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Three of the eight RNA segments encoded by the influenza A virus (IAV) undergo alternative splicing to generate distinct proteins. Previously, we found that host proteins hnRNP K and NS1-BP regulate IAV M segment splicing, but the mechanistic details were unknown. Here we show NS1-BP and hnRNP K bind M mRNA downstream of the M2 5′ splice site (5′ss). NS1-BP binds most proximal to the 5′ss, partially overlapping the U1 snRNP binding site, while hnRNP K binds further downstream and promotes U1 snRNP recruitment. Mutation of either or both the hnRNP K and NS1-BP-binding sites results in M segment mis-splicing and attenuated IAV replication. Additionally, we show that hnRNP K and NS1-BP regulate host splicing events and that viral infection causes mis-splicing of some of these transcripts. Therefore, our proposed mechanism of hnRNP K/NS1-BP mediated IAV M splicing provides potential targets of antiviral intervention and reveals novel host functions for these proteins.

Cellular homeostasis of the minor spliceosome is regulated by a negative feed-back loop that targets U11-48K and U11/U12-65K mRNAs encoding essential components of the U12-type intron-specific U11/U12 di-snRNP. This involves interaction of the U11 snRNP with an evolutionarily conserved splicing enhancer giving rise to unproductive mRNA isoforms. In the case of U11/U12-65K, this mechanism controls the length of the 3′ untranslated region (3′UTR). We show that this process is dynamically regulated in developing neurons and some other cell types, and involves a binary switch between translation-competent mRNAs with a short 3′UTR to non-productive isoforms with a long 3′UTR that are retained in the nucleus or/and spliced to the downstream amylase locus. Importantly, the choice between these alternatives is determined by alternative terminal exon definition events regulated by conserved U12- and U2-type 5′ splice sites as well as sequence signals used for pre-mRNA cleavage and polyadenylation. We additionally show that U11 snRNP binding to the U11/U12-65K mRNA species with a long 3′UTR is required for their nuclear retention. Together, our studies uncover an intricate molecular circuitry regulating the abundance of a key spliceosomal protein and shed new light on the mechanisms limiting the export of non-productively spliced mRNAs from the nucleus to the cytoplasm., Author summary The cellular homeostasis of many components of the eukaryotic RNA processing machinery is regulated via negative feed-back pathways that result in the formation of both productive and non-productive mRNA species. Typically, the formation of non-productive mRNAs species results from changes in alternative splicing that disrupt the reading frame of the protein coding region and leads to destabilization of the mRNA. Here, we have investigated the homeostasis regulation of the U11/U12-65K mRNA that encodes an essential protein component of the minor (U12-dependent) spliceosome intron recognition complex. We show that homeostasis is regulated at the level of nuclear mRNA export and mRNA 3′-end formation, and that it can be further regulated during neuronal differentiation. We describe a multilayered regulatory system utilizing alternative exon definition interactions that use the input from both spliceosomes and the polyadenylation machinery to decide between productive and non-productive mRNA formation. Because the 65K protein is an essential component of the minor spliceosome, this regulatory pathway can potentially affect the expression of ~700 genes containing U12-type introns.

Genes encoding RNA-binding proteins have frequently been implicated in various motor neuron diseases, but the particular step in RNA metabolism that is vulnerable in motor neurons remains unknown. FUS, a nuclear protein, forms cytoplasmic aggregates in cells affected by amyotrophic lateral sclerosis (ALS), and mutations disturbing the nuclear import of FUS cause the disease. It is extremely likely that the cytoplasmic aggregates are cytotoxic because they trap important factors; the nature of these factors, however, remains to be elucidated. Here we show that FUS associates in a neuronal cell line with SMN, the causative factor in spinal muscular atrophy (SMA). The two genes work on the same pathway, as FUS binds to spliceosomal snRNPs downstream of the SMN function. Pathogenic FUS mutations do not disturb snRNP binding. Instead, cytoplasmic mislocalisation of FUS causes partial mis-localisation of snRNAs to the cytoplasm, which in turn causes a change in the behaviour of the alternative splicing machinery. FUS, and especially its mutations, thus have a similar effect as SMN1 deletion in SMA, suggesting that motor neurons could indeed be particularly sensitive to changes in alternative splicing. © 2013 Elsevier Inc.

Cleavage of histone pre-mRNAs at the 3′ end requires stem–loop binding protein (SLBP) and U7 snRNP that consists of U7 snRNA and a unique Sm ring containing two U7-specific proteins: Lsm10 and Lsm11. Lsm11 interacts with FLASH and together they bring a subset of polyadenylation factors to U7 snRNP, including the CPSF73 endonuclease that cleaves histone pre-mRNA. SLBP binds to a conserved stem–loop structure upstream of the cleavage site and acts by promoting an interaction between the U7 snRNP and a sequence element located downstream from the cleavage site. We show that both human and Drosophila SLBPs stabilize U7 snRNP on histone pre-mRNA via two regions that are not directly involved in recognizing the stem–loop structure: helix B of the RNA binding domain and the C-terminal region that follows the RNA binding domain. Stabilization of U7 snRNP binding to histone pre-mRNA by SLBP requires FLASH but not the polyadenylation factors. Thus, FLASH plays two roles in 3′ end processing of histone pre-mRNAs: It interacts with Lsm11 to form a docking platform for the polyadenylation factors, and it cooperates with SLBP to recruit U7 snRNP to histone pre-mRNA.

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

Overexpression of high-mobility group A protein 1a (HMGA1a) causes aberrant exon 5 skipping of the Presenilin-2 (PS2) pre-mRNA, which is almost exclusively detected in patients with sporadic Alzheimer's disease. An electrophoretic mobility shift assay confirmed aberrant U1 small nuclear ribonucleoprotein particle (snRNP)-HMGA1a complex formation (via the U1-70K component), with RNA containing a specific HMGA1a-binding site and an adjacent 5′ splice site. Psoralen cross-linking analysis demonstrated that the binding of HMGA1a adjacent to the 5′ splice site induces unusually extended association of U1 snRNP to the 5′ splice site. As a result, spliceosome assembly across either the intron or the exon is arrested at an early ATP-independent stage. We conclude that the HMGA1a-induced aberrant exon skipping is caused by impaired dissociation of U1 snRNP from the 5′ splice site, leading to a defect in exon definition. The proposed molecular mechanism has profound implications for other known posttranscriptional modulation strategies in various organisms, all of which are triggered by aberrant U1 snRNP binding.

