L30 binds the nascent RPL30 transcript to repress U2 snRNP recruitment

Mechanisms and regulation of spliceosome-mediated pre-mRNA splicing in Saccharomyces cerevisiae

Pre-mRNA splicing, the removal of introns and ligation of flanking exons, is a crucial step in eukaryotic gene expression. The spliceosome, a macromolecular complex made up of five small nuclear RNAs (snRNAs) and dozens of proteins, assembles on introns via a complex pathway before catalyzing the two transesterification reactions necessary for splicing. All of these steps have the potential to be highly regulated to ensure correct mRNA isoform production for proper cellular function. While Saccharomyces cerevisiae (yeast) has a limited set of intron-containing genes, many of these genes are highly expressed, resulting in a large number of transcripts in a cell being spliced. As a result, splicing regulation is of critical importance for yeast. Just as in humans, yeast splicing can be influenced by protein components of the splicing machinery, structures and properties of the pre-mRNA itself, or by the action of trans-acting factors. It is likely that further analysis of the mechanisms and pathways of splicing regulation in yeast can reveal general principles applicable to other eukaryotes. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing.

pTDP-43 levels correlate with cell type specific molecular alterations in the prefrontal cortex of C9orf72 ALS/FTD patients

Repeat expansions in the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis and familial frontotemporal dementia (ALS/FTD). To identify molecular defects that take place in the dorsolateral frontal cortex of patients with C9orf72 ALS/FTD, we compared healthy controls with C9orf72 ALS/FTD donor samples staged based on the levels of cortical phosphorylated TAR DNA binding protein (pTDP-43), a neuropathological hallmark of disease progression. We identified distinct molecular changes in different cell types that take place during FTD development. Loss of neurosurveillance microglia and activation of the complement cascade take place early, when pTDP-43 aggregates are absent or very low, and become more pronounced in late stages, suggesting an initial involvement of microglia in disease progression. Reduction of layer 2-3 cortical projection neurons with high expression of CUX2/LAMP5 also occurs early, and the reduction becomes more pronounced as pTDP-43 accumulates. Several unique features were observed only in samples with high levels of pTDP-43, including global alteration of chromatin accessibility in oligodendrocytes, microglia, and astrocytes; higher ratios of premature oligodendrocytes; increased levels of the noncoding RNA NEAT1 in astrocytes and neurons, and higher amount of phosphorylated ribosomal protein S6. Our findings reveal previously unknown progressive functional changes in major cell types found in the frontal cortex of C9orf72 ALS/FTD patients that shed light on the mechanisms underlying the pathology of this disease.

The non-template functions of helper virus RNAs create optimal replication conditions to enhance the proliferation of satellite RNAs

As a type of parasitic agent, satellite RNAs (satRNAs) rely on cognate helper viruses to achieve their replication and transmission. During the infection of satRNAs, helper virus RNAs serve as templates for synthesizing viral proteins, including the replication proteins essential for satRNA replication. However, the role of non-template functions of helper virus RNAs in satRNA replication remains unexploited. Here we employed the well-studied model that is composed of cucumber mosaic virus (CMV) and its associated satRNA. In the experiments employing the CMV trans-replication system, we observed an unexpected phenomenon the replication proteins of the mild strain LS-CMV exhibited defective in supporting satRNA replication, unlike those of the severe strain Fny-CMV. Independent of translation products, all CMV genomic RNAs could enhance satRNA replication, when combined with the replication proteins of CMV. This enhancement is contingent upon the recruitment and complete replication of helper virus RNAs. Using the method developed for analyzing the satRNA recruitment, we observed a markedly distinct ability of the replication proteins from both CMV strains to recruit the positive-sense satRNA-harboring RNA3 mutant for replication. This is in agreement with the differential ability of both 1a proteins in binding satRNAs in plants. The discrepancies provide a convincing explanation for the variation of the replication proteins of both CMV strains in replicating satRNAs. Taken together, our work provides compelling evidence that the non-template functions of helper virus RNAs create an optimal replication environment to enhance satRNA proliferation.

Archaeal Kink-Turn Binding Protein Mediates Inhibition of Orthomyxovirus Splicing Biology

Despite lacking a DNA intermediate, orthomyxoviruses complete their replication cycle in the nucleus and generate multiple transcripts by usurping the host splicing machinery. This biology results in dynamic changes of relative viral transcripts over time and dictates the replicative phase of the infection. Here, we demonstrate that the family of archaeal L7Ae proteins uniquely inhibit the splicing biology of influenza A virus, influenza B virus, and Salmon isavirus, revealing a common strategy utilized by Orthomyxoviridae members to achieve this dynamic. L7Ae-mediated inhibition of virus biology was lost with the generation of a splicing-independent strain of influenza A virus and attempts to select for an escape mutant resulted in variants that conformed to host splicing biology at significant cost to their overall fitness. As L7Ae recognizes conventional kink turns in various RNAs, these data implicate the formation of a similar structure as a shared strategy adopted by this virus family to coordinate their replication cycle. IMPORTANCE Here, we demonstrate that a family of proteins from archaea specifically inhibit this splicing biology of all tested members of the Orthomyxoviridae family. We show that this inhibition extends to influenza A virus, influenza B virus, and isavirus genera, while having no significant impact on the mammalian transcriptome or proteome. Attempts to generate an escape mutant against L7Ae-mediated inhibition resulted in mutations surrounding the viral splice sites and a significant loss of viral fitness. Together, these findings reveal a unique biology shared among diverse members of the Orthomyxoviridae family that may serve as a means to generate future universal therapeutics.

Mutations in RPS19 may affect ribosome function and biogenesis in Diamond Blackfan Anemia

Ribosomes, the cellular organelles translating the genetic code to proteins, are assemblies of RNA chains and many proteins (RPs) arranged in precise fine-tuned interwoven structures. Mutated ribosomal genes cause ribosomopathies, including Diamond Blackfan Anemia (DBA, a rare heterogeneous red-cell aplasia connected to ribosome malfunction) or failed biogenesis. Combined bioinformatical, structural, and predictive analyses of potential consequences of possibly expressed mutations in eS19, the protein product of the highly mutated RPS19, suggests that mutations in its exposed surface could alter its positioning during assembly and consequently prevent biogenesis, implying a natural selective strategy to avoid malfunctions in ribosome assembly. A search for RPS19 pseudogenes indicated >90% sequence identity with the wild type, hinting at its expression in cases of absent or truncated gene products.

