Spliceosome Definition: The RNA Splicing Machine Explained
By Dr. Zubair Khalid, DVM, MS, PhD ·
What Is the Spliceosome? A Simple Definition
The spliceosome is a large, dynamic ribonucleoprotein complex—a molecular machine composed of both RNA and protein—that catalyzes the removal of non-coding sequences called introns from precursor messenger RNA (pre-mRNA) and joins the remaining coding sequences, called exons, together. This process, known as pre-mRNA splicing, is an essential step in gene expression in all eukaryotic cells, from yeast to humans.
Every protein-coding gene in a eukaryotic genome contains introns that interrupt the coding sequence. When a gene is transcribed by RNA polymerase II, the initial product is pre-mRNA, which is a faithful copy of the entire gene, including both exons and introns. If this pre-mRNA were translated directly, the resulting protein would be non-functional because the intronic sequences would introduce frameshifts and premature stop codons. The spliceosome solves this problem by precisely excising introns and stitching exons together to produce a mature messenger RNA (mRNA) that can be exported to the cytoplasm and translated into protein.
The importance of the spliceosome cannot be overstated. In humans, more than 95% of multi-exon genes undergo alternative splicing, meaning the spliceosome can join exons in different combinations to produce multiple distinct proteins from a single gene. This dramatically expands the coding capacity of the genome. The spliceosome is also remarkably abundant: it is estimated that a single human cell contains on the order of 10⁵ to 10⁶ spliceosomal complexes at any given time, reflecting the high flux of splicing that must occur to sustain cellular function.
The spliceosome is not a static structure but rather a highly dynamic assembly that forms anew on each intron, undergoes a series of conformational rearrangements, catalyzes two chemical reactions, and then disassembles. This cycle of assembly, catalysis, and disassembly is tightly regulated and consumes energy in the form of ATP hydrolysis. Understanding the spliceosome is fundamental to understanding how genetic information flows from DNA to protein and how defects in this process cause disease.
The Discovery of the Spliceosome
The story of the spliceosome begins in 1977 with a surprising discovery that overturned the prevailing view of gene structure. At that time, the central dogma of molecular biology—DNA makes RNA makes protein—was well established, but it was assumed that genes were continuous stretches of DNA that were directly copied into RNA. Two research groups independently shattered this assumption.
Phillip Sharp and his colleagues at the Massachusetts Institute of Technology were studying adenovirus, a virus that infects human cells. They used electron microscopy to visualize hybrids formed between viral DNA and the viral mRNA produced during infection. When they annealed the mRNA to the DNA template, they expected to see a continuous RNA-DNA duplex. Instead, they observed loops of single-stranded DNA that were not base-paired with the mRNA. These loops represented DNA sequences that were present in the genome but absent from the mature mRNA. In other words, the gene was split into segments, and the intervening sequences were somehow removed during RNA processing.
At the same time, Richard Roberts and his team at Cold Spring Harbor Laboratory were performing similar experiments with adenovirus and reached the same conclusion. They found that the mRNA was complementary to several distinct, non-contiguous regions of the viral DNA, indicating that the primary transcript contained sequences that were later eliminated. These intervening sequences were named introns, and the retained sequences were called exons. Sharp and Roberts shared the 1993 Nobel Prize in Physiology or Medicine for this discovery.
The immediate question was: what machinery performs this excision? In the early 1980s, several lines of evidence pointed to a large RNA-protein complex. First, it was observed that splicing occurs in the nucleus, not in the cytoplasm, and that it requires ATP. Second, experiments using cell-free extracts showed that splicing could be reproduced in vitro, allowing biochemical dissection of the process. Third, a class of small, abundant nuclear RNAs called U-rich small nuclear RNAs (snRNAs) were found to be essential for splicing. These snRNAs, named U1, U2, U4, U5, and U6, are each associated with a set of proteins to form small nuclear ribonucleoprotein particles (snRNPs, pronounced "snurps").
In 1985, the term "spliceosome" was coined to describe the large complex that assembles from these snRNPs and additional protein factors on the pre-mRNA substrate. Subsequent work, much of it from the laboratories of Joan Steitz, Christine Guthrie, and John Abelson, established that the spliceosome assembles in a stepwise manner, that the snRNAs play direct catalytic roles, and that the complex undergoes major structural rearrangements during the splicing reaction. The spliceosome was thus revealed as a ribonucleoprotein machine of extraordinary complexity, rivaling the ribosome in its sophistication.