The AD29 mutation in HPRP31 belongs to a series of mutations that were initially linked with the autosomal dominant disorder retinitis pigmentosa (RP) type 11. The HPRP31 gene encodes the hPrp31 protein that specifically associates with spliceosomal small nuclear ribonucleoprotein particles (snRNPs). Despite intensive research, it is still unclear how the AD29 (Ala216Pro) mutation causes RP. In this study, we report that the expression of this mutant protein affects cell proliferation and alters the structure of nuclear Cajal bodies that are connected with snRNP metabolism. Interestingly, these effects can be reversed by the over-expression of the hPrp6 protein, a binding partner of hPrp31. Although Ala216 is not contained within the U4 or U5 snRNP interacting domains, we present several lines of evidence that demonstrate that the association between the AD29 mutant and snRNPs in the cell nucleus is significantly reduced. Finally, we show that the stability of the AD29 mutant is severely affected resulting in its rapid degradation. Taken together, our results indicate that the Ala216Pro mutation destabilizes the hPrp31 protein structure in turn reducing its interaction with snRNP binding partners and leading to its rapid degradation. These findings significantly impact our understanding of the molecular mechanisms underlying RP and suggest that the insufficiency of the functional hPrp31 protein combined with the potential cytotoxicity associated with the expression the AD29 mutant are at least partially causative of the RP phenotype.

This work represents the first guide for using steric-block antisense oligos as tools for effective and targeted modification of RNA splicing. Comparison of several steric-block oligo types shows the properties of Morpholinos provide significant advantages over other potential splice-blocking oligos. The procedures and complications of designing effective splice-blocking Morpholino oligos are described. The design process requires complete pre-mRNA sequence for defining suitable targets, which usually generate specific predictable messengers. To validate the targeting procedure, the level and nature of transcript alteration is characterized by RT-PCR analysis of splice modification in a beta-globin splice model system. An oligo-walking study reveals that while U1 and U2 small nuclear RiboNucleoProtein (snRNP) binding sites are the most effective targets for blocking splicing, inclusion of these sites is not required to achieve effective splice modifications. The most effective targeting strategy employs simultaneously blocking snRNP binding sites and splice-junctions. The work presented here continues to be the basis for most of the successful Morpholino oligos designed for the worldwide research community to block RNA splicing.

Unlike typical cis-splicing, trans-splicing joins exons from two separate transcripts to produce chimeric mRNA and has been detected in most eukaryotes. Trans-splicing in trypanosomes and nematodes has been characterized as a spliced leader RNA-facilitated reaction; in contrast, its mechanism in higher eukaryotes remains unclear. Here we investigate mod(mdg4), a classic trans-spliced gene in Drosophila, and report that two critical RNA sequences in the middle of the last 5′ intron, TSA and TSB, promote trans-splicing of mod(mdg4). In TSA, a 13-nucleotide (nt) core motif is conserved across Drosophila species and is essential and sufficient for trans-splicing, which binds U1 small nuclear RNP (snRNP) through strong base-pairing with U1 snRNA. In TSB, a conserved secondary structure acts as an enhancer. Deletions of TSA and TSB using the CRISPR/Cas9 system result in developmental defects in flies. Although it is not clear how the 5′ intron finds the 3′ introns, compensatory changes in U1 snRNA rescue trans-splicing of TSA mutants, demonstrating that U1 recruitment is critical to promote trans-splicing in vivo. Furthermore, TSA core-like motifs are found in many other trans-spliced Drosophila genes, including lola. These findings represent a novel mechanism of trans-splicing, in which RNA motifs in the 5′ intron are sufficient to bring separate transcripts into close proximity to promote trans-splicing.

In disease-associated genes, the understanding of the functional significance of deep intronic nucleotide variants may represent a difficult challenge. We have previously reported a new disease-causing mechanism that involves an intronic splicing processing element (ISPE) in ATM, composed of adjacent consensus 5' and 3' splice sites. A GTAA deletion within ISPE maintains potential adjacent splice sites, disrupts a non-canonical U1 snRNP interaction and activates an aberrant exon. In this paper, we demonstrate that binding of U1 snRNA through complementarity within a approximately 40 nt window downstream of the ISPE prevents aberrant splicing. By selective mutagenesis at the adjacent consensus ISPE splice sites, we show that this effect is not due to a resplicing process occurring at the ISPE. Functional comparison of the ATM mouse counterpart and evaluation of the pre-mRNA splicing intermediates derived from affected cell lines and hybrid minigene assays indicate that U1 snRNP binding at the ISPE interferes with the cryptic acceptor site. Activation of this site results in a stringent 5'-3' order of intron sequence removal around the cryptic exon. Artificial U1 snRNA loading by complementarity to heterologous exonic sequences represents a potential therapeutic method to prevent the usage of an aberrant CFTR cryptic exon. Our results suggest that ISPE-like intronic elements binding U1 snRNPs may regulate correct intron processing.

The integrated human immunodeficiency virus type 1 (HIV-1) genome is transcribed in a single pre-mRNA that is alternatively spliced into more than 40 mRNAs. We characterized a novel bidirectional exonic splicing enhancer (ESE) that regulates the expression of the HIV-1 env , vpu , rev , and nef mRNAs. The ESE is localized downstream of the vpu -, env -, and nef -specific 3′ splice site no. 5. SF2/ASF and SRp40 activate the ESE and are required for efficient 3′ splice site usage and binding of the U1 snRNP to the downstream 5′ splice site no. 4. U1 snRNP binding to the 5′ splice site no. 4 is required for splicing of the rev and nef mRNAs and to increase expression of the partially spliced env mRNA. Finally, our results indicate that this ESE is necessary for the recruitment of the U1 snRNP to the 5′ splice site no. 4, even when the 5′ splice site and the U1 snRNA have been mutated to obtain a perfect complementary match. The ESE characterized here is highly conserved in most viral subtypes.