Ribosome Fate during Decoding of UGA-Sec Codons

Decoding of genetic information into polypeptides occurs during translation, generally following the codon assignment rules of the organism's genetic code. However, recoding signals in certain mRNAs can overwrite the normal rules of translation. An exquisite example of this occurs during translation of selenoprotein mRNAs, wherein UGA codons are reassigned to encode for the 21st proteogenic amino acid, selenocysteine. In this review, we will examine what is known about the mechanisms of UGA recoding and discuss the fate of ribosomes that fail to incorporate selenocysteine.

Transcription Regulation Through Nascent RNA Folding

Folding of RNA into secondary structures through intramolecular base pairing determines an RNA's three-dimensional architecture and associated function. Simple RNA structures like stem loops can provide specialized functions independent of coding capacity, such as protein binding, regulation of RNA processing and stability, stimulation or inhibition of translation. RNA catalysis is dependent on tertiary structures found in the ribosome, tRNAs and group I and II introns. While the extent to which non-coding RNAs contribute to cellular maintenance is generally appreciated, the fact that both non-coding and coding RNA can assume relevant structural states has only recently gained attention. In particular, the co-transcriptional folding of nascent RNA of all classes has the potential to regulate co-transcriptional processing, RNP (ribonucleoprotein particle) formation, and transcription itself. Riboswitches are established examples of co-transcriptionally folded coding RNAs that directly regulate transcription, mainly in prokaryotes. Here we discuss recent studies in both prokaryotes and eukaryotes showing that structure formation may carry a more widespread regulatory logic during RNA synthesis. Local structures forming close to the catalytic center of RNA polymerases have the potential to regulate transcription by reducing backtracking. In addition, stem loops or more complex structures may alter co-transcriptional RNA processing or its efficiency. Several examples of functional structures have been identified to date, and this review provides an overview of physiologically distinct processes where co-transcriptionally folded RNA plays a role. Experimental approaches such as single-molecule FRET and in vivo structural probing to further advance our insight into the significance of co-transcriptional structure formation are discussed.

Autoregulation of yeast ribosomal proteins discovered by efficient search for feedback regulation

Post-transcriptional autoregulation of gene expression is common in bacteria but many fewer examples are known in eukaryotes. We used the yeast collection of genes fused to GFP as a rapid screen for examples of feedback regulation in ribosomal proteins by overexpressing a non-regulatable version of a gene and observing the effects on the expression of the GFP-fused version. We tested 95 ribosomal protein genes and found a wide continuum of effects, with 30% showing at least a 3-fold reduction in expression. Two genes, RPS22B and RPL1B, showed over a 10-fold repression. In both cases the cis-regulatory segment resides in the 5’ UTR of the gene as shown by placing that segment of the mRNA upstream of GFP alone and demonstrating it is sufficient to cause repression of GFP when the protein is over-expressed. Further analyses showed that the intron in the 5’ UTR of RPS22B is required for regulation, presumably because the protein inhibits splicing that is necessary for translation. The 5’ UTR of RPL1B contains a sequence and structure motif that is conserved in the binding sites of Rpl1 orthologs from bacteria to mammals, and mutations within the motif eliminate repression.

Here, the authors screen for feedback regulation of ribosomal proteins by overexpressing a non- regulatable version of a gene and observing its effects on the expression of the GFP-fused version. They find that 30% show at least a 3-fold reduction in expression and two genes show a 10-fold reduction with the regulatory site being in the 5’ untranslated region of the gene.

Structural Insights into the Roles of Metazoan-Specific Splicing Factors in the Human Step 1 Spliceosome

Human spliceosomes contain numerous proteins absent in yeast, whose functions remain largely unknown. Here we report a 3D cryo-EM structure of the human spliceosomal C complex at 3.4 Å core resolution and 4.5-5.7 Å at its periphery, and aided by protein crosslinking we determine its molecular architecture. Our structure provides additional insights into the spliceosome's architecture between the catalytic steps of splicing, and how proteins aid formation of the spliceosome's catalytically active RNP (ribonucleoprotein) conformation. It reveals the spatial organization of the metazoan-specific proteins PPWD1, WDR70, FRG1, and CIR1 in human C complexes, indicating they stabilize functionally important protein domains and RNA structures rearranged/repositioned during the B^act to C transition. Structural comparisons with human B^act, C^∗, and P complexes reveal an intricate cascade of RNP rearrangements during splicing catalysis, with intermediate RNP conformations not found in yeast, and additionally elucidate the structural basis for the sequential recruitment of metazoan-specific spliceosomal proteins.

Regulation of ribosomal protein genes: An ordered anarchy

Ribosomal protein genes are among the most highly expressed genes in most cell types. Their products are generally essential for ribosome synthesis, which is the cornerstone for cell growth and proliferation. Many cellular resources are dedicated to producing ribosomal proteins and thus this process needs to be regulated in ways that carefully balance the supply of nascent ribosomal proteins with the demand for new ribosomes. Ribosomal protein genes have classically been viewed as a uniform interconnected regulon regulated in eukaryotic cells by target of rapamycin and protein kinase A pathway in response to changes in growth conditions and/or cellular status. However, recent literature depicts a more complex picture in which the amount of ribosomal proteins produced varies between genes in response to two overlapping regulatory circuits. The first includes the classical general ribosome‐producing program and the second is a gene‐specific feature responsible for fine‐tuning the amount of ribosomal proteins produced from each individual ribosomal gene. Unlike the general pathway that is mainly controlled at the level of transcription and translation, this specific regulation of ribosomal protein genes is largely achieved through changes in pre‐mRNA splicing efficiency and mRNA stability. By combining general and specific regulation, the cell can coordinate ribosome production, while allowing functional specialization and diversity. Here we review the many ways ribosomal protein genes are regulated, with special focus on the emerging role of posttranscriptional regulatory events in fine‐tuning the expression of ribosomal protein genes and its role in controlling the potential variation in ribosome functions.

