription and co‐transcriptional processes. Many transcription elongation regulators must function simultaneously to coordinate these processes, yet few strategies exist to explore the complement of factors regulating specific stages of transcription. To this end, we developed a strategy to purify Pol II elongation complexes from subgenic regions of a single gene, namely the 5′ and 3′ regions, using sequences in the nascent RNA. Applying this strategy to Saccharomyces cerevisiae, we determined the specific set of factors that interact with Pol II at precise stages during transcription. We identify many known region‐specific factors as well as determine unappreciated associations of regulatory factors during early and late stages of transcription. These data reveal a role for the transcription termination factor, Rai1, in regulating the early stages of transcription genome‐wide and support the role of Bye1 as a negative regulator of early elongation. We also demonstrate a role for the ubiquitin ligase, Bre1, in regulating Pol II dynamics during the latter stages of transcription. These data and our approach to analyze subgenic transcription elongation complexes will shed new light on the myriad factors that regulate the different stages of transcription and coordinate co‐transcriptional processes.

CDK regulation of transcription by RNAP II: Not over 'til it's over?

Transcription by RNA polymerase (RNAP) II is regulated at multiple steps by phosphorylation, catalyzed mainly by members of the cyclin-dependent kinase (CDK) family. The CDKs involved in transcription have overlapping substrate specificities, but play largely non-redundant roles in coordinating gene expression. Novel functions and targets of CDKs have recently emerged at the end of the transcription cycle, when the primary transcript is cleaved, and in most cases polyadenylated, and transcription is terminated by the action of the "torpedo" exonuclease Xrn2, which is a CDK substrate. Collectively, various functions have been ascribed to CDKs or CDK-mediated phosphorylation: recruiting cleavage and polyadenylation factors, preventing premature termination within gene bodies while promoting efficient termination of full-length transcripts, and preventing extensive readthrough transcription into intergenic regions or neighboring genes. The assignment of precise functions to specific CDKs is still in progress, but recent advances suggest ways in which the CDK network and RNAP II machinery might cooperate to ensure timely exit from the transcription cycle.

Modulation of mRNA and lncRNA expression dynamics by the Set2-Rpd3S pathway

H3K36 methylation by Set2 targets Rpd3S histone deacetylase to transcribed regions of mRNA genes, repressing internal cryptic promoters and slowing elongation. Here we explore the function of this pathway by analysing transcription in yeast undergoing a series of carbon source shifts. Approximately 80 mRNA genes show increased induction upon SET2 deletion. A majority of these promoters have overlapping lncRNA transcription that targets H3K36me3 and deacetylation by Rpd3S to the mRNA promoter. We previously reported a similar mechanism for H3K4me2-mediated repression via recruitment of the Set3C histone deacetylase. Here we show that the distance between an mRNA and overlapping lncRNA promoter determines whether Set2–Rpd3S or Set3C represses. This analysis also reveals many previously unreported cryptic ncRNAs induced by specific carbon sources, showing that cryptic promoters can be environmentally regulated. Therefore, in addition to repression of cryptic transcription and modulation of elongation, H3K36 methylation maintains optimal expression dynamics of many mRNAs and ncRNAs.

H3K36 methylation by Set2 targets Rpd3S histone deacetylase to transcribed mRNA genes, repressing internal cryptic promoters and modulating elongation. Here, the authors provide evidence that the Set2-Rpd3S pathway also regulates dynamic expression of mRNAs and lncRNAs.

Exon junction complex proteins bind nascent transcripts independently of pre-mRNA splicing in Drosophila melanogaster

Although it is currently understood that the exon junction complex (EJC) is recruited on spliced mRNA by a specific interaction between its central protein, eIF4AIII, and splicing factor CWC22, we found that eIF4AIII and the other EJC core proteins Y14 and MAGO bind the nascent transcripts of not only intron-containing but also intronless genes on Drosophila polytene chromosomes. Additionally, Y14 ChIP-seq demonstrates that association with transcribed genes is also splicing-independent in Drosophila S2 cells. The association of the EJC proteins with nascent transcripts does not require CWC22 and that of Y14 and MAGO is independent of eIF4AIII. We also show that eIF4AIII associates with both polysomal and monosomal RNA in S2 cell extracts, whereas Y14 and MAGO fractionate separately. Cumulatively, our data indicate a global role of eIF4AIII in gene expression, which would be independent of Y14 and MAGO, splicing, and of the EJC, as currently understood.