The aberrant splicing isoform (PS2V), generated by exon 5 skipping of the Presenilin-2 (PS2) gene transcript, is a diagnostic feature of sporadic Alzheimer's disease (AD). We found PS2V is hypoxia-inducible in human neuroblastoma SK-N-SH cells. We purified a responsible trans-acting factor based on its binding to an exon 5 fragment. The factor was identified as the high mobility group A1a protein (HMGA1a; formerly HMG-I). HMGA1a bound to a specific sequence on exon 5, located upstream of the 5' splice site. HMGA1a expression was induced by hypoxia and the protein was accumulated in the nuclear speckles with the endogenous splicing factor SC35. Overexpression of HMGA1a generated PS2V, but PS2V was repressed by cotransfection with the U1 snRNP 70K protein that has a strong affinity to HMGA1a. HMGA1a could interfere with U1 snRNP binding to the 5' splice site and caused exon 5 skipping. HMGA1a levels were significantly increased in the brain tissue from sporadic AD patients. We propose a novel mechanism of sporadic AD that involves HMGA1a-induced aberrant splicing of PS2 pre-mRNA in the absence of any mutations.

The U1 small nuclear ribonucleoprotein particle (snRNP)/5' splice site (5'SS) interaction in yeast is essential for the splicing process and depends on the formation of a short RNA duplex between the 5' arm of U1 snRNA and the 1st intronic nucleotides. This RNA/RNA interaction is characterized by the presence of a mismatch that occurs with almost all yeast introns and concerns nucleotides 4 on the pre-mRNA (a U) and 5 on U1 snRNA (a Psi). The latter nucleotide is well conserved from yeast to vertebrates, but its role in yeast and the significance of the associated mismatch in the U1 snRNA/5'SS interaction have never been fully explained. We report here that the presence of this mismatch is a determinant of stability that mainly affects the off rate of the interaction. To our knowledge this is the first report assigning a function to this noncanonical interaction. We also performed SELEX (systematic evolution of ligands by exponential enrichment) experiments by immunoprecipitating U1 snRNP and the associated RNA. The artificial phylogeny derived from these experiments allows the isolation of the selective pressure due to U1 snRNP binding on the 5'SS of yeast introns.

Disease-causing splicing mutations described in the literature primarily produce changes in splice sites and, to a lesser extent, variations in exon-regulatory sequences such as the enhancer elements. The gene ATM is mutated in individuals with ataxia-telangiectasia; we have identified the aberrant inclusion of a cryptic exon of 65 bp in one affected individual with a deletion of four nucleotides (GTAA) in intron 20. The deletion is located 12 bp downstream and 53 bp upstream from the 5' and 3' ends of the cryptic exon, respectively. Through analysis of the splicing defect using a hybrid minigene system, we identified a new intron-splicing processing element (ISPE) complementary to U1 snRNA, the RNA component of the U1 small nuclear ribonucleoprotein (snRNP). This element mediates accurate intron processing and interacts specifically with U1 snRNP particles. The 4-nt deletion completely abolished this interaction, causing activation of the cryptic exon. On the basis of this analysis, we describe a new type of U1 snRNP binding site in an intron that is essential for accurate intron removal. Deletion of this sequence is directly involved in the splicing processing defect.

The 3' ends of most eukaryotic pre-mRNAs are generated by 3' endonucleolytic cleavage and subsequent polyadenylation. 3'-end formation can be influenced positively or negatively by various factors. In particular, U1 snRNP acts as an inhibitor when bound to a 5' splice site located either upstream of the 3'-end formation signals of bovine papilloma virus (BPV) late transcripts or downstream of the 3'-end processing signals in the 5' LTR of the HIV-1 provirus. Previous work showed that in BPV it is not the first step, 3' cleavage, that is affected by U1 snRNP, but rather the second step, polyadenylation, that is inhibited. Since in HIV-1 the biological requirement is to produce transcripts that read through the 5' LTR cleavage site rather than being cleaved there, this mechanism seemed unlikely to apply. The obvious difference between the two examples was the relative orientation of the 3'-end formation signals and the U1 snRNP-binding site. In vitro assays were therefore used to assess the effect of U1 snRNP bound at various locations relative to a cleavage/polyadenylation site on the 3' cleavage reaction. U1 snRNP was found to inhibit cleavage when bound to a 5' splice site downstream of the cleavage/polyadenylation site, as in the HIV-1 LTR. U1 snRNP binding at this location was shown not to affect the recruitment of multiple cleavage/polyadenylation factors to the cleavage substrate, indicating that inhibition is unlikely to be due to steric hindrance. Interactions between U1A, U1 70K, and poly(A) polymerase, which mediate the effect of U1 snRNP on polyadenylation of other pre-mRNAs, were shown not to be required for cleavage inhibition. Therefore, U1 snRNP bound to a 5' splice site can inhibit cleavage and polyadenylation in two mechanistically different ways depending on whether the 5' splice site is located upstream or downstream of the cleavage site.

The 3′ end of histone mRNA is formed by an endonucleolytic cleavage of the primary transcript after a conserved stem-loop sequence. The cleavage reaction requires at least two trans-acting factors: the stem-loop binding protein (SLBP), which binds the stem-loop sequence, and the U7 snRNP that interacts with a sequence downstream from the cleavage site. Removal of SLBP from a nuclear extract abolishes 3′-end processing, and the addition of recombinant SLBP restores processing activity of the depleted extract. To determine the regions of human SLBP necessary for 3′ processing, various deletion mutants of the protein were tested for their ability to complement the SLBP-depleted extract. The entire N-terminal domain and the majority of the C-terminal domain of human SLBP are dispensable for processing. The minimal protein that efficiently supports cleavage of histone pre-mRNA consists of 93 amino acids containing the 73-amino-acid RNA-binding domain and 20 amino acids located immediately next to its C terminus. Replacement of these 20 residues with an unrelated sequence in the context of the full-length SLBP reduces processing >90%. Coimmunoprecipitation experiments with the anti-SLBP antibody demonstrated that SLBP and U7 snRNP form a stable complex only in the presence of pre-mRNA substrates containing a properly positioned U7 snRNP binding site. One role of SLBP is to stabilize the interaction of the histone pre-mRNA with U7 snRNP.