This article is categorized under:

Translation > Ribosome Biogenesis

Translation > Ribosome Structure/Function

Translation > Translation Regulation

Revisiting the window of opportunity for cotranscriptional splicing in budding yeast

We reported previously that, in budding yeast, transcription rate affects both the efficiency and fidelity of pre-mRNA splicing, especially of ribosomal protein transcripts. Here, we report that the majority of ribosomal protein transcripts with non-consensus 5' splice sites are spliced less efficiently when transcription is faster, and more efficiently with slower transcription. These results support the "window of opportunity" model, and we suggest a possible mechanism to explain these findings.

Genistein Represses HOTAIR/Chromatin Remodeling Pathways to Suppress Kidney Cancer

BACKGROUND/AIMS

Genistein, a soy isoflavone, has been shown to have anti-cancer effects in various cancers including renal cancer. Long non-coding RNA, HOX transcript antisense RNA (HOTAIR), is involved in cancer progression and metastasis, such as renal cancer. Our aim was to investigate the effects of genistein on HOTAIR chromatin remodeling functions.

METHODS

We used MTS assays and Transwell migration assays to study the effects of genistein on cell proliferation and migration respectively in human renal cell carcinoma (RCC) cell lines. We used Western blots to analyze SNAIL and ZO-1 expression. We performed chromatin immunoprecipitation (ChIP) assays to study recruitment of the polycomb repressive complex 2 (PRC2) to the ZO-1 promoter. We performed RNA immunoprecipitation (RIP) assays to study interaction between HOTAIR and PRC2, SMARCB1 or ARID1A. We also performed transfection experiments to overexpress EED, HOTAIR and knockdown SMARCB1.

RESULTS

Genistein reduced cell proliferation and migration of human renal cell carcinoma cell lines. ChIP assays indicated that genistein reduces recruitment of the PRC2 to the ZO-1 promoter and increased its expression. RIP assays showed that genistein inhibits HOTAIR interaction with PRC2, leading to tumor suppression. Immunoprecipitation also revealed that genistein reduced EED levels in PRC2, suggesting that decreased EED levels suppress HOTAIR interaction with PRC2. EED overexpression in the presence of genistein restored PRC2 interaction with HOTAIR and reduced ZO-1 transcription, suggesting genistein activates ZO-1 by inhibiting HOTAIR/PRC2 functions. RIP assays also showed that HOTAIR interacts with SMARCB1 and ARID1A, subunits of the human SWI/SNF chromatin remodeling complex and genistein reduces this interaction. Combination of HOTAIR overexpression and SMARCB1 knockdown in the presence of genistein revealed that genistein inhibits SNAIL transcription via the HOTAIR/SMARCB1 pathway.

CONCLUSION

Genistein suppresses EED levels in PRC2 and inhibits HOTAIR/PRC2 interaction. Genistein suppresses HOTAIR/PRC2 recruitment to the ZO-1 promoter and enhances ZO-1 transcription. Genistein also inhibits SNAIL transcription via reducing HOTAIR/SMARCB1 interaction. We demonstrate that the reduction of HOTAIR interaction with chromatin remodeling factors by genistein represses HOTAIR/chromatin remodeling pathways to suppress RCC malignancy.

New Directions for Understanding the Codon Redefinition Required for Selenocysteine Incorporation

The fact that selenocysteine (Sec) is delivered to the elongating ribosome by a tRNA that recognizes a UGA stop codon makes it unique and a thorn in the side of what was originally thought to be a universal genetic code. The mechanism by which this redefinition occurs has been slowly coming to light over the past 30 years, but key questions remain. This review seeks to highlight the prominent mechanistic questions that will guide the direction of work in the near future. These questions arise from two major aspects of Sec incorporation: (1) novel functions for the Sec insertion sequence (SECIS) that resides in all selenoprotein mRNAs and (2) the myriad of RNA-binding proteins, both known and yet to be discovered, that act in concert to modify the translation elongation process to allow Sec incorporation.

Cus2 enforces the first ATP-dependent step of splicing by binding to yeast SF3b1 through a UHM-ULM interaction

Stable recognition of the intron branchpoint (BP) by the U2 snRNP to form the pre-spliceosome is the first ATP-dependent step of splicing. Genetic and biochemical data from yeast indicate that Cus2 aids U2 snRNA folding into the stem IIa conformation prior to pre-spliceosome formation. Cus2 must then be removed by an ATP-dependent function of Prp5 before assembly can progress. However, the location from which Cus2 is displaced and the nature of its binding to the U2 snRNP are unknown. Here, we show that Cus2 contains a conserved UHM (U2AF homology motif) that binds Hsh155, the yeast homolog of human SF3b1, through a conserved ULM (U2AF ligand motif). Mutations in either motif block binding and allow pre-spliceosome formation without ATP. A 2.0 Å resolution structure of the Hsh155 ULM in complex with the UHM of Tat-SF1, the human homolog of Cus2, and complementary binding assays show that the interaction is highly similar between yeast and humans. Furthermore, we show that Tat-SF1 can replace Cus2 function by enforcing ATP dependence of pre-spliceosome formation in yeast extracts. Cus2 is removed before pre-spliceosome formation, and both Cus2 and its Hsh155 ULM binding site are absent from available cryo-EM structure models. However, our data are consistent with the apparent location of the disordered Hsh155 ULM between the U2 stem-loop IIa and the HEAT repeats of Hsh155 that interact with Prp5. We propose a model in which Prp5 uses ATP to remove Cus2 from Hsh155 such that extended base-pairing between U2 snRNA and the intron BP can occur.

Heterogeneity and specialized functions of translation machinery: from genes to organisms

Regulation of mRNA translation offers the opportunity to diversify the expression and abundance of proteins made from individual gene products in cells, tissues and organisms. Emerging evidence has highlighted variation in the composition and activity of several large, highly conserved translation complexes as a means to differentially control gene expression. Heterogeneity and specialized functions of individual components of the ribosome and of the translation initiation factor complexes eIF3 and eIF4F, which are required for recruitment of the ribosome to the mRNA 5' untranslated region, have been identified. In this Review, we summarize the evidence for selective mRNA translation by components of these macromolecular complexes as a means to dynamically control the translation of the proteome in time and space. We further discuss the implications of this form of gene expression regulation for a growing number of human genetic disorders associated with mutations in the translation machinery.