Transcription factors that influence RNA polymerases I and II: To what extent is mechanism of action conserved?

In eukaryotic cells, nuclear RNA synthesis is accomplished by at least three unique, multisubunit RNA polymerases. The roles of these enzymes are generally partitioned into the synthesis of the three major classes of RNA: rRNA, mRNA, and tRNA for RNA polymerases I, II, and III respectively. Consistent with their unique cellular roles, each enzyme has a complement of specialized transcription factors and enzymatic properties. However, not all transcription factors have evolved to affect only one eukaryotic RNA polymerase. In fact, many factors have been shown to influence the activities of multiple nuclear RNA polymerases. This review focuses on a subset of these factors, specifically addressing the mechanisms by which these proteins influence RNA polymerases I and II.

snRNA 3' End Processing by a CPSF73-Containing Complex Essential for Development in Arabidopsis

Uridine-rich small nuclear RNAs (snRNAs) are the basal components of the spliceosome and play essential roles in splicing. The biogenesis of the majority of snRNAs involves 3′ end endonucleolytic cleavage of the nascent transcript from the elongating DNA-dependent RNA ploymerase II. However, the protein factors responsible for this process remain elusive in plants. Here, we show that DEFECTIVE in snRNA PROCESSING 1 (DSP1) is an essential protein for snRNA 3′ end maturation in Arabidopsis. A hypomorphic dsp1-1 mutation causes pleiotropic developmental defects, impairs the 3′ end processing of snRNAs, increases the levels of snRNA primary transcripts (pre-snRNAs), and alters the occupancy of Pol II at snRNA loci. In addition, DSP1 binds snRNA loci and interacts with Pol-II in a DNA/RNA-dependent manner. We further show that DSP1 forms a conserved complex, which contains at least four additional proteins, to catalyze snRNA 3′ end maturation in Arabidopsis. The catalytic component of this complex is likely the cleavage and polyadenylation specificity factor 73 kDa-I (CSPF73-I), which is the nuclease cleaving the pre-mRNA 3′ end. However, the DSP1 complex does not affect pre-mRNA 3′ end cleavage, suggesting that plants may use different CPSF73-I-containing complexes to process snRNAs and pre-mRNAs. This study identifies a complex responsible for the snRNA 3′ end maturation in plants and uncovers a previously unknown function of CPSF73 in snRNA maturation.

This study identifies a protein complex in plants that is responsible for the maturation of the 3′ ends of spliceosomal snRNAs and uncovers a novel function for the mRNA 3′ cleavage nuclease CPSF73.

snRNAs form the RNA components of the spliceosome and are required for spliceosome formation and splicing. The generation of snRNAs involves 3′ end endonucleolytic cleavage of primary snRNA transcripts (pre-snRNAs). The factors responsible for pre-snRNA 3′ end cleavage are known in metazoans, but many of these components are missing in plants. Therefore, the proteins that catalyze pre-snRNA cleavage in plants and the mechanism leading to plant snRNA 3′ maturation are unknown. Here, we show that a DSP1 complex (containing DSP1, DSP2, DSP3, DSP4, and CPFS73-I) is responsible for pre-snRNA 3′ end cleavage in Arabidopsis. We further show that CPSF73-I, which is known to cleave the pre-mRNA 3′ end, is likely the enzyme also catalyzing snRNA 3′ end maturation in plants. Interestingly, plants appear to use two different CPSF73-I-containing complexes to catalyze the maturation of mRNAs and snRNAs. The study thereby identifies an snRNA-processing complex in plants and also elucidates a new role for CPSF73-I in this process.

Site-specific methylation and acetylation of lysine residues in the C-terminal domain (CTD) of RNA polymerase II

Dynamic modification of heptad-repeats with the consensus sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 of RNA polymerase II (RNAPII) C-terminal domain (CTD) regulates transcription-coupled processes. Mass spectrometry analysis revealed that K7-residues in non-consensus repeats of human RNAPII are modified by acetylation, or mono-, di-, and tri-methylation. K7ac, K7me2, and K7me3 were found exclusively associated with phosphorylated CTD peptides, while K7me1 occurred also in non-phosphorylated CTD. The monoclonal antibody 1F5 recognizes K7me1/2 residues in CTD and reacts with RNAPIIA. Treatment of cellular extracts with phosphatase or of cells with the kinase inhibitor flavopiridol unmasked the K7me1/2 epitope in RNAPII0, consistent with the association of K7me1/2 marks with phosphorylated CTD peptides. Genome-wide profiling revealed high levels of K7me1/2 marks at the transcriptional start site of genes for sense and antisense transcribing RNAPII. The new K7 modifications further expand the mammalian CTD code to allow regulation of differential gene expression.

