RNA ligands generated against complex nuclear targets indicate a role for U1 snRNP in co-ordinating transcription and RNA splicing

Gene Architecture Facilitates Intron-Mediated Enhancement of Transcription

Introns impact several vital aspects of eukaryotic organisms like proteomic plasticity, genomic stability, stress response and gene expression. A role for introns in the regulation of gene expression at the level of transcription has been known for more than thirty years. The molecular basis underlying the phenomenon, however, is still not entirely clear. An important clue came from studies performed in budding yeast that indicate that the presence of an intron within a gene results in formation of a multi-looped gene architecture. When looping is defective, these interactions are abolished, and there is no enhancement of transcription despite normal splicing. In this review, we highlight several potential mechanisms through which looping interactions may enhance transcription. The promoter-5′ splice site interaction can facilitate initiation of transcription, the terminator-3′ splice site interaction can enable efficient termination of transcription, while the promoter-terminator interaction can enhance promoter directionality and expedite reinitiation of transcription. Like yeast, mammalian genes also exhibit an intragenic interaction of the promoter with the gene body, especially exons. Such promoter-exon interactions may be responsible for splicing-dependent transcriptional regulation. Thus, the splicing-facilitated changes in gene architecture may play a critical role in regulation of transcription in yeast as well as in higher eukaryotes.

Concerted effects of heterogeneous nuclear ribonucleoprotein C1/C2 to control vitamin D-directed gene transcription and RNA splicing in human bone cells

Traditionally recognized as an RNA splicing regulator, heterogeneous nuclear ribonucleoprotein C1/C2 (hnRNPC1/C2) can also bind to double-stranded DNA and function in trans as a vitamin D response element (VDRE)-binding protein. As such, hnRNPC1/C2 may couple transcription induced by the active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D) with subsequent RNA splicing. In MG63 osteoblastic cells, increased expression of the 1,25(OH)₂D target gene CYP24A1 involved immunoprecipitation of hnRNPC1/C2 with CYP24A1 chromatin and RNA. Knockdown of hnRNPC1/C2 suppressed expression of CYP24A1, but also increased expression of an exon 10-skipped CYP24A1 splice variant; in a minigene model the latter was attenuated by a functional VDRE in the CYP24A1 promoter. In genome-wide analyses, knockdown of hnRNPC1/C2 resulted in 3500 differentially expressed genes and 2232 differentially spliced genes, with significant commonality between groups. 1,25(OH)₂D induced 324 differentially expressed genes, with 187 also observed following hnRNPC1/C2 knockdown, and a further 168 unique to hnRNPC1/C2 knockdown. However, 1,25(OH)₂D induced only 10 differentially spliced genes, with no overlap with differentially expressed genes. These data indicate that hnRNPC1/C2 binds to both DNA and RNA and influences both gene expression and RNA splicing, but these actions do not appear to be linked through 1,25(OH)₂D-mediated induction of transcription.

How Are Short Exons Flanked by Long Introns Defined and Committed to Splicing?

The splice sites (SSs) delimiting an intron are brought together in the earliest step of spliceosome assembly yet it remains obscure how SS pairing occurs, especially when introns are thousands of nucleotides long. Splicing occurs in vivo in mammals within minutes regardless of intron length, implying that SS pairing can instantly follow transcription. Also, factors required for SS pairing, such as the U1 small nuclear ribonucleoprotein (snRNP) and U2AF65, associate with RNA polymerase II (RNAPII), while nucleosomes preferentially bind exonic sequences and associate with U2 snRNP. Based on recent publications, we assume that the 5' SS-bound U1 snRNP can remain tethered to RNAPII until complete synthesis of the downstream intron and exon. An additional U1 snRNP then binds the downstream 5' SS, whereas the RNAPII-associated U2AF65 binds the upstream 3' SS to facilitate SS pairing along with exon definition. Next, the nucleosome-associated U2 snRNP binds the branch site to advance splicing complex assembly. This may explain how RNAPII and chromatin are involved in spliceosome assembly and how introns lengthened during evolution with a relatively minimal compromise in splicing.