MS2-Affinity Purification Coupled With RNA Sequencing Approach in the Human Pathogen Staphylococcus aureus

Staphylococcus aureus is a Gram-positive major human pathogen involved in a wide range of human infectious diseases (from minor skin infections to septicemia, endocarditis or toxic shock syndrome). The treatment of S. aureus infections is very challenging due to the emergence of multiple antibiotic-resistant isolates. The high diversity of clinical symptoms caused by S. aureus depends on the precise expression of numerous virulence factors and stress response pathways, which are tightly regulated at every level (transcriptional, posttranscriptional, translational, and posttranslational). During the last two decades, it has become evident that small regulatory RNAs (sRNAs) play a major role in fast adaptive responses, mainly by targeting mRNA translation. sRNAs act as antisense RNAs by forming noncontiguous pairings with their target mRNAs and their mechanisms of action vary according to the interaction site. To obtain a global and detailed view of the regulatory networks involved in the adaptive processes of S. aureus, we have adapted the MAPS approach to get individual sRNA targetomes. We also set up different strategies to validate MAPS results and establish sRNA regulatory activities. As this method has been first developed in Gram-negative bacteria, we provide here a protocol for its application in S. aureus and highlight underlying differences. Finally, we discuss several points that have been and could be further improved and provide a workflow file for the automatic analysis of the sequencing in Galaxy.

A bacterial three-hybrid assay detects Escherichia coli Hfq-sRNA interactions in vivo

The interaction of RNA molecules with proteins is a critical aspect of gene regulation across all domains of life. Here, we report the development of a bacterial three-hybrid (B3H) assay to genetically detect RNA-protein interactions. The basis for this three-hybrid assay is a transcription-based bacterial two-hybrid assay that has been used widely to detect and dissect protein-protein interactions. In the three-hybrid assay, a DNA-bound protein with a fused RNA-binding moiety (the coat protein of bacteriophage MS2 (MS2CP)) is used to recruit a hybrid RNA upstream of a test promoter. The hybrid RNA consists of a constant region that binds the tethered MS2CP and a variable region. Interaction between the variable region of the hybrid RNA and a target RNA-binding protein that is fused to a subunit of Escherichia coli RNA polymerase (RNAP) stabilizes the binding of RNAP to the test promoter, thereby activating transcription of a reporter gene. We demonstrate that this three-hybrid assay detects interaction between non-coding small RNAs (sRNAs) and the hexameric RNA chaperone Hfq from E. coli and enables the identification of Hfq mutants with sRNA-binding defects. Our findings suggest that this B3H assay will be broadly applicable for the study of RNA-protein interactions.

Structure and Conformational Dynamics of the Human Spliceosomal Bact Complex

The spliceosome is a highly dynamic macromolecular complex that precisely excises introns from pre-mRNA. Here we report the cryo-EM 3D structure of the human B^act spliceosome at 3.4 Å resolution. In the B^act state, the spliceosome is activated but not catalytically primed, so that it is functionally blocked prior to the first catalytic step of splicing. The spliceosomal core is similar to the yeast B^act spliceosome; important differences include the presence of the RNA helicase aquarius and peptidyl prolyl isomerases. To examine the overall dynamic behavior of the purified spliceosome, we developed a principal component analysis-based approach. Calculating the energy landscape revealed eight major conformational states, which we refined to higher resolution. Conformational differences of the highly flexible structural components between these eight states reveal how spliceosomal components contribute to the assembly of the spliceosome, allowing it to generate a dynamic interaction network required for its subsequent catalytic activation.

Introns provide a platform for intergenic regulatory feedback of RPL22 paralogs in yeast

Ribosomal protein genes (RPGs) in Saccharomyces cerevisiae are a remarkable regulatory group that may serve as a model for understanding genetic redundancy in evolutionary adaptations. Most RPGs exist as pairs of highly conserved functional paralogs with divergent untranslated regions and introns. We examined the roles of introns in strains with various combinations of intron and gene deletions in RPL22, RPL2, RPL16, RPL37, RPL17, RPS0, and RPS18 paralog pairs. We found that introns inhibited the expression of their genes in the RPL22 pair, with the RPL22B intron conferring a much stronger effect. While the WT RPL22A/RPL22B mRNA ratio was 93/7, the rpl22aΔi/RPL22B and RPL22A/rpl22bΔi ratios were >99/<1 and 60/40, respectively. The intron in RPL2A stimulated the expression of its own gene, but the removal of the other introns had little effect on expression of the corresponding gene pair. Rpl22 protein abundances corresponded to changes in mRNAs.

Using splicing reporters containing endogenous intron sequences, we demonstrated that these effects were due to the inhibition of splicing by Rpl22 proteins but not by their RNA-binding mutant versions. Indeed, only WT Rpl22A/Rpl22B proteins (but not the mutants) interacted in a yeast three-hybrid system with an RPL22B intronic region between bp 165 and 236. Transcriptome analysis showed that both the total level of Rpl22 and the A/B ratio were important for maintaining the WT phenotype. The data presented here support the contention that the Rpl22B protein has a paralog-specific role.

The RPL22 singleton of Kluyveromyces lactis, which did not undergo whole genome duplication, also responded to Rpl22-mediated inhibition in K. lactis cells. Vice versa, the overproduction of the K. lactis protein reduced the expression of RPL22A/B in S. cerevisiae. The extraribosomal function of of the K. lactis Rpl22 suggests that the loop regulating RPL22 paralogs of S. cerevisiae evolved from autoregulation.

Extremely fast and incredibly close: cotranscriptional splicing in budding yeast

RNA splicing, an essential part of eukaryotic pre-messenger RNA processing, can be simultaneous with transcription by RNA polymerase II. Here, we compare and review independent next-generation sequencing methods that jointly quantify transcription and splicing in budding yeast. For many yeast transcripts, splicing is fast, taking place within seconds of intron transcription, while polymerase is within a few dozens of nucleotides of the 3' splice site. Ribosomal protein transcripts are spliced particularly fast and cotranscriptionally. However, some transcripts are spliced inefficiently or mainly post-transcriptionally. Intron-mediated regulation of some genes is likely to be cotranscriptional. We suggest that intermediates of the splicing reaction, missing from current data sets, may hold key information about splicing kinetics.