Components of the Spliceosome: snRNPs and Proteins
The spliceosome is composed of five major snRNPs—U1, U2, U4/U6, and U5—along with numerous non-snRNP protein factors. In total, the human spliceosome contains over 150 distinct proteins, making it one of the most complex molecular machines in the cell. The Spliceosome Structure is organized around a core of snRNA-protein complexes that provide the scaffolding for the catalytic center.
snRNPs: The Core Building Blocks
Each snRNP consists of a small nuclear RNA molecule (approximately 100–300 nucleotides long) complexed with a set of proteins. The snRNAs are uridine-rich, hence the "U" designation. The protein components include both common proteins shared by all snRNPs and proteins unique to each snRNP.
The common proteins are the seven Sm proteins, named B/B', D1, D2, D3, E, F, and G. These form a ring-shaped structure around a conserved sequence motif on the snRNA called the Sm site. The Sm ring is a hallmark of the Sm-class snRNPs and is essential for their biogenesis, stability, and nuclear import.
Each snRNP has a distinct function in splicing:
- U1 snRNP: Contains U1 snRNA, which has a 5' end sequence complementary to the 5' splice site of the intron. U1 snRNP recognizes and base-pairs with the 5' splice site, initiating spliceosome assembly. The U1-specific proteins include U1-70K, U1-A, and U1-C.
- U2 snRNP: Contains U2 snRNA, which base-pairs with the branch point sequence within the intron. This interaction bulges out the branch point adenosine, positioning it for the first catalytic step. U2-specific proteins include U2-A' and U2-B''.
- U4/U6 snRNP: U4 and U6 snRNAs are extensively base-paired with each other in a single snRNP particle. U6 is the catalytic core of the spliceosome, while U4 acts as a chaperone that keeps U6 in an inactive conformation. During activation, the U4/U6 interaction is disrupted, and U6 base-pairs with U2 to form the active site. U4 and U5 snRNPs associate through protein-protein interactions.
- U5 snRNP: Contains U5 snRNA, which interacts with exon sequences at both the 5' and 3' splice sites, helping to align the two exons for ligation. U5-specific proteins include U5-200kD, U5-116kD, and U5-102kD.
The Spliceosome Composed of these snRNPs also includes the Lsm proteins, which are Sm-like proteins that associate with U6 snRNA and are important for U6 stability and function.
Non-snRNP Splicing Factors
In addition to the snRNPs, the spliceosome requires a large number of non-snRNP proteins. These include:
- The SF1/BBP complex: The branch point binding protein (BBP) in yeast, or splicing factor 1 (SF1) in humans, binds to the branch point sequence early in assembly.
- The U2AF complex: The U2 auxiliary factor binds to the polypyrimidine tract and the 3' splice site, helping to recruit U2 snRNP. U2AF is a heterodimer of a 65 kDa subunit (U2AF65) and a 35 kDa subunit (U2AF35).
- The Prp19/CDC5 complex (NTC): The nineteen complex (NTC) is a large protein assembly that is essential for spliceosome activation. It stabilizes the catalytic core and is required for the first transesterification reaction.
- The RES complex: A smaller complex involved in the recognition of the 3' splice site in some introns.
- DExD/H-box RNA helicases: These enzymes use ATP hydrolysis to remodel RNA-RNA and RNA-protein interactions during spliceosome assembly and disassembly. Key examples include Prp5, Prp28, Brr2, Prp2, Prp16, Prp22, and Prp43. These helicases are responsible for the conformational changes that drive the splicing cycle forward.
- The Prp8 protein: A large (280 kDa) protein that is a central component of the catalytic core. Prp8 interacts with the 5' splice site, the branch point, and the 3' splice site simultaneously and is thought to coordinate the two catalytic steps.
The Spliceosome Proteins list is extensive, and many of these factors are highly conserved from yeast to humans, underscoring the fundamental importance of the splicing machinery.