The negative regulator of splicing (NRS) from Rous sarcoma virus suppresses viral RNA splicing and is one of several cis elements that account for the accumulation of large amounts of unspliced RNA for use as gag-pol mRNA and progeny virion genomic RNA. The NRS can also inhibit splicing of heterologous introns in vivo and in vitro. Previous data showed that the splicing factors SF2/ASF and U1, U2, and U11 small nuclear ribonucleoproteins (snRNPs) bind the NRS, and a correlation was established between SF2/ASF and U11 binding and activity, suggesting that these factors are important for function. These observations, and the finding that a large spliceosome-like complex (NRS-C) assembles on NRS RNA in nuclear extract, led to the proposal that the NRS is recognized as a minor-class 5′ splice site. One model to explain NRS splicing inhibition holds that the NRS interacts nonproductively with and sequesters U2-dependent 3′ splice sites. In this study, we provide evidence that the NRS interacts with an adenovirus 3′ splice site. The interaction was dependent on the integrity of the branch point and pyrimidine tract of the 3′ splice site, and it was sensitive to a mutation that was previously shown to abolish U11 snRNP binding and NRS function. However, further mutational analyses of NRS sequences have identified a U1 binding site that overlaps the U11 site, and the interaction with the 3′ splice site correlated with U1, not U11, binding. These results show that the NRS can interact with a 3′ splice site and suggest that U1 is of primary importance for NRS splicing inhibition.

Splicing of Rous sarcoma virus RNA is regulated in part by a cis-acting intronic RNA element called the negative regulator of splicing (NRS). An NRS mutant affecting nt 916-923 disrupts U11 snRNP binding and reduces NRS activity (Gontarek et al., 1993, Genes & Dev 7:1926-1936). However, we observed that a U15' splice site-like sequence, which overlapped the U11 site, was also disrupted by this mutation. To determine whether the U1 or the U11 site was essential for NRS activity, we analyzed twelve additional mutants involving nt 915-926. All mutations that disrupted the potential base pairing between U1 snRNA and the NRS reduced NRS activity, including single point mutations at nt 915, 916, and 919. The point mutation at nt 919 was partially suppressed by a compensatory base change mutation in U1 snRNA. In contrast, a mutation which strengthened the potential base pairing between the U1 site and the NRS increased NRS activity. Surprisingly, mutations that specifically targeted the U115' splice site consensus sequence increased the levels of unspliced RNA, suggesting U11 binding plays an antagonistic role to NRS activity. We propose that U1 snRNP binding to the NRS inhibits splicing and is regulated by U11 snRNP binding to the overlapping sequence. Competition between U1 and U11 snRNPs would result in the appropriate balance of spliced to unspliced RNAs for optimal viral replication. Further, a virus mutated in the U1/U11 region of the NRS was found to have delayed replication.

All precursor mRNAs transcribed by RNA polymerase II have a 5′ cap (m7G[5′]ppp[5′]N) added cotranscriptionally (63, 68) by enzymes associated with RNA polymerase II (8, 45). The 5′ cap protects pre-mRNA from 5′ exoribonucleases (17, 20, 70) and has been shown to have an important role in mRNA translation (62, 69). The cap also has roles in pre-mRNA processing; it is implicated in both polyadenylation and export from the nucleus, even if it is not essential for these processes (12, 15, 21, 28, 43, 51, 77, 78), and it has an important role in splicing. A role for the 5′ cap structure in pre-mRNA splicing was shown by early studies in extracts. In HeLa whole-cell extracts, substrate pre-mRNA was spliced efficiently only when capped (32), and splicing reactions could be inhibited by adding low levels of m7GpppG or m7GTP cap analogues to the extract. In HeLa nuclear extracts, splicing was reduced, but only by one-third, in the absence of a cap (37, 52). Subsequent work showed that cap dependence was enhanced if nuclear extracts were preincubated in the presence of magnesium before the splicing reaction (13). The splicing of introns other than the cap-proximal intron is not affected much by the 5′ cap (25, 55) because of the presence of a polypyrimidine tract in an upstream intron (42). This is consistent with the suggestion that the cap functions in a manner analogous to that of an upstream polypyrimidine tract in the definition of the adjacent exon (1, 23, 24, 42, 58). The effects of the cap on pre-mRNA processing and export are mediated by a cap-binding complex (CBC). In mammals, this comprises two proteins of 80 and 20 kDa (26, 30, 54). Immunodepletion of the CBC led to a loss of spliceosome assembly (26), including the loss of most U1 snRNP base pairing with the 5′ splice site (42). In Saccharomyces cerevisiae, loss of either capping enzyme activity or a CBC component affected splicing efficiency in vivo (9, 16, 66); in vitro, depletion of CBC reduced commitment complex formation and splicing (9, 41), but the absence of a 5′ cap on the pre-mRNA did not affect the efficiency of splicing (9, 66). The finding that the CBC was required for efficient interactions of U1 snRNA with the 5′ splice site seemed to be inconsistent with other evidence. Not only are there reports that uncapped RNA can be spliced, but in some cases uncapped RNA has been used intentionally to detect complexes at 5′ splice sites (52, 53, 61). Furthermore, there are numerous reports of unaided interactions between pure U1 snRNPs and 5′ splice sites (5, 22, 27, 31, 49, 60, 75). These findings could be reconciled if the cap-CBC interaction was not required constitutively but only at specific sites where U1 snRNP binding is hindered by, for example, weak 5′ splice sites or sequestration of sites by proteins or RNA secondary structure. If this is true, then the exact mechanism of the cap effect on U1 snRNP binding would be expected to affect the selection of alternative 5′ splice sites in a 5′ exon. Thus, if binding of U1 snRNPs was enhanced by the cap, and the cap-proximal site was favored, then splicing would favor that site. If the cap enhanced binding at all sites, then weak sites would behave like strong sites and the presence of the cap would lead to a shift in splicing preferences towards the cap-distal (downstream) site, according to the model described in reference 14. If the cap enhanced binding only at specific sequestered sites, then the presence of the cap would lead to a shift towards those sites at the expense of the site favored in the absence of the cap. These schemes all suggest that the presence of the cap or the concentration of cap-binding proteins may have important effects on 5′ splice site selection. Previous studies have shown that the cap cannot just promote cap-proximal U1 snRNP binding with alternative strong 5′ splice sites, because initial U1 snRNP binding did not appear to depend on the proximity of the sites to the cap and the cap-distal site was spliced (14, 50), but discriminatory effects of the cap cannot be discounted for weaker sites, where the initial binding of U1 snRNPs is less well characterized (5, 74) and the cap-proximal site is sometimes preferred (57). The uncertainties about the generality of the cap requirement for U1 snRNP binding have such important implications for splice site selection that we sought to determine whether the cap is required for binding and splicing at all alternative splice sites, including strong consensus splice sites. We investigated U1 snRNP binding to alternative 5′ splice sites on pre-mRNA capped with m7GpppG or ApppG. Splicing at all the sites was cap dependent. These experiments revealed that there were two kinetic modes for U1 snRNP binding, a rapid binding mode that depended on the 5′ cap and a slower mode that was seen in the absence of the correct cap or at sites that were not used for splicing. The cap itself did not appear to determine which site was bound rapidly and spliced. Strikingly, the m7GpppG cap was required for efficient interactions of U6 snRNA at active 5′ splice sites.