Large-scale remodeling of a repressed exon ribonucleoprotein to an exon definition complex active for splicing

Polypyrimidine-tract binding protein PTBP1 can repress splicing during the exon definition phase of spliceosome assembly, but the assembly steps leading to an exon definition complex (EDC) and how PTBP1 might modulate them are not clear. We found that PTBP1 binding in the flanking introns allowed normal U2AF and U1 snRNP binding to the target exon splice sites but blocked U2 snRNP assembly in HeLa nuclear extract. Characterizing a purified PTBP1-repressed complex, as well as an active early complex and the final EDC by SILAC-MS, we identified extensive PTBP1-modulated changes in exon RNP composition. The active early complex formed in the absence of PTBP1 proceeded to assemble an EDC with the eviction of hnRNP proteins, the late recruitment of SR proteins, and binding of the U2 snRNP. These results demonstrate that during early stages of splicing, exon RNP complexes are highly dynamic with many proteins failing to bind during PTBP1 arrest.

DOI:http://dx.doi.org/10.7554/eLife.19743.001

The regulation and functions of the nuclear RNA exosome complex

The RNA exosome complex is the most versatile RNA-degradation machine in eukaryotes. The exosome has a central role in several aspects of RNA biogenesis, including RNA maturation and surveillance. Moreover, it is emerging as an important player in regulating the expression levels of specific mRNAs in response to environmental cues and during cell differentiation and development. Although the mechanisms by which RNA is targeted to (or escapes from) the exosome are still not fully understood, general principles have begun to emerge, which we discuss in this Review. In addition, we introduce and discuss novel, previously unappreciated functions of the nuclear exosome, including in transcription regulation and in the maintenance of genome stability.

Regulation of mRNA Levels by Decay-Promoting Introns that Recruit the Exosome Specificity Factor Mmi1

In eukaryotic cells, inefficient splicing is surprisingly common and leads to the degradation of transcripts with retained introns. How pre-mRNAs are committed to nuclear decay is unknown. Here, we uncover a mechanism by which specific intron-containing transcripts are targeted for nuclear degradation in fission yeast. Sequence elements within these "decay-promoting" introns co-transcriptionally recruit the exosome specificity factor Mmi1, which induces degradation of the unspliced precursor and leads to a reduction in the levels of the spliced mRNA. This mechanism negatively regulates levels of the RNA helicase DDX5/Dbp2 to promote cell survival in response to stress. In contrast, fast removal of decay-promoting introns by co-transcriptional splicing precludes Mmi1 recruitment and relieves negative expression regulation. We propose that decay-promoting introns facilitate the regulation of gene expression. Based on the identification of multiple additional Mmi1 targets, including mRNAs, long non-coding RNAs, and sn/snoRNAs, we suggest a general role in RNA regulation for Mmi1 through transcript degradation.

Eukaryotic translation initiation factor 6 is a novel regulator of reactive oxygen species-dependent megakaryocyte maturation

BACKGROUND

Ribosomopathies constitute a class of inherited disorders characterized by defects in ribosome biogenesis and function. Classically, bone marrow (BM) failure is a clinical symptom shared between these syndromes, including Shwachman-Bodian-Diamond syndrome (SBDS). Eukaryotic translation initiation factor 6 (eIF6) is a critical translation factor that rescues the quasilethal effect of the loss of the SBDS protein.

OBJECTIVES

To determine whether eIF6 activity is necessary for BM development.

METHODS

We used eIF6(+/-) mice and primary BM megakaryocytes to investigate the involvement of eIF6 in the regulation of hematopoiesis.

RESULTS

We provide evidence that reduced eIF6 expression negatively impacts on megakaryopoiesis. We show that inhibition of eIF6 leads to a reduction in cell size and mean ploidy level of megakaryocytes and a delay in megakaryocyte maturation by blocking the G1 /S transition. Consistent with this phenotype, only few megakaryocyte-forming proplatelets were found in eIF6(+/-) cells. We also discovered that, in eIF6(+/-) cells, the steady-state abundance of mitochondrial respiratory chain complex I-encoding mRNAs is decreased, resulting in decreased reactive oxygen species (ROS) production. Intriguingly, connectivity map analysis showed that eIF6-mediated changes overlap with specific translational inhibitors. eIF6 is a translation factor acting downstream of insulin/phorbol 12-myristate 13-acetate (PMA) stimulation. PMA treatment significantly restored eIF6(+/-) megakaryocyte maturation, indicating that activation of eIF6 is essential for the rescue of the phenotype.

CONCLUSIONS

Taken together, our results show a role for eIF6-driven translation in megakaryocyte development, and unveil the novel connection between translational control and ROS production in this cell subset.

Ribosomal proteins as novel players in tumorigenesis

Ribosome biogenesis is the most demanding energetic and metabolic expenditure of the cell. The nucleolus, a nuclear compartment, coordinates rRNA transcription, maturation, and assembly into ribosome subunits. The transcription process is highly coordinated with ribosome biogenesis. In this context, ribosomal proteins (RPs) play a crucial role. In the last decade, an increasing number of studies have associated RPs with extraribosomal functions related to proliferation. Importantly, the expression of RPs appears to be deregulated in several human disorders due, at least in part, to genetic mutations. Although the deregulation of RPs in human malignancies is commonly observed, a more complex mechanism is believed to be involved, favoring the tumorigenic process, its progression and metastasis. This review explores the roles of the most frequently mutated oncogenes and tumor suppressor genes in human cancer that modulate ribosome biogenesis, including their interaction with RPs. In this regard, we propose a new focus for novel therapies.

U1A regulates 3' processing of the survival motor neuron mRNA

Insufficient expression of the survival motor neuron (SMN) protein causes spinal muscular atrophy, a neurodegenerative disease characterized by loss of motor neurons. Despite the importance of maintaining adequate SMN levels, little is known about factors that control SMN expression, particularly 3' end processing of the SMN pre-mRNA. In this study, we identify the U1A protein as a key regulator of SMN expression. U1A, a component of the U1 snRNP, is known to inhibit polyadenylation upon direct binding to mRNA. We show that U1A binds directly and with high affinity and specificity to the SMN 3'-UTR adjacent to the polyadenylation site, independent of the U1 snRNP (U1 small nuclear ribonucleoprotein). Binding of U1A inhibits polyadenylation of the SMN pre-mRNA by specifically inhibiting 3' cleavage by the cleavage and polyadenylation specificity factor. Expression of U1A in excess of U1 snRNA causes inhibition of SMN polyadenylation and decreases SMN protein levels. This work reveals a new mechanism for regulating SMN levels and provides new insight into the roles of U1A in 3' processing of mRNAs.

