A quantitative, high-throughput reverse genetic screen reveals novel connections between Pre-mRNA splicing and 5' and 3' end transcript determinants

DEAD-box ATPase Dbp2 is the key enzyme in an mRNP assembly checkpoint at the 3'-end of genes and involved in the recycling of cleavage factors

mRNA biogenesis in the eukaryotic nucleus is a highly complex process. The numerous RNA processing steps are tightly coordinated to ensure that only fully processed transcripts are released from chromatin for export from the nucleus. Here, we present the hypothesis that fission yeast Dbp2, a ribonucleoprotein complex (RNP) remodelling ATPase of the DEAD-box family, is the key enzyme in an RNP assembly checkpoint at the 3'-end of genes. We show that Dbp2 interacts with the cleavage and polyadenylation complex (CPAC) and localises to cleavage bodies, which are enriched for 3'-end processing factors and proteins involved in nuclear RNA surveillance. Upon loss of Dbp2, 3'-processed, polyadenylated RNAs accumulate on chromatin and in cleavage bodies, and CPAC components are depleted from the soluble pool. Under these conditions, cells display an increased likelihood to skip polyadenylation sites and a delayed transcription termination, suggesting that levels of free CPAC components are insufficient to maintain normal levels of 3'-end processing. Our data support a model in which Dbp2 is the active component of an mRNP remodelling checkpoint that licenses RNA export and is coupled to CPAC release.

Yeast Small Ubiquitin-Like Modifier (SUMO) Protease Ulp2 is Involved in RNA Splicing

In eukaryotes, RNA splicing, an essential biological process, is crucial for precise gene expression. Inaccurate RNA splicing can cause aberrant mRNA production, disrupting protein synthesis. To regulate splicing efficiency, some splicing factors are reported to undergo Ubiquitin-like Modifier (SUMO)ylation. Our data indicate that in Saccharomyces cerevisiae, the SUMO protease, Ulp2, is involved in splicing. In the ulp2Δ mutant, some ribosomal protein (RP) transcripts exhibited a significant increase in the levels of intron-containing pre-mRNA because of improper splicing. Moreover, we confirmed Ulp2 protein binding to the intronic regions of RP genes. These findings highlight a critical Ulp2 role in RP transcript splicing.

Regulation of alternative splicing and polyadenylation in neurons

Cell-type-specific gene expression is a fundamental feature of multicellular organisms and is achieved by combinations of regulatory strategies. Although cell-restricted transcription is perhaps the most widely studied mechanism, co-transcriptional and post-transcriptional processes are also central to the spatiotemporal control of gene functions. One general category of expression control involves the generation of multiple transcript isoforms from an individual gene, whose balance and cell specificity are frequently tightly regulated via diverse strategies. The nervous system makes particularly extensive use of cell-specific isoforms, specializing the neural function of genes that are expressed more broadly. Here, we review regulatory strategies and RNA-binding proteins that direct neural-specific isoform processing. These include various classes of alternative splicing and alternative polyadenylation events, both of which broadly diversify the neural transcriptome. Importantly, global alterations of splicing and alternative polyadenylation are characteristic of many neural pathologies, and recent genetic studies demonstrate how misregulation of individual neural isoforms can directly cause mutant phenotypes.

Relevance and Regulation of Alternative Splicing in Plant Heat Stress Response: Current Understanding and Future Directions

Alternative splicing (AS) is a major mechanism for gene expression in eukaryotes, increasing proteome diversity but also regulating transcriptome abundance. High temperatures have a strong impact on the splicing profile of many genes and therefore AS is considered as an integral part of heat stress response. While many studies have established a detailed description of the diversity of the RNAome under heat stress in different plant species and stress regimes, little is known on the underlying mechanisms that control this temperature-sensitive process. AS is mainly regulated by the activity of splicing regulators. Changes in the abundance of these proteins through transcription and AS, post-translational modifications and interactions with exonic and intronic cis-elements and core elements of the spliceosomes modulate the outcome of pre-mRNA splicing. As a major part of pre-mRNAs are spliced co-transcriptionally, the chromatin environment along with the RNA polymerase II elongation play a major role in the regulation of pre-mRNA splicing under heat stress conditions. Despite its importance, our understanding on the regulation of heat stress sensitive AS in plants is scarce. In this review, we summarize the current status of knowledge on the regulation of AS in plants under heat stress conditions. We discuss possible implications of different pathways based on results from non-plant systems to provide a perspective for researchers who aim to elucidate the molecular basis of AS under high temperatures.