Co-transcriptional splicing and the CTD code

Transcription and splicing are fundamental steps in gene expression. These processes have been studied intensively over the past four decades, and very recent findings are challenging some of the formerly established ideas. In particular, splicing was shown to occur much faster than previously thought, with the first spliced products observed as soon as splice junctions emerge from RNA polymerase II (Pol II). Splicing was also found coupled to a specific phosphorylation pattern of Pol II carboxyl-terminal domain (CTD), suggesting a new layer of complexity in the CTD code. Moreover, phosphorylation of the CTD may be scarcer than expected, and other post-translational modifications of the CTD are emerging with unanticipated roles in gene expression regulation.

Covalent targeting of remote cysteine residues to develop CDK12 and CDK13 inhibitors

Cyclin-dependent kinases 12 and 13 (CDK12 and CDK13) play critical roles in the regulation of gene transcription. However, the absence of CDK12 and CDK13 inhibitors has hindered the ability to investigate the consequences of their inhibition in healthy cells and cancer cells. Here we describe the rational design of a first-in-class CDK12 and CDK13 covalent inhibitor, THZ531. Co-crystallization of THZ531 with CDK12-cyclin K indicates that THZ531 irreversibly targets a cysteine located outside the kinase domain. THZ531 causes a loss of gene expression with concurrent loss of elongating and hyperphosphorylated RNA polymerase II. In particular, THZ531 substantially decreases the expression of DNA damage response genes and key super-enhancer-associated transcription factor genes. Coincident with transcriptional perturbation, THZ531 dramatically induced apoptotic cell death. Small molecules capable of specifically targeting CDK12 and CDK13 may thus help identify cancer subtypes that are particularly dependent on their kinase activities.

The mRNA capping enzyme of Saccharomyces cerevisiae has dual specificity to interact with CTD of RNA Polymerase II

RNA Polymerase II (RNAPII) uniquely possesses an extended carboxy terminal domain (CTD) on its largest subunit, Rpb1, comprising a repetitive Tyr1Ser2Pro3Thr4 Ser5Pro6Ser7 motif with potential phosphorylation sites. The phosphorylation of the CTD serves as a signal for the binding of various transcription regulators for mRNA biogenesis including the mRNA capping complex. In eukaryotes, the 5 prime capping of the nascent transcript is the first detectable mRNA processing event, and is crucial for the productive transcript elongation. The binding of capping enzyme, RNA guanylyltransferases to the transcribing RNAPII is known to be primarily facilitated by the CTD, phosphorylated at Ser5 (Ser5P). Here we report that the Saccharomyces cerevesiae RNA guanylyltransferase (Ceg1) has dual specificity and interacts not only with Ser5P but also with Ser7P of the CTD. The Ser7 of CTD is essential for the unconditional growth and efficient priming of the mRNA capping complex. The Arg159 and Arg185 of Ceg1 are the key residues that interact with the Ser5P, while the Lys175 with Ser7P of CTD. These interactions appear to be in a specific pattern of Ser5PSer7PSer5P in a tri-heptad CTD (YSPTSPPS YSPTSPSP YSPTSPPS) and provide molecular insights into the Ceg1-CTD interaction for mRNA transcription.

Engineered Covalent Inactivation of TFIIH-Kinase Reveals an Elongation Checkpoint and Results in Widespread mRNA Stabilization

During transcription initiation, the TFIIH-kinase Kin28/Cdk7 marks RNA polymerase II (Pol II) by phosphorylating the C-terminal domain (CTD) of its largest subunit. Here we describe a structure-guided chemical approach to covalently and specifically inactivate Kin28 kinase activity in vivo. This method of irreversible inactivation recapitulates both the lethal phenotype and the key molecular signatures that result from genetically disrupting Kin28 function in vivo. Inactivating Kin28 impacts promoter release to differing degrees and reveals a "checkpoint" during the transition to productive elongation. While promoter-proximal pausing is not observed in budding yeast, inhibition of Kin28 attenuates elongation-licensing signals, resulting in Pol II accumulation at the +2 nucleosome and reduced transition to productive elongation. Furthermore, upon inhibition, global stabilization of mRNA masks different degrees of reduction in nascent transcription. This study resolves long-standing controversies on the role of Kin28 in transcription and provides a rational approach to irreversibly inhibit other kinases in vivo.