FUS functions in coupling transcription to splicing by mediating an interaction between RNAP II and U1 snRNP

Pre-mRNA splicing is coupled to transcription by RNA polymerase II (RNAP II). We previously showed that U1 small nuclear ribonucleoprotein (snRNP) associates with RNAP II, and both RNAP II and U1 snRNP are also the most abundant factors associated with the protein fused-in-sarcoma (FUS), which is mutated to cause the neurodegenerative disease amyotrophic lateral sclerosis. Here, we show that an antisense morpholino that base-pairs to the 5' end of U1 snRNA blocks splicing in the coupled system and completely disrupts the association between U1 snRNP and both FUS and RNAP II, but has no effect on the association between FUS and RNAP II. Conversely, we found that U1 snRNP does not interact with RNAP II in FUS knockdown extracts. Moreover, using these extracts, we found that FUS must be present during the transcription reaction in order for splicing to occur. Together, our data lead to a model that FUS functions in coupling transcription to splicing via mediating an interaction between RNAP II and U1 snRNP.

Promoter-proximal polyadenylation sites reduce transcription activity

Gene expression relies on the functional communication between mRNA processing and transcription. We previously described the negative impact of a point-mutated splice donor (SD) site on transcription. Here we demonstrate that this mutation activates an upstream cryptic polyadenylation (CpA) site, which in turn causes reduced transcription. Functional depletion of U1 snRNP in the context of the wild-type SD triggers the same CpA event accompanied by decreased RNA levels. Thus, in accordance with recent findings, U1 snRNP can shield premature pA sites. The negative impact of unshielded pA sites on transcription requires promoter proximity, as demonstrated using artificial constructs and supported by a genome-wide data set. Importantly, transcription down-regulation can be recapitulated in a gene context devoid of splice sites by placing a functional bona fide pA site/transcription terminator within ~500 base pairs of the promoter. In contrast, promoter-proximal positioning of a pA site-independent histone gene terminator supports high transcription levels. We propose that optimal communication between a pA site-dependent gene terminator and its promoter critically depends on gene length and that short RNA polymerase II-transcribed genes use specialized termination mechanisms to maintain high transcription levels.

InTRONs in biotech

Eukaryotic gene expression relies on several complex molecular machineries that act in a highly coordinated fashion. These machineries govern all the different steps of mRNA maturation, from gene transcription and pre-mRNA processing in the nucleus to the export of the mRNA to the cytoplasm and its translation. In particular, the pre-mRNA splicing process consists in the joining together of sequences (known as “exons”) that have to be differentiated from their intervening sequences commonly referred to as “introns.” The complex required to perform this process is a very dynamic macromolecular ribonucleoprotein assembly that functions as an enzyme, and is called the “spliceosome.” Because of its flexibility, the splicing process represents one of the main mechanisms of qualitative and quantitative regulation of gene expression in eukaryotic genomes. This flexibility is mainly due to the possibility of alternatively recognizing the various exons that are present in a pre-mRNA molecule and therefore enabling the possibility of obtaining multiple transcripts from the same gene. However, regulation of gene expression by the spliceosome is also achieved through its ability to influence many other gene expression steps that include transcription, mRNA export, mRNA stability, and even protein translation. Therefore, from a biotechnological point of view the splicing process can be exploited to improve production strategies and processes of molecules of interest. In this work, we have aimed to provide an overview on how biotechnology applications may benefit from the introduction of introns within a sequence of interest.

A fraction of the transcription factor TAF15 participates in interactions with a subset of the spliceosomal U1 snRNP complex

RNA/ssDNA-binding proteins comprise an emerging class of multifunctional proteins with an anticipated role in coupling transcription with RNA processing. We focused here on the highly related transcription factors of the TET sub-class: TLS/FUS, EWS and in particular the least studied member TAF15. An extensive array of immunoprecipitation studies on differentially extracted HeLa nuclei revealed the specific association of TAF15 with the spliceosomal U1 snRNP complex, as deduced by the co-precipitating U1 snRNA, U1-70K and Sm proteins. Additionally, application of anti-U1 RNP autoantibodies identified TAF15 in the immunoprecipitates. Minor fractions of nuclear TAF15 and U1 snRNP were involved in this association. Pull-down assays using recombinant TAF15 and U1 snRNP-specific proteins (U1-70K, U1A and U1C) provided in vitro evidence for a direct protein-protein interaction between TAF15 and U1C, which required the N-terminal domain of TAF15. The ability of TAF15 to directly contact RNA, most likely RNA pol II transcripts, was supported by in vivo UV cross-linking studies in the presence of α-amanitin. By all findings, the existence of a functionally discrete subset of U1 snRNP in association with TAF15 was suggested and provided further support for the involvement of U1 snRNP components in early steps of coordinated gene expression.