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

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

The histone variant H2A.Z promotes efficient cotranscriptional splicing in S. cerevisiae

In eukaryotes, a dynamic ribonucleic protein machine known as the spliceosome catalyzes the removal of introns from premessenger RNA (pre-mRNA). Recent studies show the processes of RNA synthesis and RNA processing to be spatio-temporally coordinated, indicating that RNA splicing takes place in the context of chromatin. H2A.Z is a highly conserved histone variant of the canonical histone H2A. In Saccharomyces cerevisiae, H2A.Z is deposited into chromatin by the SWR-C complex, is found near the 5' ends of protein-coding genes, and has been implicated in transcription regulation. Here we show that splicing of intron-containing genes in cells lacking H2A.Z is impaired, particularly under suboptimal splicing conditions. Cells lacking H2A.Z are especially dependent on a functional U2 snRNP (small nuclear RNA [snRNA] plus associated proteins), as H2A.Z shows extensive genetic interactions with U2 snRNP-associated proteins, and RNA sequencing (RNA-seq) reveals that introns with nonconsensus branch points are particularly sensitive to H2A.Z loss. Consistently, H2A.Z promotes efficient spliceosomal rearrangements involving the U2 snRNP, as H2A.Z loss results in persistent U2 snRNP association and decreased recruitment of downstream snRNPs to nascent RNA. H2A.Z impairs transcription elongation, suggesting that spliceosome rearrangements are tied to H2A.Z's role in elongation. Depletion of disassembly factor Prp43 suppresses H2A.Z-mediated splice defects, indicating that, in the absence of H2A.Z, stalled spliceosomes are disassembled, and unspliced RNAs are released. Together, these data demonstrate that H2A.Z is required for efficient pre-mRNA splicing and indicate a role for H2A.Z in coordinating the kinetics of transcription elongation and splicing.

Multilayered Control of Alternative Splicing Regulatory Networks by Transcription Factors

Networks of coordinated alternative splicing (AS) events play critical roles in development and disease. However, a comprehensive knowledge of the factors that regulate these networks is lacking. We describe a high-throughput system for systematically linking trans-acting factors to endogenous RNA regulatory events. Using this system, we identify hundreds of factors associated with diverse regulatory layers that positively or negatively control AS events linked to cell fate. Remarkably, more than one-third of the regulators are transcription factors. Further analyses of the zinc finger protein Zfp871 and BTB/POZ domain transcription factor Nacc1, which regulate neural and stem cell AS programs, respectively, reveal roles in controlling the expression of specific splicing regulators. Surprisingly, these proteins also appear to regulate target AS programs via binding RNA. Our results thus uncover a large "missing cache" of splicing regulators among annotated transcription factors, some of which dually regulate AS through direct and indirect mechanisms.

Bdf1 Bromodomains Are Essential for Meiosis and the Expression of Meiotic-Specific Genes

Bromodomain and Extra-terminal motif (BET) proteins play a central role in transcription regulation and chromatin signalling pathways. They are present in unicellular eukaryotes and in this study, the role of the BET protein Bdf1 has been explored in Saccharomyces cerevisiae. Mutation of Bdf1 bromodomains revealed defects on both the formation of spores and the meiotic progression, blocking cells at the exit from prophase, before the first meiotic division. This phenotype is associated with a massive deregulation of the transcription of meiotic genes and Bdf1 bromodomains are required for appropriate expression of the key meiotic transcription factor NDT80 and almost all the Ndt80-inducible genes, including APC complex components. Bdf1 notably accumulates on the promoter of Ndt80 and its recruitment is dependent on Bdf1 bromodomains. In addition, the ectopic expression of NDT80 during meiosis partially bypasses this dependency. Finally, purification of Bdf1 partners identified two independent complexes with Bdf2 or the SWR complex, neither of which was required to complete sporulation. Taken together, our results unveil a new role for Bdf1 –working independently from its predominant protein partners Bdf2 and the SWR1 complex–as a regulator of meiosis-specific genes.