It has previously been shown in vivo that bovine papillomavirus represses its late gene expression via a 5' splice site sequence located upstream of the late polyadenylation signal. Here, the mechanism of repression is determined by in vitro analysis. U1 snRNP binding to the 5' splice site results in inhibition of polyadenylation via a direct interaction with poly(A) polymerase (PAP). Although the inhibitory mechanism is similar to that used in U1A autoregulation, U1A within the U1 snRNP does not contribute to PAP inhibition. Instead the U1 70K protein, when bound to U1 snRNA, both interacts with and inhibits PAP. Conservation of the U1 70K inhibitory domains suggests that polyadenylation regulation via PAP inhibition may be more widespread than previously thought.

The first AG dinucleotide downstream from the branchpoint sequence (BPS) is chosen as the 3' splice site during catalytic step II of the splicing reaction. The mechanism and factors involved in selection of this AG are not known. Early in mammalian spliceosome assembly, U2AF65 binds to the pyrimidine tract between the BPS and AG. Here we show that U2AF65 crosslinking is replaced by crosslinking of three proteins of 110, 116 and 220 kDa prior to catalytic step II, and we provide evidence that all three proteins are components of U5 snRNP. These proteins interact with pre-mRNA in the region spanning from immediately downstream of U2 snRNP's binding site at the BPS to just beyond the 3' splice site. We also demonstrate that there are strict constraints on both the sequence and the distance between the BPS and AG for catalytic step II. Together, these observations suggest that U5 snRNP is positioned on the 3' splice site by an interaction (direct or indirect) with U2 snRNP bound at the BPS and by a direct interaction with the pyrimidine tract. The functional AG for catalytic step II may be specified, in turn, by its location with respect to the U5 snRNP binding site.

The mechanism by which intron-containing RNAs are recognized by the splicing machinery is only partly understood. A nuclear cap-binding complex (CBC), which specifically recognizes the monomethyl guanosine cap structure carried by RNA polymerase II transcripts, has previously been shown to play a role in pre-mRNA splicing. Using a combination of splicing complex and psoralen cross-linking analysis we demonstrate that CBC is required for efficient recognition of the 5' splice site by U1 snRNP during formation of E (early) complex on a pre-mRNA containing a single intron. However, in a pre-mRNA containing two introns, CBC is not required for splicing of the cap distal intron. In this case, the presence of an intact polypyrimidine tract in the cap-proximal intron renders splicing of the cap-distal intron independent of CBC. These results support models in which the splice sites in a pre-mRNA are originally recognized by interactions spanning exons. The defects in splicing and U1 snRNP binding caused by CBC depletion can be specifically reversed by recombinant CBC. In summary, efficient recognition of the cap-proximal 5' splice site by U1 snRNP is facilitated by CBC in what may be one of the earliest steps in pre-mRNA recognition. Data in Colot et al. (this issue) indicate that this function of CBC is conserved in humans and yeast.

U1 snRNP (U1 small nuclear ribonucleoprotein) is a well-characterized splicing factor. Besides, when U1 snRNP binds close to a putative polyadenylation site, mostly located in introns, it prevents premature cleavage and polyadenylation and controls the length of most cellular mRNAs. On the other hand, U1 snRNP binding close to the 3'-end of some mRNAs, inhibits polyadenylation and, therefore, gene expression. The inhibition of polyadenylation by U1 snRNP is the basis of U1i (U1 snRNP-based inhibition), a technique used to inhibit gene expression. U1i consists of the expression of a U1 snRNP modified to interact with a target mRNA and inhibit target gene expression. U1i has been used to inhibit the expression of reporter or endogenous genes both in tissue culture and in animal models. Furthermore, combination of U1i and RNA interference (RNAi) results in synergistic increased inhibitions which allow the dose of inhibitors to be decreased whilst at the same time obtaining good inhibitions with fewer unwanted secondary effects. The combination of RNAi and U1i is of special interest for antiviral therapy, as a functional decrease of the expression of replicative viral RNAs may require high inhibition and the combination of two or more inhibitors should decrease the possibility of escape mutants resistant to treatment. In fact, a therapy with U1i combined with RNAi is currently being developed for the treatment of HBV infections. We believe that this review will clarify the hallmarks of U1i technology and will encourage many laboratories to use U1i for functional studies and therapeutic applications.

The Sm binding sites of different spliceosomal U small nuclear RNAs (snRNAs), the RNA structural elements required for interaction with common snRNP proteins, have been considered to be similar or identical. Here we show that this is not the case. Instead, structural and sequence features unique to U1 or U5 snRNAs that contribute to common protein binding are identified. The determinants of Sm protein binding in both RNAs are complex, consisting in U5 of minimally two and in U1 of minimally four separate structural elements. Even the most conserved features of the two RNAs, single-stranded regions whose generalized sequence is PuA(U)nGPu, are not functionally interchangeable in protein binding. At least one of the newly defined RNA elements functions in assembly with the common proteins, but is not required for their stable binding thereafter. U1, but not U5, snRNP requires a trimethyl guanosine cap structure for its transport to the nucleus. This is not a consequence of the differences in common snRNP binding to the two RNAs, but is due to structural features of U1 RNA that do not contribute to Sm protein binding.