Splicing-independent recruitment of U1 snRNP to a transcription unit in living cells

Numerous non-coding RNAs are known to be involved in the regulation of gene expression. In this work, we analyzed RNAs that co-immunoprecipitated with human RNA polymerase II from mitotic cell extracts and identified U1 small nuclear RNA (snRNA) as a major species. To investigate a possible splicing-independent recruitment of U1 snRNA to transcription units, we established cell lines having integrated a reporter gene containing a functional intron or a splicing-deficient construction. Recruitment of U snRNAs and some splicing factors to transcription sites was evaluated using fluorescence in situ hybridization (FISH) and immunofluorescence. To analyze imaging data, we developed a quantitative procedure, 'radial analysis', based on averaging data from multiple fluorescence images. The major splicing snRNAs (U2, U4 and U6 snRNAs) as well as the U2AF65 and SC35 splicing factors were found to be recruited only to transcription units containing a functional intron. By contrast, U1 snRNA, the U1-70K (also known as snRNP70) U1-associated protein as well as the ASF/SF2 (also known as SFRS1) serine/arginine-rich (SR) protein were efficiently recruited both to normally spliced and splicing-deficient transcription units. The constitutive association of U1 small nuclear ribonucleoprotein (snRNP) with the transcription machinery might play a role in coupling transcription with pre-mRNA maturation.

DExD/H box RNA helicases: multifunctional proteins with important roles in transcriptional regulation

The DExD/H box family of proteins includes a large number of proteins that play important roles in RNA metabolism. Members of this family have been shown to act as RNA helicases or unwindases, using the energy from ATP hydrolysis to unwind RNA structures or dissociate RNA–protein complexes in cellular processes that require modulation of RNA structures. However, it is clear that several members of this family are multifunctional and, in addition to acting as RNA helicases in processes such as pre-mRNA processing, play important roles in transcriptional regulation. In this review I shall concentrate on RNA helicase A (Dhx9), DP103 (Ddx20), p68 (Ddx5) and p72 (Ddx17), proteins for which there is a strong body of evidence showing that they play important roles in transcription, often as coactivators or corepressors through their interaction with key components of the transcriptional machinery, such as CREB-binding protein, p300, RNA polymerase II and histone deacetylases.

Functional coupling of RNAP II transcription to spliceosome assembly

The pathway of gene expression in higher eukaryotes involves a highly complex network of physical and functional interactions among the different machines involved in each step of the pathway. Here we established an efficient in vitro system to determine how RNA polymerase II (RNAP II) transcription is functionally coupled to pre-mRNA splicing. Strikingly, our data show that nascent pre-messenger RNA (pre-mRNA) synthesized by RNAP II is immediately and quantitatively directed into the spliceosome assembly pathway. In contrast, nascent pre-mRNA synthesized by T7 RNA polymerase is quantitatively assembled into the nonspecific H complex, which consists of heterogeneous nuclear ribonucleoprotein (hnRNP) proteins and is inhibitory for spliceosome assembly. Consequently, RNAP II transcription results in a dramatic increase in both the kinetics of splicing and overall yield of spliced mRNA relative to that observed for T7 transcription. We conclude that RNAP II mediates the functional coupling of transcription to splicing by directing the nascent pre-mRNA into spliceosome assembly, thereby bypassing interaction of the pre-mRNA with the inhibitory hnRNP proteins.

Des oncogènes à l’interface entre transcription et épissage

Oncogene activity ranges from transduction signals to transcription factors. Altered expression of oncogenes, either by chromosomal translocation, proviral insertion or point mutations, can lead to tumor formation. More specifically, data accumulated through the last two decades have shown that disregulation of oncogenic transcription factors can interfere with regulatory cascades that control the growth, differentiation, and survival of normal cells. There is also evidence that alterations of oncogene activity are associated with pre-mRNA splicing defects. The insights gained from the pivotal role of RNA polymerase II in coupling transcription and splicing have instigated a new line of research regarding the possible role of oncogenic transcription factors in pre-mRNA splicing regulation. This review focuses on recent advances addressing this question. Understanding the impact of alterations in the expression and/or function of oncogenes have important prognostic implications that can guide the design of new therapeutic drugs to promote differentiation and/or apoptosis over cell proliferation.