Chromatin modifying proteins play a central role in transcription regulation and chromatin signalling. In this study we investigated the functional role of the bromodomains of the chromatin protein Bdf1 during yeast gametogenesis. Our results show that the bromodomains of Bdf1 are essential for meiotic progression and the formation of mature spores. Bdf1 bromodomains are required for the expression of key meiotic genes and the master regulator NDT80. Forced expression of NDT80 can partially rescue the formation of spores when Bdf1 bromodomains are mutated. The results presented here indicate that Bdf1 forms two exclusive complexes, with Bdf2 or with the SWR complex. However, none of these complexes are required for sporulation progression. To conclude, our findings suggest that Bdf1 is a new regulator of the meiotic transcription program and of the expression of the master regulator NDT80.

The bromodomain protein BRD4 regulates splicing during heat shock

The cellular response to heat stress is an ancient and evolutionarily highly conserved defence mechanism characterised by the transcriptional up-regulation of cyto-protective genes and a partial inhibition of splicing. These features closely resemble the proteotoxic stress response during tumor development. The bromodomain protein BRD4 has been identified as an integral member of the oxidative stress as well as of the inflammatory response, mainly due to its role in the transcriptional regulation process. In addition, there are also several lines of evidence implicating BRD4 in the splicing process. Using RNA-sequencing we found a significant increase in splicing inhibition, in particular intron retentions (IR), following heat treatment in BRD4-depleted cells. This leads to a decrease of mRNA abundancy of the affected transcripts, most likely due to premature termination codons. Subsequent experiments revealed that BRD4 interacts with the heat shock factor 1 (HSF1) such that under heat stress BRD4 is recruited to nuclear stress bodies and non-coding SatIII RNA transcripts are up-regulated. These findings implicate BRD4 as an important regulator of splicing during heat stress. Our data which links BRD4 to the stress induced splicing process may provide novel mechanisms of BRD4 inhibitors in regard to anti-cancer therapies.

Interconnections Between RNA-Processing Pathways Revealed by a Sequencing-Based Genetic Screen for Pre-mRNA Splicing Mutants in Fission Yeast

Pre-mRNA splicing is an essential component of eukaryotic gene expression and is highly conserved from unicellular yeasts to humans. Here, we present the development and implementation of a sequencing-based reverse genetic screen designed to identify nonessential genes that impact pre-mRNA splicing in the fission yeast Schizosaccharomyces pombe, an organism that shares many of the complex features of splicing in higher eukaryotes. Using a custom-designed barcoding scheme, we simultaneously queried ∼3000 mutant strains for their impact on the splicing efficiency of two endogenous pre-mRNAs. A total of 61 nonessential genes were identified whose deletions resulted in defects in pre-mRNA splicing; enriched among these were factors encoding known or predicted components of the spliceosome. Included among the candidates identified here are genes with well-characterized roles in other RNA-processing pathways, including heterochromatic silencing and 3ʹ end processing. Splicing-sensitive microarrays confirm broad splicing defects for many of these factors, revealing novel functional connections between these pathways.

Comprehensive RNA Polymerase II Interactomes Reveal Distinct and Varied Roles for Each Phospho-CTD Residue

Transcription controls splicing and other gene regulatory processes, yet mechanisms remain obscure due to our fragmented knowledge of the molecular connections between the dynamically phosphorylated RNA polymerase II (Pol II) C-terminal domain (CTD) and regulatory factors. By systematically isolating phosphorylation states of the CTD heptapeptide repeat (Y1S2P3T4S5P6S7), we identify hundreds of protein factors that are differentially enriched, revealing unappreciated connections between the Pol II CTD and co-transcriptional processes. These data uncover a role for threonine-4 in 3' end processing through control of the transition between cleavage and termination. Furthermore, serine-5 phosphorylation seeds spliceosomal assembly immediately downstream of 3' splice sites through a direct interaction with spliceosomal subcomplex U1. Strikingly, threonine-4 phosphorylation also impacts splicing by serving as a mark of co-transcriptional spliceosome release and ensuring efficient post-transcriptional splicing genome-wide. Thus, comprehensive Pol II interactomes identify the complex and functional connections between transcription machinery and other gene regulatory complexes.