Somatic inhibition restricts splicing of the Drosophila P-element third intron (IVS3) to the germ line. We have exploited this simple system to provide a model for a mechanism of alternative pre-mRNA splicing. Biochemical complementation experiments revealed that Drosophila somatic extracts inhibited U1 snRNP binding to the 5' splice site. Using sensitive RNase protection and modification-interference assays, we found that U1 snRNP bound to a pseudo-5' splice site in the 5' exon and that multiprotein complexes bound to an adjacent site. Binding of these factors appeared to mediate the inhibitory effect, because mutations in the pseudo-5' splice sites blocked binding and activated splicing in vitro. Likewise, wild-type, but not mutant, 5' exon RNA titrated inhibitory factors away from the pre-mRNA and activated splicing. Thus, we have defined the pseudo-5' splice sites as crucial components of the regulatory element, correlated the inhibitory activity with specific RNA binding factors from Drosophila somatic cells, and provided a mechanistic description of somatic inhibition. Because the inhibitory activity involves general splicing functions such as protein recognition of 5' splice site sequences and changes in the distribution of bound U1 snRNP, our data may also provide insights into how splice sites are selected.

A two-site model for the binding of U1 small nuclear ribonucleoprotein particle (U1 snRNP) was tested in order to understand how exon partners are selected in complex pre-mRNAs containing alternative exons. In this model, it is proposed that two U1 snRNPs define a functional unit of splicing by base pairing to the 3' boundary of the downstream exon as well as the 5' boundary of the intron to be spliced. Three-exon substrates contained the alternatively spliced exon 4 (E4) region of the preprotachykinin gene. Combined 5' splice site mutations at neighboring exons demonstrate that weakened binding of U1 snRNP at the downstream site and improved U1 snRNP binding at the upstream site result in the failure to rescue splicing of the intron between the mutations. These results indicate the stringency of the requirement for binding a second U1 snRNP to the downstream 5' splice site for these substrates as opposed to an alternative model in which a certain threshold level of U1 snRNP can be provided at either site. Further support for the two-site model is provided by single-site mutations in the 5' splice site of the third exon, E5, that weaken base complementarity to U1 RNA. These mutations block E5 branchpoint formation and, surprisingly, generate novel branchpoints that are specified chiefly by their proximity to a cryptic 5' splice site located at the 3' terminus of the pre-mRNA. The experiments shown here demonstrate a true stimulation of 3' splice site activity by the downstream binding of U1 snRNP and suggest a possible mechanism by which combinatorial patterns of exon selection are achieved for alternatively spliced pre-mRNAs.

We have studied the consequences of decreasing the donor site-branch site distance on splicing factor-splice site interactions by analyzing alternative splicing of adenovirus E1A pre-mRNAs in vitro. We show that the proximal 13S donor site has a cis-inhibiting effect on the 9S and 12S mRNA reactions when it is brought too close to the common branch site, suggesting that the factor interactions in the common 3' part of the intron are impaired by the U1 small nuclear ribonucleoprotein particle (snRNP) binding to the displaced 13S donor site. Further analysis of the interactions was carried out by studying complex assembly and the accessibility to micrococcal nuclease digestion of 5'-truncated E1A substrates containing only splice sites for the 13S mRNA reaction. A deletion which brings the donor site- branch site distance to 49 nucleotides, which is just below the minimal functional distance, results in a complete block of the U4-U5-U6 snRNP binding, whereas a deletion 15 nucleotides larger results in a severe inhibition of the formation of the U2 snRNP-containing complexes. Sequence accessibility analyses performed by using the last mini-intron-containing transcript demonstrate that the interactions of U2 snRNP with the branch site are strongly impaired whereas the initial bindings of U1 snRNP to the donor site and of specific factors to the 3' splice site are not significantly modified. Our results strongly suggest that the interaction of U1 snRNP with the donor site of a mini-intron is stable enough in vitro to affect the succession of events leading to U2 snRNP binding with the branch site.

U1 interference (U1i) is a novel method to block gene expression. U1i requires expression of a 5'-end-mutated U1 snRNA designed to base pair to the 3'-terminal exon of the target gene's pre-mRNA that leads to inhibition of polyadenylation. Here, we show U1i is robust (> or =95%) and a 10-nt target length is sufficient for good silencing. Surprisingly, longer U1 snRNAs, which could increase annealing to the target, fail to improve silencing. Extensive mutagenesis of the 10-bp U1 snRNA:target duplex shows that any single mismatch different from GU at positions 3-8, destroys silencing. However, mismatches within the other positions give partial silencing, suggesting that off-target inhibition could occur. The specificity of U1i may be enhanced, however, by the fact that silencing is impaired by RNA secondary structure or by splicing factors binding nearby, the latter mediated by Arginine-Serine (RS) domains. U1i inhibition can be reconstituted in vivo by tethering of RS domains of U1-70K and U2AF65. These results help to: (i) define good target sites for U1i; (ii) identify and understand natural cellular examples of U1i; (iii) clarify the contribution of hydrogen bonding to U1i and to U1 snRNP binding to 5' splice sites and (iv) understand the mechanism of U1i.

The 5' splice site of a pre-mRNA is recognized by U1 small nuclear ribonucleoprotein particles (snRNP) through base pairing with the 5' end of U1 small nuclear RNA (snRNA). Single-base substitutions within a 9-nucleotide 5'-splice-site sequence can abolish or attenuate use of that site and, in higher eukaryotes, can also activate nearby "cryptic" 5' splice sites. Here we show that the effects of single-base substitutions within a 5' splice site can be completely or partially suppressed by cis mutations that improve the overall complementarity of the site to U1 snRNA. We further show that in the presence of the normal 5' splice site, a cryptic 5' splice site can be activated by increasing its complementarity to U1 snRNA. U1 snRNP binding experiments confirm that cryptic 5' splice sites are activated when their affinity for U1 snRNP approaches that of the authentic 5' splice site. Based upon these results, we propose a spliceosome competition model for 5'-splice-site selection and cryptic 5'-splice-site activation. We discuss our results with regard to the factors involved in 5'-splice-site recognition.