The ribosomal protein Rpl22 controls ribosome composition by directly repressing expression of its own paralog, Rpl22l1

Most yeast ribosomal protein genes are duplicated and their characterization has led to hypotheses regarding the existence of specialized ribosomes with different subunit composition or specifically-tailored functions. In yeast, ribosomal protein genes are generally duplicated and evidence has emerged that paralogs might have specific roles. Unlike yeast, most mammalian ribosomal proteins are thought to be encoded by a single gene copy, raising the possibility that heterogenous populations of ribosomes are unique to yeast. Here, we examine the roles of the mammalian Rpl22, finding that Rpl22^−/− mice have only subtle phenotypes with no significant translation defects. We find that in the Rpl22^−/− mouse there is a compensatory increase in Rpl22-like1 (Rpl22l1) expression and incorporation into ribosomes. Consistent with the hypothesis that either ribosomal protein can support translation, knockdown of Rpl22l1 impairs growth of cells lacking Rpl22. Mechanistically, Rpl22 regulates Rpl22l1 directly by binding to an internal hairpin structure and repressing its expression. We propose that ribosome specificity may exist in mammals, providing evidence that one ribosomal protein can influence composition of the ribosome by regulating its own paralog.

Translation is the process by which proteins are made within a cell. Ribosomes are the main macromolecular complexes involved in this process. Ribosomes are composed of ribosomal RNA and ribosomal proteins. Ribosomal proteins are generally thought to be structural components of the ribosome but recent findings have suggested that they might have a regulatory function as well. A growing number of human diseases have been linked to mutations in genes encoding factors involved in ribosome biogenesis and translation. These include developmental malformations, inherited bone marrow failure syndromes and cancer in a variety of organisms. Here, we describe the role of one ribosomal protein regulating another. We provide evidence that ribosomal proteins can influence the composition of the ribosome, which we hypothesize, may impact the function of the ribosome.

Identification of nucleotides and amino acids that mediate the interaction between ribosomal protein L30 and the SECIS element

Background

Ribosomal protein L30 belongs to the L7Ae family of RNA-binding proteins, which recognize diverse targets. L30 binds to kink-turn motifs in the 28S ribosomal RNA, L30 pre-mRNA, and mature L30 mRNA. L30 has a noncanonical function as a component of the UGA recoding machinery that incorporates selenocysteine (Sec) into selenoproteins during translation. L30 binds to a putative kink-turn motif in the Sec Insertion Sequence (SECIS) element in the 3’ UTR of mammalian selenoprotein mRNAs. The SECIS also interacts with SECIS-binding protein 2 (SBP2), an essential factor for Sec incorporation. Previous studies showed that L30 and SBP2 compete for binding to the SECIS in vitro. The SBP2:SECIS interaction has been characterized but much less is known about how L30 recognizes the SECIS.

Results

Here we use enzymatic RNA footprinting to define the L30 binding site on the SECIS. Like SBP2, L30 protects nucleotides in the 5’ side of the internal loop, the 5’ side of the lower helix, and the SECIS core, including the GA tandem base pairs that are predicted to form a kink-turn. However, L30 has additional determinants for binding as it also protects nucleotides in the 3’ side of the internal loop, which are not protected by SBP2. In support of the competitive binding model, we found that purified L30 repressed UGA recoding in an in vitro translation system, and that this inhibition was rescued by SBP2. To define the amino acid requirements for SECIS-binding, site-specific mutations in L30 were generated based on published structural studies of this protein in a complex with its canonical target, the L30 pre-mRNA. We identified point mutations that selectively inhibited binding of L30 to the SECIS, to the L30 pre-mRNA, or both RNAs, suggesting that there are subtle differences in how L30 interacts with the two targets.

Conclusions

This study establishes that L30 and SBP2 bind to overlapping but non-identical sites on the SECIS. The amino acid requirements for the interaction of L30 with the SECIS differ from those that mediate binding to the L30 pre-mRNA. Our results provide insight into how L7Ae family members recognize their cognate RNAs.

Identification of maize genes associated with host plant resistance or susceptibility to Aspergillus flavus infection and aflatoxin accumulation

BACKGROUND

Aspergillus flavus infection and aflatoxin contamination of maize pose negative impacts in agriculture and health. Commercial maize hybrids are generally susceptible to this fungus. Significant levels of host plant resistance have been observed in certain maize inbred lines. This study was conducted to identify maize genes associated with host plant resistance or susceptibility to A. flavus infection and aflatoxin accumulation.

RESULTS

Genome wide gene expression levels with or without A. flavus inoculation were compared in two resistant maize inbred lines (Mp313E and Mp04:86) in contrast to two susceptible maize inbred lines (Va35 and B73) by microarray analysis. Principal component analysis (PCA) was used to find genes contributing to the larger variances associated with the resistant or susceptible maize inbred lines. The significance levels of gene expression were determined by using SAS and LIMMA programs. Fifty candidate genes were selected and further investigated by quantitative RT-PCR (qRT-PCR) in a time-course study on Mp313E and Va35. Sixteen of the candidate genes were found to be highly expressed in Mp313E and fifteen in Va35. Out of the 31 highly expressed genes, eight were mapped to seven previously identified quantitative trait locus (QTL) regions. A gene encoding glycine-rich RNA binding protein 2 was found to be associated with the host hypersensitivity and susceptibility in Va35. A nuclear pore complex protein YUP85-like gene was found to be involved in the host resistance in Mp313E.

CONCLUSION

Maize genes associated with host plant resistance or susceptibility were identified by a combination of microarray analysis, qRT-PCR analysis, and QTL mapping methods. Our findings suggest that multiple mechanisms are involved in maize host plant defense systems in response to Aspergillus flavus infection and aflatoxin accumulation. These findings will be important in identification of DNA markers for breeding maize lines resistant to aflatoxin accumulation.