TINTIN, at the interface of chromatin, transcription elongation, and mRNA processing

Recent work including high-resolution genome-wide analysis uncovered a new trimeric complex involved in transcription elongation, both as an integral part of the NuA4 histone acetyltransferase and as an independent functional entity. The complex is conserved in eukaryotes and is named TINTIN, for Trimer Independent of NuA4 for transcription Interactions with Nucleosomes. This point of view covers the current knowledge regarding TINTIN's function in modulating chromatin structure and influencing transcription elongation in eukaryotes. It also points to several physical and functional links to co-transcriptional processes, including interactions with the mRNA splicing machinery and the nuclear exosome.

Histone H3K36 methylation regulates pre-mRNA splicing in Saccharomyces cerevisiae

Co-transcriptional splicing takes place in the context of a highly dynamic chromatin architecture, yet the role of chromatin restructuring in coordinating transcription with RNA splicing has not been fully resolved. To further define the contribution of histone modifications to pre-mRNA splicing in Saccharomyces cerevisiae, we probed a library of histone point mutants using a reporter to monitor pre-mRNA splicing. We found that mutation of H3 lysine 36 (H3K36) - a residue methylated by Set2 during transcription elongation - exhibited phenotypes similar to those of pre-mRNA splicing mutants. We identified genetic interactions between genes encoding RNA splicing factors and genes encoding the H3K36 methyltransferase Set2 and the demethylase Jhd1 as well as point mutations of H3K36 that block methylation. Consistent with the genetic interactions, deletion of SET2, mutations modifying the catalytic activity of Set2 or H3K36 point mutations significantly altered expression of our reporter and reduced splicing of endogenous introns. These effects were dependent on the association of Set2 with RNA polymerase II and H3K36 dimethylation. Additionally, we found that deletion of SET2 reduces the association of the U2 and U5 snRNPs with chromatin. Thus, our study provides the first evidence that H3K36 methylation plays a role in co-transcriptional RNA splicing in yeast.

An Updated Collection of Sequence Barcoded Temperature-Sensitive Alleles of Yeast Essential Genes

Systematic analyses of essential gene function using mutant collections in Saccharomyces cerevisiae have been conducted using collections of heterozygous diploids, promoter shut-off alleles, through alleles with destabilized mRNA, destabilized protein, or bearing mutations that lead to a temperature-sensitive (ts) phenotype. We previously described a method for construction of barcoded ts alleles in a systematic fashion. Here we report the completion of this collection of alleles covering 600 essential yeast genes. This resource covers a larger gene repertoire than previous collections and provides a complementary set of strains suitable for single gene and genomic analyses. We use deep sequencing to characterize the amino acid changes leading to the ts phenotype in half of the alleles. We also use high-throughput approaches to describe the relative ts behavior of the alleles. Finally, we demonstrate the experimental usefulness of the collection in a high-content, functional genomic screen for ts alleles that increase spontaneous P-body formation. By increasing the number of alleles and improving the annotation, this ts collection will serve as a community resource for probing new aspects of biology for essential yeast genes.

Cwc21p promotes the second step conformation of the spliceosome and modulates 3' splice site selection

Pre-mRNA splicing involves two transesterification steps catalyzed by the spliceosome. How RNA substrates are positioned in each step and the molecular rearrangements involved, remain obscure. Here, we show that mutations in PRP16, PRP8, SNU114 and the U5 snRNA that affect this process interact genetically with CWC21, that encodes the yeast orthologue of the human SR protein, SRm300/SRRM2. Our microarray analysis shows changes in 3′ splice site selection at elevated temperature in a subset of introns in cwc21Δ cells. Considering all the available data, we propose a role for Cwc21p positioning the 3′ splice site at the transition to the second step conformation of the spliceosome, mediated through its interactions with the U5 snRNP. This suggests a mechanism whereby SRm300/SRRM2, might influence splice site selection in human cells.

Structural basis for recognition of intron branchpoint RNA by yeast Msl5 and selective effects of interfacial mutations on splicing of yeast pre-mRNAs

Saccharomyces cerevisiae Msl5 orchestrates spliceosome assembly by binding the intron branchpoint sequence 5'-UACUAAC and, with its heterodimer partner protein Mud2, establishing cross intron-bridging interactions with the U1 snRNP at the 5' splice site. Here we define the central Msl5 KH-QUA2 domain as sufficient for branchpoint RNA recognition. The 1.8 Å crystal structure of Msl5-(KH-QUA2) bound to the branchpoint highlights an extensive network of direct and water-mediated protein-RNA and intra-RNA atomic contacts at the interface that illuminate how Msl5 recognizes each nucleobase of the UACUAAC element. The Msl5 structure rationalizes a large body of mutational data and inspires new functional studies herein, which reveal how perturbations of the Msl5·RNA interface impede the splicing of specific yeast pre-mRNAs. We also identify interfacial mutations in Msl5 that bypass the essentiality of Sub2, a DExD-box ATPase implicated in displacing Msl5 from the branchpoint in exchange for the U2 snRNP. These studies establish an atomic resolution framework for understanding splice site selection and early spliceosome dynamics.