Adeno-associated virus type 5 (AAV5) is unique among human AAV serotypes in that it uses a polyadenylation site [(pA)p] within the single small intron in the center of the genome. We previously reported that inhibition of polyadenylation at (pA)p, necessary for read-through of P41-generated capsid gene pre-mRNAs which are subsequently spliced, requires binding of U1 snRNP to the upstream donor. Inhibition was reduced as the distance between the cap site and the donor was increased (increasing the size of the 5′ exon). Here, we have demonstrated that U1-70K is a key component of U1 snRNP that mediates inhibition of polyadenylation at (pA)p. Furthermore, introduction of a U-rich stretch, predicted to target TIA-1 and thus increase the affinity of U1 snRNP binding to the intervening donor site, significantly augmented inhibition of (pA)p, while depletion of TIA-1 by siRNA increased (pA)p read-through. Finally, artificially tethering the cap binding complex (CBC) components CBP80 and CBP20 upstream of the intron donor increased inhibition of polyadenylation at (pA)p. Our results suggest that interaction with the CBC strengthens U1 snRNP binding to the downstream intron donor in a manner inversely proportional to the size of the 5′ exon, thus governing the competition between intron splicing and polyadenylation at (pA)p. This competition must be optimized to program both the levels of polyadenylation of P7- and P19-generated RNA at (pA)p required to produce proper levels of the essential Rep proteins and the splicing of P41-generated RNAs to produce the proper ratio of capsid proteins during AAV5 infection.

The Rous sarcoma virus (RSV) negative regulator of splicing (NRS) is an RNA element that represses splicing and promotes polyadenylation of viral RNA. The NRS acts as a pseudo 5′ splice site (ss), and serine-arginine (SR) proteins, U1snRNP, and U6 small nuclear ribonucleoproteins (snRNPs) are implicated in its function. The NRS also efficiently binds U11 snRNP of the U12-dependent splicing pathway, which is interesting, because U11 binds only poorly to authentic substrates that lack a U12-type 3′ splice site. It is of considerable interest to understand how the low abundance U11 snRNP binds the NRS so well. Here we show that U11 can bind the NRS as a mono-snRNP in vitro and that a G-rich element located downstream of the U11 site is required for efficient binding. Mutational analyses indicated that two of four G tracts in this region were important for optimal U11 binding and that the G-rich region did not function indirectly by promoting U1 snRNP binding to an overlapping site. Surprisingly, inactivation of U2 snRNP also decreased U11 binding to the NRS. The NRS harbors a branch point-like/pyrimidine tract sequence (BP/Py) just upstream of the U1/U11 site that is characteristic of 3′ splice sites. Deletion of this region decreased U2 and U11 binding, and deletion of the G-rich region also reduced U2 binding. The G element, but not the BP/Py sequence, was also required for U11 binding to the NRS in vivo as assessed by minor class splicing from the NRS to a minor class 3′ss from the P120 gene. These results indicate that efficient U11 binding to the isolated NRS involves at least two elements in addition to the U11 consensus sequence and may have implications for U11 binding to authentic splicing substrates.

Adeno-associated virus type 5 is unique among adeno-associated virus serotypes in that it uses a polyadenylation site in the center of the genome. The great majority of transcripts generated from the upstream P7 and P19 promoters are polyadenylated at a site in the central intron ((pA)p); however, most of the viral transcripts generated by the proximal P41 promoter are polyadenylated at the distal polyadenylation site at the 3′ end of the genome (pA)d and subsequently spliced. Polyadenylation at (pA)p increases as the distance between the RNA initiation site and the intron and (pA)p site is increased. The steady-state level of RNAs polyadenylated at (pA)p is independent of the promoter used or of the intervening sequence but is dependent upon competition with splicing, inhibition by U1 snRNP binding to the intron donor, and the intrinsic efficiency of the cleavage/polyadenylation reaction. Each of these determinants shows a marked dependence on the distance between the RNA initiation site and the intron and (pA)p. Finally, unlike other reported systems, inhibition of (pA)p by U1 snRNP binding to the intron donor is decreased as the distance between the donor and (pA)p is increased.

P-element somatic inhibitor (PSI) is a KH domain-containing splicing factor highly expressed in Drosophila somatic tissues. Here we have identified a direct association of PSI with the spliceosomal U1 small nuclear ribonucleoprotein (snRNP) particle in somatic nuclear extracts. This interaction is mediated by highly conserved residues within the PSI C-terminal AB motif and the U1 snRNP-specific 70K protein. Through the AB motif, PSI modulates U1 snRNP binding on the P-element third intron (IVS3) 5′ splice site and its upstream exonic regulatory element. Ectopic expression experiments in the Drosophila female germline demonstrate that the AB motif also contributes to IVS3 splicing inhibition in vivo. These data show that the processing of specific target transcripts, such as the P-element mRNA, is regulated by a functional PSI-U1 snRNP interaction in Drosophila .

The first component known to recognize and discriminate among potential 5' splice sites (5'SSs) in pre-mRNA is the U1 snRNP. However, the relative levels of U1 snRNP binding to alternative 5'SSs do not necessarily determine the splicing outcome. Strikingly, SF2/ASF, one of the essential SR protein-splicing factors, causes a dose-dependent shift in splicing to a downstream (intron-proximal) site, and yet it increases U1 snRNP binding at upstream and downstream sites simultaneously. We show here that hnRNP A1, which shifts splicing towards an upstream 5'SS, causes reduced U1 snRNP binding at both sites. Nonetheless, the importance of U1 snRNP binding is shown by proportionality between the level of U1 snRNP binding to the downstream site and its use in splicing. With purified components, hnRNP A1 reduces U1 snRNP binding to 5'SSs by binding cooperatively and indiscriminately to the pre-mRNA. Mutations in hnRNP A1 and SF2/ASF show that the opposite effects of the proteins on 5'SS choice are correlated with their effects on U1 snRNP binding. Cross-linking experiments show that SF2/ASF and hnRNP A1 compete to bind pre-mRNA, and we conclude that this competition is the basis of their functional antagonism; SF2/ASF enhances U1 snRNP binding at all 5'SSs, the rise in simultaneous occupancy causing a shift in splicing towards the downstream site, whereas hnRNP A1 interferes with U1 snRNP binding such that 5'SS occupancy is lower and the affinities of U1 snRNP for the individual sites determine the site of splicing.