CDK1-Cyclin B1 Activates RNMT, Coordinating mRNA Cap Methylation with G1 Phase Transcription

The creation of translation-competent mRNA is dependent on RNA polymerase II transcripts being modified by addition of the 7-methylguanosine (m7G) cap. The factors that mediate splicing, nuclear export, and translation initiation are recruited to the transcript via the cap. The cap structure is formed by several activities and completed by RNMT (RNA guanine-7 methyltransferase), which catalyzes N7 methylation of the cap guanosine. We report that CDK1-cyclin B1 phosphorylates the RNMT regulatory domain on T77 during G2/M phase of the cell cycle. RNMT T77 phosphorylation activates the enzyme both directly and indirectly by inhibiting interaction with KPNA2, an RNMT inhibitor. RNMT T77 phosphorylation results in elevated m7G cap methyltransferase activity at the beginning of G1 phase, coordinating mRNA capping with the burst of transcription that occurs following nuclear envelope reformation. RNMT T77 phosphorylation is required for the production of cohort of proteins, and inhibiting T77 phosphorylation reduces the cell proliferation rate.

Molecular basis of RNA guanine-7 methyltransferase (RNMT) activation by RAM

Maturation and translation of mRNA in eukaryotes requires the addition of the 7-methylguanosine cap. In vertebrates, the cap methyltransferase, RNA guanine-7 methyltransferase (RNMT), has an activating subunit, RNMT-Activating Miniprotein (RAM). Here we report the first crystal structure of the human RNMT in complex with the activation domain of RAM. A relatively unstructured and negatively charged RAM binds to a positively charged surface groove on RNMT, distal to the active site. This results in stabilisation of a RNMT lobe structure which co-evolved with RAM and is required for RAM binding. Structure-guided mutagenesis and molecular dynamics simulations reveal that RAM stabilises the structure and positioning of the RNMT lobe and the adjacent α-helix hinge, resulting in optimal positioning of helix A which contacts substrates in the active site. Using biophysical and biochemical approaches, we observe that RAM increases the recruitment of the methyl donor, AdoMet (S-adenosyl methionine), to RNMT. Thus we report the mechanism by which RAM allosterically activates RNMT, allowing it to function as a molecular rheostat for mRNA cap methylation.

Phospho-site mutants of the RNA Polymerase II C-terminal domain alter subtelomeric gene expression and chromatin modification state in fission yeast

Eukaryotic gene expression requires that RNA Polymerase II (RNAP II) gain access to DNA in the context of chromatin. The C-terminal domain (CTD) of RNAP II recruits chromatin modifying enzymes to promoters, allowing for transcription initiation or repression. Specific CTD phosphorylation marks facilitate recruitment of chromatin modifiers, transcriptional regulators, and RNA processing factors during the transcription cycle. However, the readable code for recruiting such factors is still not fully defined and how CTD modifications affect related families of genes or regional gene expression is not well understood. Here, we examine the effects of manipulating the Y₁S₂P₃T₄S₅P₆S₇ heptapeptide repeat of the CTD of RNAP II in Schizosaccharomyces pombe by substituting non-phosphorylatable alanines for Ser2 and/or Ser7 and the phosphomimetic glutamic acid for Ser7. Global gene expression analyses were conducted using splicing-sensitive microarrays and validated via RT-qPCR. The CTD mutations did not affect pre-mRNA splicing or snRNA levels. Rather, the data revealed upregulation of subtelomeric genes and alteration of the repressive histone H3 lysine 9 methylation (H3K9me) landscape. The data further indicate that H3K9me and expression status are not fully correlated, suggestive of CTD-dependent subtelomeric repression mechansims that act independently of H3K9me levels.