Eaf5/7/3 form a functionally independent NuA4 submodule linked to RNA polymerase II-coupled nucleosome recycling

The NuA4 histone acetyltransferase complex is required for gene regulation, cell cycle progression, and DNA repair. Dissection of the 13-subunit complex reveals that the Eaf7 subunit bridges Eaf5 with Eaf3, a H3K36me3-binding chromodomain protein, and this Eaf5/7/3 trimer is anchored to NuA4 through Eaf5. This trimeric subcomplex represents a functional module, and a large portion exists in a native form outside the NuA4 complex. Gene-specific and genome-wide location analyses indicate that Eaf5/7/3 correlates with transcription activity and is enriched over the coding region. In agreement with a role in transcription elongation, the Eaf5/7/3 trimer interacts with phosphorylated RNA polymerase II and helps its progression. Loss of Eaf5/7/3 partially suppresses intragenic cryptic transcription arising in set2 mutants, supporting a role in nucleosome destabilization. On the other hand, loss of the trimer leads to an increase of replication-independent histone exchange over the coding region of transcribed genes. Taken together, these results lead to a model where Eaf5/7/3 associates with elongating polymerase to promote the disruption of nucleosomes in its path, but also their refolding in its wake.

Adventures in time and space: splicing efficiency and RNA polymerase II elongation rate

Control of pre-mRNA splicing is a critical part of the eukaryotic gene expression process. Extensive evidence indicates that transcription and splicing are spatiotemporally coordinated and that most splicing events occur co-transcriptionally. A kinetic coupling model has been proposed in metazoans to describe how changing RNA Polymerase II (RNAPII) elongation rate can impact which alternative splice sites are used. In Saccharomyces cerevisiae, in which most spliced genes have only a single intron and splice sites adhere to a strong consensus sequence, we recently observed that splicing efficiency was sensitive to mutations in RNAPII that increase or decrease its elongation rate. Our data revealed that RNAPII speed and splicing efficiency are generally anti-correlated: at many genes, increased elongation rate caused decreased splicing efficiency, while decreased elongation rate increased splicing efficiency. An improved splicing phenotype was also observed upon deletion of SUB1, a condition in which elongation rate is slowed. We discuss these data in the context of a growing field and expand the kinetic coupling model to apply to both alternative splicing and splicing efficiency.

A functional screen reveals an extensive layer of transcriptional and splicing control underlying RAS/MAPK signaling in Drosophila

A global RNAi screening approach in Drosophila cells identifies a large group of transcription and splicing factors that modulate RAS/MAPK signaling by altering the expression of MAPK.

The small GTPase RAS is among the most prevalent oncogenes. The evolutionarily conserved RAF-MEK-MAPK module that lies downstream of RAS is one of the main conduits through which RAS transmits proliferative signals in normal and cancer cells. Genetic and biochemical studies conducted over the last two decades uncovered a small set of factors regulating RAS/MAPK signaling. Interestingly, most of these were found to control RAF activation, thus suggesting a central regulatory role for this event. Whether additional factors are required at this level or further downstream remains an open question. To obtain a comprehensive view of the elements functionally linked to the RAS/MAPK cascade, we used a quantitative assay in Drosophila S2 cells to conduct a genome-wide RNAi screen for factors impacting RAS-mediated MAPK activation. The screen led to the identification of 101 validated hits, including most of the previously known factors associated to this pathway. Epistasis experiments were then carried out on individual candidates to determine their position relative to core pathway components. While this revealed several new factors acting at different steps along the pathway—including a new protein complex modulating RAF activation—we found that most hits unexpectedly work downstream of MEK and specifically influence MAPK expression. These hits mainly consist of constitutive splicing factors and thereby suggest that splicing plays a specific role in establishing MAPK levels. We further characterized two representative members of this group and surprisingly found that they act by regulating mapk alternative splicing. This study provides an unprecedented assessment of the factors modulating RAS/MAPK signaling in Drosophila. In addition, it suggests that pathway output does not solely rely on classical signaling events, such as those controlling RAF activation, but also on the regulation of MAPK levels. Finally, it indicates that core splicing components can also specifically impact alternative splicing.