Frontotemporal dementia accounts for a significant fraction of dementia cases. Frontotemporal dementia with parkinsonism linked to chromosome 17 is associated with either exonic or intronic mutations in the tau gene. This highlights the involvement of aberrant pre-mRNA splicing in the pathogenesis of neurodegenerative disorders. Little is known about the molecular mechanisms of the splicing defects underlying these diseases. To establish a model system for studying the role of pre-mRNA splicing in neurodegenerative diseases, we have constructed a tau minigene that reproduces tau alternative splicing in both cultured cells and in vitro biochemical assays. We demonstrate that mutations in a nonconserved intronic region of the human tau gene lead to increased splicing between exon 10 and exon 11. Systematic biochemical analyses indicate the importance of U1 snRNP and, to a lesser extent, U6 snRNP in differentially recognizing wild-type versus intron mutant tau pre-mRNAs. Gel mobility shift assays with purified U1 snRNP and oligonucleotide-directed RNase H cleavage experiments support the idea that the intronic mutations destabilize a stem-loop structure that sequesters the 5' splice site downstream of exon 10 in tau pre-mRNA, leading to increases in U1 snRNP binding and in splicing between exon 10 and exon 11. Thus, mutations in nonconserved intronic regions that increase rather than decrease alternative splicing can be an important pathogenic mechanism for the development of human diseases.

The chicken β-tropomyosin pre-mRNA is spliced in a tissue-specific manner. Internal exons 6B and 6A are specifically used in skeletal muscle and non-skeletal muscle cells, respectively. Pre-mRNA secondary structure around exon 6B has been shown to be part of the mechanism that inhibits exon 6B to 7 splicing in HeLa nuclear extract. We analyse the influence of pre-mRNA folding on the different steps of spliceosome assembly under different conditions. At 3 mM MgCl 2 , conditions that favour RNA structure formation, the interactions of U1, U2, U4, U5 and U6 small nuclear ribonucleoprotein particles (snRNPs) with the pre-mRNA are all affected. The study of several mutants destabilising some proposed stem-loop structures shows that the in vitro splicing activation is correlated with an increased binding of snRNPs on pre-mRNA molecules. At 1 mM MgCl 2 , conditions that allow a partial relaxation of the inhibitory structure, U1 snRNP binding on exon 6B 5′ splice site occurs very efficiently. Nonetheless,if this first step of spliceosome assembly is derepressed, U2, U4, U5 and U6 snRNP interaction processes remain inhibited. Altogether, these results suggest that the choice between exon 6A and 6B donor sites is a complex process not simply directed by a difference in the efficiency of interaction between U1 snRNP and alternative 5′ splice sites.

A cis-acting negative regulator of splicing (NRS) within the gag gene of RSV is involved in control of the relative levels of spliced and unspliced viral mRNAs. Insertion of the NRS into the intron of an adenovirus pre-mRNA resulted in inhibition of splicing in vitro before the first cleavage step. Analyses of spliceosome assembly with this substrate showed that it formed large RNP complexes that did not migrate like mature spliceosomes on native gels. Affinity selection of the RNP complexes formed on NRS-containing pre-mRNAs showed an association with U11 and U12 snRNPs, as well as with the spliceosomal snRNPs. Immunoprecipitation with antisera specific for U1 and U2 snRNPS showed binding of both snRNPs to NRS RNA. A 7-nucleotide missense mutation in the NRS that prevented binding of U11 and U12 snRNPs impaired NRS activity in vivo, suggesting a functional role for U11 and U12 snRNPs in the inhibition of splicing mediated by the RSV NRS RNA.

We have used protection against ribonuclease H to investigate the mechanisms by which U1 small nuclear ribonucleoprotein particles (snRNPs) determine the use of two alternative 5' splice sites. The initial binding of U1 snRNPs to alternative consensus splice sites was indiscriminate, and on a high proportion of pre-mRNA molecules both sites were occupied simultaneously. When the sites were close, this inhibited splicing. We propose that double occupancy leads to the use of the downstream site for splicing and that this is the cause of the proximity effect seen with strong alternative splice sites. This model predicts that splicing to an upstream site of any strength requires a low affinity of U1 snRNPs for the downstream site. This prediction was tested both by cleaving the 5' end of U1 snRNA and by altering the sequence of the downstream site of an adenovirus E1A gene. The enhancement of downstream 5' splice site use by splicing factor SF2/ASF appears to be mediated by an increase in the strength of U1 snRNP binding to all sites indiscriminately.

The ability of purified U1 small nuclear RNA-protein complexes (U1 snRNPs) to bind in vitro to two RNAs transcribed from recombinant DNA clones by bacteriophage T7 RNA polymerase has been studied. A transcript which contains sequences corresponding to the small intron and flanking exons of the major mouse beta-globin gene is bound in marked preference to an RNA devoid of splice site sequences. The site of U1 snRNP binding to the globin RNA has been defined by T1 ribonuclease digestion of the RNA-U1 snRNP complex. A 15-17-nucleotide region, including the 5' splice site, remains undigested and complexed with the snRNP such that it can be co-precipitated by antibodies directed against the U1 snRNP. Partial proteinase K digestion of the U1 snRNP abolishes interaction with the globin RNA, indicating that the snRNP proteins contribute significantly to RNA binding. No RNA cleavage, splicing, or recognition of the 3' splice site by U1 snRNPs has been detected. Our results are discussed in terms of the probable role of U1 snRNPs in the messenger RNA splicing of eucaryotic cell nuclei.

We have studied the assembly, composition and structure of splicing complexes using biotin-avidin affinity chromatography and RNase protection assays. We find that U1, U2, U4, U5 and U6 snRNPs associate with the pre-mRNA and are in the mature, functional complex. Association of U1 snRNP with the pre-mRNA is rapid and ATP independent; binding of all other snRNPs occurs subsequently and is ATP dependent. Efficient binding of U1 and U2 snRNPs requires a 5' splice site or a 3' splice site/branch point region, respectively. Both sequence elements are required for efficient U4, U5 and U6 snRNP binding. Mutant RNA substrates containing only a 5' splice site or a 3' splice site/branch point region are assembled into 'partial' splicing complexes, which contain a subset of these five snRNPs. RNase protection experiments indicate that in contrast to U1 and U2 snRNPs, U4, U5 and U6 snRNPs do not contact the pre-mRNA. Based upon the time course of snRNP binding and the composition of sucrose gradient fractionated splicing complexes we suggest an assembly pathway proceeding from a 20S (U1 snRNP only) through a 40S (U1 and U2 snRNPs) to the functional 60S splicing complex (U1, U2, U4, U5 and U6 snRNPs).