Evidence providing new insights into TOB-promoted deadenylation and supporting a link between TOB's deadenylation-enhancing and antiproliferative activities

The mammalian TOB1 and TOB2 proteins have emerged as key players in repressing cell proliferation. Accumulating evidence indicates that TOBs regulate mRNA deadenylation. A recruitment model was proposed in which TOBs promote deadenylation by recruiting CAF1-CCR4 deadenylase complex to the 3' end of mRNAs by simultaneously binding CAF1 and PABP. However, the exact molecular mechanism underlying TOB-promoted deadenylation remains unclear. It is also unclear whether TOBs' antiproliferative and deadenylation-promoting activities are connected. Here, we combine biochemical analyses with a functional assay directly monitoring deadenylation and mRNA decay to characterize the effects of tethering TOBs or their mutant derivatives to mRNAs. The results provide direct evidence supporting the recruitment model and reveal a link between TOBs' antiproliferative and deadenylation-promoting activities. We also find that TOBs' actions in deadenylation are independent of the phosphorylation state of three serines known to regulate antiproliferative actions, suggesting that TOBs arrest cell growth through at least two different mechanisms. TOB1 and TOB2 were interchangeable in the properties tested here, indicating considerable functional redundancy between the two proteins. We propose that their multiple modes of modulating mRNA turnover and arresting cell growth permit the TOB proteins to coordinate their diverse roles in controlling cell growth and differentiation.

Regulated pre-mRNA splicing: the ghostwriter of the eukaryotic genome

Intron removal is at the heart of mRNA synthesis. It is mediated by one of the cell's largest complexes, the spliceosome. Yet, the fundamental chemistry involved is simple. In this review we will address how the spliceosome acts in diverse ways to optimize gene expression in order to meet the cell's needs. This is done largely by regulating the splicing of key transcripts encoding products that control gene expression pathways. This widespread role is evident even in the yeast Saccharomyces cerevisiae, where many introns appear to have been lost; yet how this control is being achieved is known only in a few cases. Here we explore the relevant examples and posit hypotheses whereby regulated splicing fine-tunes gene expression pathways to maintain cell homeostasis. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

A Pre-mRNA degradation pathway that selectively targets intron-containing genes requires the nuclear poly(A)-binding protein

General discard pathways eliminate unprocessed and irregular pre-mRNAs to control the quality of gene expression. In contrast to such general pre-mRNA decay, we describe here a nuclear pre-mRNA degradation pathway that controls the expression of select intron-containing genes. We show that the fission yeast nuclear poly(A)-binding protein, Pab2, and the nuclear exosome subunit, Rrp6, are the main factors involved in this polyadenylation-dependent pre-mRNA degradation pathway. Transcriptome analysis and intron swapping experiments revealed that inefficient splicing is important to dictate susceptibility to Pab2-dependent pre-mRNA decay. We also show that negative splicing regulation can promote the poor splicing efficiency required for this pre-mRNA decay pathway, and in doing so, we identified a mechanism of cross-regulation between paralogous ribosomal proteins through nuclear pre-mRNA decay. Our findings unveil a layer of regulation in the nucleus in which the turnover of specific pre-mRNAs, besides the turnover of mature mRNAs, is used to control gene expression.

Crystal structure of the eukaryotic 60S ribosomal subunit in complex with initiation factor 6

Protein synthesis in all organisms is catalyzed by ribosomes. In comparison to their prokaryotic counterparts, eukaryotic ribosomes are considerably larger and are subject to more complex regulation. The large ribosomal subunit (60S) catalyzes peptide bond formation and contains the nascent polypeptide exit tunnel. We present the structure of the 60S ribosomal subunit from Tetrahymena thermophila in complex with eukaryotic initiation factor 6 (eIF6), cocrystallized with the antibiotic cycloheximide (a eukaryotic-specific inhibitor of protein synthesis), at a resolution of 3.5 angstroms. The structure illustrates the complex functional architecture of the eukaryotic 60S subunit, which comprises an intricate network of interactions between eukaryotic-specific ribosomal protein features and RNA expansion segments. It reveals the roles of eukaryotic ribosomal protein elements in the stabilization of the active site and the extent of eukaryotic-specific differences in other functional regions of the subunit. Furthermore, it elucidates the molecular basis of the interaction with eIF6 and provides a structural framework for further studies of ribosome-associated diseases and the role of the 60S subunit in the initiation of protein synthesis.

Secondary structure is required for 3' splice site recognition in yeast

Higher order RNA structures can mask splicing signals, loop out exons, or constitute riboswitches all of which contributes to the complexity of splicing regulation. We identified a G to A substitution between branch point (BP) and 3′ splice site (3′ss) of Saccharomyces cerevisiae COF1 intron, which dramatically impaired its splicing. RNA structure prediction and in-line probing showed that this mutation disrupted a stem in the BP-3′ss region. Analyses of various COF1 intron modifications revealed that the secondary structure brought about the reduction of BP to 3′ss distance and masked potential 3′ss. We demonstrated the same structural requisite for the splicing of UBC13 intron. Moreover, RNAfold predicted stable structures for almost all distant BP introns in S. cerevisiae and for selected examples in several other Saccharomycotina species. The employment of intramolecular structure to localize 3′ss for the second splicing step suggests the existence of pre-mRNA structure-based mechanism of 3′ss recognition.

SUS1 introns are required for efficient mRNA nuclear export in yeast

Efficient coupling between mRNA synthesis and export is essential for gene expression. Sus1/ENY2, a component of the SAGA and TREX-2 complexes, is involved in both transcription and mRNA export. While most yeast genes lack introns, we previously reported that yeast SUS1 bears two. Here we show that this feature is evolutionarily conserved and critical for Sus1 function. We determine that while SUS1 splicing is inefficient, it responds to cellular conditions, and intronic mutations either promoting or blocking splicing lead to defects in mRNA export and cell growth. Consistent with this, we find that an intron-less SUS1 only partially rescues sus1Δ phenotypes. Remarkably, splicing of each SUS1 intron is also affected by the presence of the other and by SUS1 exonic sequences. Moreover, by following SUS1 RNA and protein levels we establish that nonsense-mediated decay (NMD) pathway and the splicing factor Mud2 both play a role in SUS1 expression. Our data (and those of the accompanying work by Hossain et al.) provide evidence of the involvement of splicing, translation, and decay in the regulation of early events in mRNP biogenesis; and imply the additional requirement for a balance in splicing isoforms from a single gene.