The RAS/MAPK pathway is a cornerstone of the cell proliferation signaling apparatus. It has a notable involvement in cancer as mutations in the components of the pathway are associated with aberrant proliferation. Previous work has focused predominantly on post-translational regulation of RAS/MAPK signaling such that a large and intricate network of factors is now known to act on core pathway components. However, regulation at the pre-translational level has not been examined nearly as extensively and is comparatively poorly understood. In this study, we used an unbiased and global screening approach to survey the Drosophila genome—using Drosophila cultured cells—for novel regulators of this pathway. Surprisingly, a majority of our hits were associated to either transcription or mRNA splicing. We used a series of secondary screening assays to determine which part of the RAS/MAPK pathway these candidates target. We found that these factors were not equally distributed along the pathway, but rather converged predominantly on mapk mRNA expression and processing. Our findings raise the intriguing possibility that regulation of mapk transcript production is a key step for a diverse set of regulatory inputs, and may play an important part in RAS/MAPK signaling dynamics.

H2B ubiquitylation modulates spliceosome assembly and function in budding yeast

BACKGROUND INFORMATION

Commitment to splicing occurs co-transcriptionally, but a major unanswered question is the extent to which various modifications of chromatin, the template for transcription in vivo, contribute to the regulation of splicing.

RESULTS

Here, we perform genome-wide analyses showing that inhibition of specific marks - H2B ubiquitylation, H3K4 methylation and H3K36 methylation - perturbs splicing in budding yeast, with each modification exerting gene-specific effects. Furthermore, semi-quantitative mass spectrometry on purified nuclear mRNPs and chromatin immunoprecipitation analysis on intron-containing genes indicated that H2B ubiquitylation, but not Set1-, Set2- or Dot1-dependent H3 methylation, stimulates recruitment of the early splicing factors, namely U1 and U2 snRNPs, onto nascent RNAs.

CONCLUSIONS

These results suggest that histone modifications impact splicing of distinct subsets of genes using distinct pathways.

Application of Titration-Based Screening for the Rapid Pilot Testing of High-Throughput Assays

Pilot testing of an assay intended for high-throughput screening (HTS) with small compound sets is a necessary but often time-consuming step in the validation of an assay protocol. When the initial testing concentration is less than optimal, this can involve iterative testing at different concentrations to further evaluate the pilot outcome, which can be even more time-consuming. Quantitative HTS (qHTS) enables flexible and rapid collection of assay performance statistics, hits at different concentrations, and concentration-response curves in a single experiment. Here we describe the qHTS process for pilot testing in which eight-point concentration-response curves are produced using an interplate asymmetric dilution protocol in which the first four concentrations are used to represent the range of typical HTS screening concentrations and the last four concentrations are added for robust curve fitting to determine potency/efficacy values. We also describe how these data can be analyzed to predict the frequency of false-positives, false-negatives, hit rates, and confirmation rates for the HTS process as a function of screening concentration. By taking into account the compound pharmacology, this pilot-testing paradigm enables rapid assessment of the assay performance and choosing the optimal concentration for the large-scale HTS in one experiment.

Spliceosome-mediated decay (SMD) regulates expression of nonintronic genes in budding yeast

We uncovered a novel role for the spliceosome in regulating mRNA expression levels that involves splicing coupled to RNA decay, which we refer to as spliceosome-mediated decay (SMD). Our transcriptome-wide studies identified numerous transcripts that are not known to have introns but are spliced by the spliceosome at canonical splice sites in Saccharomyces cerevisiae. Products of SMD are primarily degraded by the nuclear RNA surveillance machinery. We demonstrate that SMD can significantly down-regulate mRNA levels; splicing at canonical splice sites in the bromodomain factor 2 (BDF2) transcript reduced transcript levels roughly threefold by generating unstable products that are rapidly degraded by the nuclear surveillance machinery. Regulation of BDF2 mRNA levels by SMD requires Bdf1, a functionally redundant Bdf2 paralog that plays a role in recruiting the spliceosome to the BDF2 mRNA. Interestingly, mutating BDF2 5' splice site and branch point consensus sequences partially suppresses the bdf1Δ temperature-sensitive phenotype, suggesting that maintaining proper levels of Bdf2 via SMD is biologically important. We propose that the spliceosome can also repress protein-coding gene expression by promoting nuclear turnover of spliced RNA products and provide an insight for coordinated regulation of Bdf1 and Bdf2 levels in the cell.