MAZ elements alter transcription elongation and silencing of the fibroblast growth factor receptor 2 exon IIIb

The fibroblast growth factor receptor 2 (FGFR2) gene exons IIIb and IIIc are alternatively spliced in a mutually exclusive and cell type-specific manner. FGFR2 exon choice depends on both activation and silencing. Exon IIIb silencing requires cis-acting elements upstream and downstream of the exon. To examine the influence of transcription on exon IIIb silencing, the putative RNA polymerase II (RNAPII)-pausing MAZ4 element was inserted at different positions within the FGFR2 minigene construct. MAZ4 insertions 5' to the upstream silencing elements or between exon IIIb and downstream silencing elements result in decreased silencing. An insertion 3' of the downstream silencing elements, however, has no effect on splicing. An RT-PCR elongation assay shows that the MAZ4 site in these constructs is likely to be a RNAPII pause site. Insertion of another RNAPII pause site into the minigene has a similar effect on exon IIIb silencing. Transfection of in vitro transcribed RNA demonstrates that the cell type specificity of FGFR2 alternative splicing requires co-transcriptional splicing. Additionally, changing the promoter alters both FGFR2 minigene splicing and the MAZ4 effect. We propose that RNAPII pauses at the MAZ4 elements resulting in a change in the transcription elongation complex that influences alternative splicing decisions downstream.

Perturbation of transcription elongation influences the fidelity of internal exon inclusion in Saccharomyces cerevisiae

Unknown mechanisms exist to ensure that exons are not skipped during biogenesis of mRNA. Studies have connected transcription elongation with regulated alternative exon inclusion. To determine whether the relative rates of transcription elongation and spliceosome assembly might play a general role in enforcing constitutive exon inclusion, we measured exon skipping for a natural two-intron gene in which the internal exon is constitutively included in the mRNA. Mutations in this gene that subtly reduce recognition of the intron 1 branchpoint cause exon skipping, indicating that rapid recognition of the first intron is important for enforcing exon inclusion. To test the role of transcription elongation, we treated cells to increase or decrease the rate of transcription elongation. Consistent with the "first come, first served" model, we found that exon skipping in vivo is inhibited when transcription is slowed by RNAP II mutants or when cells are treated with inhibitors of elongation. Expression of the elongation factor TFIIS stimulates exon skipping, and this effect is eliminated when lac repressor is targeted to DNA encoding the second intron. A mutation in U2 snRNA promotes exon skipping, presumably because a delay in recognition of the first intron allows elongating RNA polymerase to transcribe the downstream intron. This indicates that the relative rates of elongation and splicing are tuned so that the fidelity of exon inclusion is enhanced. These findings support a general role for kinetic coordination of transcription elongation and splicing during the transcription-dependent control of splicing.

Functional interactions between the transcription and mRNA 3' end processing machineries mediated by Ssu72 and Sub1

Transcription and processing of pre-mRNA are coupled events. By using a combination of biochemical, molecular, and genetic methods, we have found that the phylogenetically conserved transcription factor Ssu72 is a component of the cleavage/polyadenylation factor (CPF) of Saccharomyces cerevisiae. Our results demonstrate that Ssu72 is required for 3' end cleavage of pre-mRNA but is dispensable for poly(A) addition and RNAP II termination. The in vitro cleavage defect caused by depletion of Ssu72 from cells can be rescued by addition of recombinant Ssu72. Ssu72 interacts physically and genetically with the Pta1 subunit of CPF. Overexpression of PTA1 causes synthetic lethality in an ssu72-3 mutant. Moreover, Sub1, which has been implicated in transcription initiation and termination, also interacts with Pta1, and overexpression of SUB1 suppresses the growth and processing defect of a pta1 mutation. Physical interactions of Ssu72 and Sub1 with Pta1 are mutually exclusive. Based on the interactions of Ssu72 and Sub1 with both the Pta1 of CPF and the TFIIB component of the initiation complex, we present a model describing how these novel connections between the transcription and 3' end processing machineries might facilitate transitions in the RNAP II transcription cycle.

Promoter proximal splice sites enhance transcription

Reconstruction of a gene with its introns removed results in reduced levels of cytoplasmic mRNA. This is partly explained by introns promoting the export of mRNA through coupling splicing to nuclear export processes. However, we show here that splicing signals can have a direct role in enhancing gene transcription. Removal of promoter proximal splice signals from a mammalian gene or the excision of introns from two different yeast genes results in a marked reduction in levels of nascent transcription, based on both nuclear run-on and direct image analysis. This further establishes that mRNA processing and transcription are tightly coupled mechanisms.