RPL30 regulation of splicing reveals distinct roles for Cbp80 in U1 and U2 snRNP cotranscriptional recruitment

Pre-mRNA splicing is catalyzed by the spliceosome, and its control is essential for correct gene expression. While splicing repressors typically interfere with transcript recognition by spliceosomal components, the yeast protein L30 blocks spliceosomal rearrangements required for the engagement of U2 snRNP (small ribonucleoprotein particle) to its own transcript RPL30. Using a mutation in the RPL30 binding site that disrupts this repression, we have taken a genetic approach to reveal that regulation of splicing is restored in this mutant by deletion of the cap-binding complex (CBC) component Cbp80. Indeed, our data indicate that Cbp80 plays distinct roles in the recognition of the intron by U1 and U2 snRNP. It promotes the initial 5' splice site recognition by U1 and, independently, facilitates U2 recruitment, depending on sequences located in the vicinity of the 5' splice site. These results reveal a novel function for CBC in splicing and imply that these molecular events can be the target of a splicing regulator.

Ribosomal protein S19 binds to its own mRNA with reduced affinity in Diamond-Blackfan anemia

Heterozygous mutations in the ribosomal protein S19 (RPS19) gene are associated with Diamond-Blackfan anemia (DBA). The mechanism by which RPS19 mediates anemia are still unclear, as well as the regulation of RPS19 expression. We show herein that RPS19 binds specifically to the 5' untranslated region of its own mRNA with an equilibrium binding constant (K(D)) of 4.1+/-1.9 nM. We investigated the mRNA binding properties of two mutant RPS19 proteins (W52R and R62W) identified in DBA patients. We observed a significant increase in K(D) for both proteins (16.1+/-2.1 and 14.5+/-4.9 nM, respectively), indicating a reduced RNA binding capability (p<0.05). We suggest that the binding of RPS19 to its mRNA has a regulatory function and hypothesize that the weaker RNA binding of mutant rRPS19 may have implications for the pathophysiological mechanisms in DBA.

Threading the needle: getting selenocysteine into proteins

The co-translational incorporation of selenocysteine (Sec) requires that UGA be recognized as a sense rather than a nonsense codon. This is accomplished by the concerted action of a Sec insertion sequence (SECIS) element, SECIS binding protein 2, and a ternary complex of the Sec specific elongation factor, Sec-tRNA(Sec), and GTP. The mechanism by which they alter the canonical protein synthesis reaction has been elusive. Here we present an overview of the mechanistic perspective on Sec incorporation, highlighting recent advances in the field.

Role of RNA structure in regulating pre-mRNA splicing

Pre-mRNA splicing involves removing non-coding introns from RNA transcripts. It is carried out by the spliceosome, along with other auxiliary factors. In general, research in splicing has focused on the sequences within the pre-mRNA, without considering the structures that these sequences might form. We propose that the role of RNA structure deserves more consideration when thinking about splicing mechanisms. RNA structures can inhibit or aid binding of spliceosomal components to the pre-mRNA, or can increase splicing efficiency by bringing important sequences into close proximity. Recent reports have identified proteins and small molecules that can regulate splicing by modulating RNA structures, thereby expanding our knowledge of the mechanisms used to regulate splicing.

Modulation of alternative splicing by long-range RNA structures in Drosophila

Accurate and efficient recognition of splice sites during pre-mRNA splicing is essential for proper transcriptome expression. Splice site usage can be modulated by secondary structures, but it is unclear if this type of modulation is commonly used or occurs to a significant degree with secondary structures forming over long distances. Using phlyogenetic comparisons of intronic sequences among 12 Drosophila genomes, we elucidated a group of 202 highly conserved pairs of sequences, each at least nine nucleotides long, capable of forming stable stem structures. This set was highly enriched in alternatively spliced introns and introns with weak acceptor sites and long introns, and most occurred over long distances (>150 nucleotides). Experimentally, we analyzed the splicing of several of these introns using mini-genes in Drosophila S2 cells. Wild-type splicing patterns were changed by mutations that opened the stem structure, and restored by compensatory mutations that re-established the base-pairing potential, demonstrating that these secondary structures were indeed implicated in the splice site choice. Mechanistically, the RNA structures masked splice sites, brought together distant splice sites and/or looped out introns. Thus, base-pairing interactions within introns, even those occurring over long distances, are more frequent modulators of alternative splicing than is currently assumed.

Decrypting the genome's alternative messages

Alternative splicing of messenger RNA (mRNA) precursors affects the majority of human genes, has a considerable impact on eukaryotic gene function and offers distinct opportunities for regulation. Alterations in alternative splicing can cause or modify the progression of a significant number of pathologies. Recent high-throughput technologies have uncovered a wealth of transcript diversity generated by alternative splicing, as well as examples for how this diversity can be established and become misregulated. A variety of mechanisms modulate splice site choice coordinately with other cellular processes, from transcription and mRNA editing or decay to miRNA-based regulation and telomerase function. Alternative splicing studies can contribute to our understanding of multiple biological processes, including genetic diversity, speciation, cell/stem cell differentiation, nervous system function, neuromuscular disorders and tumour progression.

The quest for a message: budding yeast, a model organism to study the control of pre-mRNA splicing

Removal of introns during pre-mRNA splicing is a critical process in gene expression, and understanding its control at both single-gene and genomic levels is one of the great challenges in Biology. Splicing takes place in a dynamic, large ribonucleoprotein complex known as the spliceosome. Combining Genetics and Biochemistry, Saccharomyces cerevisiae provides insights into its mechanisms, including its regulation by RNA-protein interactions. Recent genome-wide analyses indicate that regulated splicing is broad and biologically relevant even in organisms with a relatively simple intronic structure, such as yeast. Furthermore, the possibility of coordination in splicing regulation at genomic level is becoming clear in this model organism. This should provide a valuable system to approach the complex problem of the role of regulated splicing in genomic expression.