Splicing functions and global dependency on fission yeast slu7 reveal diversity in spliceosome assembly

The multiple short introns in Schizosaccharomyces pombe genes with degenerate cis sequences and atypically positioned polypyrimidine tracts make an interesting model to investigate canonical and alternative roles for conserved splicing factors. Here we report functions and interactions of the S. pombe slu7(+) (spslu7(+)) gene product, known from Saccharomyces cerevisiae and human in vitro reactions to assemble into spliceosomes after the first catalytic reaction and to dictate 3' splice site choice during the second reaction. By using a missense mutant of this essential S. pombe factor, we detected a range of global splicing derangements that were validated in assays for the splicing status of diverse candidate introns. We ascribe widespread, intron-specific SpSlu7 functions and have deduced several features, including the branch nucleotide-to-3' splice site distance, intron length, and the impact of its A/U content at the 5' end on the intron's dependence on SpSlu7. The data imply dynamic substrate-splicing factor relationships in multiintron transcripts. Interestingly, the unexpected early splicing arrest in spslu7-2 revealed a role before catalysis. We detected a salt-stable association with U5 snRNP and observed genetic interactions with spprp1(+), a homolog of human U5-102k factor. These observations together point to an altered recruitment and dependence on SpSlu7, suggesting its role in facilitating transitions that promote catalysis, and highlight the diversity in spliceosome assembly.

RRP1B is a metastasis modifier that regulates the expression of alternative mRNA isoforms through interactions with SRSF1

RRP1B (ribosomal RNA processing 1 homolog B) was first identified as a metastasis susceptibility gene in breast cancer through its ability to modulate gene expression in a manner that can be used to accurately predict prognosis in breast cancer. However, the mechanism(s) by which RRP1B modulates gene expression is currently unclear. Many RRP1B binding candidates are involved in alternative splicing, a mechanism of gene expression regulation that is increasingly recognized to be involved in cancer progression and metastasis. One such target is SRSF1 (SF2/ASF), an essential splicing regulator that also functions as an oncoprotein. Earlier studies demonstrated that splicing and transcription occur concurrently and are coupled processes. Given that RRP1B regulates transcriptional activity, we hypothesized that RRP1B also regulates the expression of alternative mRNA isoforms through its interaction with SRSF1. Interaction between RRP1B and SRSF1 was verified by co-immunoprecipitation and co-immunofluorescence. Treatment of cells with transcriptional inhibitors significantly increased this interaction, demonstrating that the association of these two proteins is transcriptionally regulated. To assess the role of RRP1B in the regulation of alternative isoform expression, RNA-seq data were generated from control and Rrp1b-knockdown cells. Knockdown of Rrp1b induced a significant change in isoform expression in over 600 genes compared to control cell lines. This was verified by qRT-PCR using isoform-specific primers. Pathway enrichment analyses identified cell cycle and checkpoint regulation to be those most affected by Rrp1b knockdown. These data suggest that RRP1B suppresses metastatic progression by altering the transcriptome through its interaction with splicing regulators such as SRSF1.

Perceiving the epigenetic landscape through histone readers

Post-translational modifications (PTMs) of histones provide a fine-tuned mechanism for regulating chromatin structure and dynamics. PTMs can alter direct interactions between histones and DNA and serve as docking sites for protein effectors, or readers, of these PTMs. Binding of the readers recruits or stabilizes various components of the nuclear signaling machinery at specific genomic sites, mediating fundamental DNA-templated processes, including gene transcription and DNA recombination, replication and repair. In this review, we highlight the latest advances in characterizing histone-binding mechanisms and identifying new epigenetic readers and summarize the functional significance of PTM recognition.