Strand-specific, high-resolution mapping of modified RNA polymerase II

Reversible modification of the RNAPII C‐terminal domain links transcription with RNA processing and surveillance activities. To better understand this, we mapped the location of RNAPII carrying the five types of CTD phosphorylation on the RNA transcript, providing strand‐specific, nucleotide‐resolution information, and we used a machine learning‐based approach to define RNAPII states. This revealed enrichment of Ser5P, and depletion of Tyr1P, Ser2P, Thr4P, and Ser7P in the transcription start site (TSS) proximal ~150 nt of most genes, with depletion of all modifications close to the poly(A) site. The TSS region also showed elevated RNAPII relative to regions further 3′, with high recruitment of RNA surveillance and termination factors, and correlated with the previously mapped 3′ ends of short, unstable ncRNA transcripts. A hidden Markov model identified distinct modification states associated with initiating, early elongating and later elongating RNAPII. The initiation state was enriched near the TSS of protein‐coding genes and persisted throughout exon 1 of intron‐containing genes. Notably, unstable ncRNAs apparently failed to transition into the elongation states seen on protein‐coding genes.

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

Specific threonine-4 phosphorylation and function of RNA polymerase II CTD during M phase progression

Dynamic phosphorylation of Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 heptad-repeats in the C-terminal domain (CTD) of the large subunit coordinates progression of RNA polymerase (Pol) II through the transcription cycle. Here, we describe an M phase-specific form of Pol II phosphorylated at Thr4, but not at Tyr1, Ser2, Ser5, and Ser7 residues. Thr4 phosphorylated Pol II binds to centrosomes and midbody and interacts with the Thr4-specific Polo-like kinase 1. Binding of Pol II to centrosomes does not require the CTD but may involve subunits of the non-canonical R2TP-Prefoldin-like complex, which bind to and co-localize with Pol II at centrosomes. CTD Thr4 mutants, but not Ser2 and Ser5 mutants, display severe mitosis and cytokinesis defects characterized by multipolar spindles and polyploid cells. We conclude that proper M phase progression of cells requires binding of Pol II to centrosomes to facilitate regulation of mitosis and cytokinesis in a CTD Thr4-P dependent manner.

Functional interplay between MSL1 and CDK7 controls RNA polymerase II Ser5 phosphorylation

Proper gene expression requires coordinated interplay among transcriptional coactivators, transcription factors and the general transcription machinery. We report here that MSL1, a central component of the dosage compensation complex in Drosophila melanogaster and Drosophila virilis, displays evolutionarily conserved sex-independent binding to promoters. Genetic and biochemical analyses reveal a functional interaction of MSL1 with CDK7, a subunit of the Cdk-activating kinase (CAK) complex of the general transcription factor TFIIH. Importantly, MSL1 depletion leads to decreased phosphorylation of Ser5 of RNA polymerase II. In addition, we demonstrate that MSL1 is a phosphoprotein, and transgenic flies expressing MSL1 phosphomutants show mislocalization of the histone acetyltransferase MOF and histone H4 K16 acetylation, thus ultimately causing male lethality due to a failure of dosage compensation. We propose that, by virtue of its interaction with components of the general transcription machinery, MSL1 exists in different phosphorylation states, thereby modulating transcription in flies.

Histone H2B ubiquitylation represses gametogenesis by opposing RSC-dependent chromatin remodeling at the ste11 master regulator locus

In fission yeast, the ste11 gene encodes the master regulator initiating the switch from vegetative growth to gametogenesis. In a previous paper, we showed that the methylation of H3K4 and consequent promoter nucleosome deacetylation repress ste11 induction and cell differentiation (Materne et al., 2015) but the regulatory steps remain poorly understood. Here we report a genetic screen that highlighted H2B deubiquitylation and the RSC remodeling complex as activators of ste11 expression. Mechanistic analyses revealed more complex, opposite roles of H2Bubi at the promoter where it represses expression, and over the transcribed region where it sustains it. By promoting H3K4 methylation at the promoter, H2Bubi initiates the deacetylation process, which decreases chromatin remodeling by RSC. Upon induction, this process is reversed and efficient NDR (nucleosome depleted region) formation leads to high expression. Therefore, H2Bubi represses gametogenesis by opposing the recruitment of RSC at the promoter of the master regulator ste11 gene.

DOI:http://dx.doi.org/10.7554/eLife.13500.001

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.