RNA helicases in splicing

In eukaryotic cells, introns are spliced from pre-mRNAs by the spliceosome. Both the composition and the structure of the spliceosome are highly dynamic, and eight DExD/H RNA helicases play essential roles in controlling conformational rearrangements. There is evidence that the various helicases are functionally and physically connected with each other and with many other factors in the spliceosome. Understanding the dynamics of those interactions is essential to comprehend the mechanism and regulation of normal as well as of pathological splicing. This review focuses on recent advances in the characterization of the splicing helicases and their interactions, and highlights the deep integration of splicing helicases in global mRNP biogenesis pathways.

The yeast SR-like protein Npl3 links chromatin modification to mRNA processing

Eukaryotic gene expression involves tight coordination between transcription and pre–mRNA splicing; however, factors responsible for this coordination remain incompletely defined. Here, we explored the genetic, functional, and biochemical interactions of a likely coordinator, Npl3, an SR-like protein in Saccharomyces cerevisiae that we recently showed is required for efficient co-transcriptional recruitment of the splicing machinery. We surveyed the NPL3 genetic interaction space and observed a significant enrichment for genes involved in histone modification and chromatin remodeling. Specifically, we found that Npl3 genetically interacts with both Bre1, which mono-ubiquitinates histone H2B as part of the RAD6 Complex, and Ubp8, the de-ubiquitinase of the SAGA Complex. In support of these genetic data, we show that Bre1 physically interacts with Npl3 in an RNA–independent manner. Furthermore, using a genome-wide splicing microarray, we found that the known splicing defect of a strain lacking Npl3 is exacerbated by deletion of BRE1 or UBP8, a phenomenon phenocopied by a point mutation in H2B that abrogates ubiquitination. Intriguingly, even in the presence of wild-type NPL3, deletion of BRE1 exhibits a mild splicing defect and elicits a growth defect in combination with deletions of early and late splicing factors. Taken together, our data reveal a connection between Npl3 and an extensive array of chromatin factors and describe an unanticipated functional link between histone H2B ubiquitination and pre–mRNA splicing.

Pre-messenger RNA splicing is the process by which an intron is identified and removed from a transcript and the protein-coding exons are ligated together. It is carried out by the spliceosome, a large and dynamic molecular machine that catalyzes the splicing reaction. It is now apparent that most splicing occurs while the transcript is still engaged with RNA polymerase, implying that the biologically relevant splicing substrate is chromatin-associated. Here, we used a genetic approach to understand which factors participate in the coordination of transcription and splicing. Having recently shown that the Npl3 protein is involved in the recruitment of splicing factors to chromatin-associated transcripts, we performed a systematic screen for genetically interacting factors. Interestingly, we identified factors that influence the ubiquitin modification of histone H2B, a mark involved in transcription initiation and elongation. We show that disruption of the H2B ubiquitination/de-ubiquitination cycle results in defects in splicing, particularly in the absence of Npl3. Furthermore, the ubiquitin ligase, Bre1, shows genetic interactions with other, more canonical spliceosomal factors. Taken together with the myriad Npl3 interaction partners we found, our data suggest an extensive cross-talk between the spliceosome and chromatin.

Alternative transcription and alternative splicing in cancer

In recent years, the notion of "one gene makes one protein that functions in one signaling pathway" in mammalian cells has been shown to be overly simplistic. Recent genome-wide studies suggest that at least half of the human genes, including many therapeutic target genes, produce multiple protein isoforms through alternative splicing and alternative usage of transcription initiation and/or termination. For example, alternative splicing of the vascular endothelial growth factor gene (VEGFA) produces multiple protein isoforms, which display either pro-angiogenic or anti-angiogenic activities. Similarly, for the majority of human genes, the inclusion or exclusion of exonic sequences enhances the generation of transcript variants and/or protein isoforms that can vary in structure and functional properties. Many of the isoforms produced in this manner are tightly regulated during normal development but are misregulated in cancer cells. Altered expression of transcript variants and protein isoforms for numerous genes is linked with disease and its prognosis, and cancer cells manipulate regulatory mechanisms to express specific isoforms that confer drug resistance and survival advantages. Emerging insights indicate that modulating the expression of transcript and protein isoforms of a gene may hold the key to impeding tumor growth and act as a model for efficient targeting of disease-associated genes at the isoform level. This review highlights the role and regulation of alternative transcription and splicing mechanisms in generating the transcriptome, and the misuse and diagnostic/prognostic potential of alternative transcription and splicing in cancer.