How the Spliceosome Works: The Splicing Cycle
The splicing reaction proceeds through a well-defined cycle of assembly, catalysis, and disassembly. This cycle is driven by ATP-dependent conformational changes and involves the sequential association and dissociation of snRNPs. The entire process can be reproduced in vitro using nuclear extracts, ATP, and a radiolabeled pre-mRNA substrate, typically incubated at 30°C for 30–90 minutes.
Step 1: Recognition of the 5' Splice Site
The first step in spliceosome assembly is the recognition of the 5' splice site by U1 snRNP. The U1 snRNA contains a sequence at its 5' end that is complementary to the conserved 9-nucleotide consensus sequence at the 5' splice site of the intron (typically AG/GURAGU, where the slash indicates the exon-intron boundary, R is a purine, and the GU dinucleotide is invariant). Base-pairing between U1 snRNA and the pre-mRNA commits the intron to the splicing pathway.
Simultaneously, the branch point binding protein SF1 recognizes the branch point sequence (typically UACUAAC in yeast, with the adenosine that will form the lariat being the underlined A). The U2AF heterodimer binds to the polypyrimidine tract immediately downstream of the branch point and to the conserved AG dinucleotide at the 3' splice site. This early complex is called the E complex (early complex) or commitment complex.
Step 2: Branch Point Recognition and Lariat Formation
The next step is the ATP-dependent recruitment of U2 snRNP to the branch point. The U2 snRNA base-pairs with the branch point sequence, displacing SF1 and bulging out the branch point adenosine. This interaction positions the 2' hydroxyl group of the branch point adenosine for the first catalytic step. The resulting complex, containing U1, U2, and the pre-mRNA, is called the A complex.
The U4/U6.U5 tri-snRNP is then recruited to form the B complex. This recruitment is mediated by protein-protein interactions and requires the U5 snRNP protein Prp8, which contacts all three splice sites. The B complex is catalytically inactive; the U4/U6 interaction must be disrupted to release U6 for catalysis.
Spliceosome activation involves the action of the RNA helicase Prp28, which destabilizes the U1-5' splice site interaction, allowing U6 to base-pair with the 5' splice site. The helicase Brr2 then unwinds the U4/U6 duplex, releasing U4 snRNP and allowing U6 to fold into its catalytically active conformation. U6 base-pairs with U2 to form the U2/U6 helix, which constitutes the catalytic core of the spliceosome. This activated complex is called the B* complex.
The first transesterification reaction then occurs: the 2' hydroxyl group of the branch point adenosine attacks the phosphate at the 5' splice site. This cleaves the pre-mRNA at the 5' splice site and joins the 5' end of the intron to the branch point adenosine via a 2'-5' phosphodiester bond. This creates a lariat structure—a looped intron with a branched nucleotide—and releases the 5' exon. The resulting complex is called the C complex.
Step 3: Exon Ligation and Spliceosome Recycling
After the first catalytic step, the spliceosome must rearrange to position the 3' splice site for the second transesterification. The helicase Prp16 promotes this conformational change, which involves the movement of U5 snRNP to align the two exons. The U5 snRNA base-pairs with exon sequences at both the 5' and 3' splice sites, holding the two exons in close proximity.
The second transesterification reaction then occurs: the 3' hydroxyl group of the 5' exon attacks the phosphate at the 3' splice site. This cleaves the intron at the 3' splice site and simultaneously joins the two exons together. The lariat intron is released, and the mature mRNA is formed.
The post-catalytic complex must then be disassembled. The helicase Prp22 releases the mRNA from the spliceosome, and the helicase Prp43 disassembles the remaining complex, releasing the lariat intron for degradation and recycling the snRNPs for another round of splicing. The lariat intron is debranched by the enzyme Dbr1 and degraded.
The entire splicing cycle, from initial recognition to disassembly, takes approximately 30 seconds to several minutes in vivo. The Spliceosome Assembly pathway is highly ordered, and each step is subject to proofreading by the helicases, which can reject incorrect substrates and promote discard of aberrant complexes.
Alternative Splicing: One Gene, Many Proteins
The spliceosome is not a rigid machine that always removes every intron in the same way. Instead, it can choose different splice sites, skip exons, or retain introns, generating multiple mRNA isoforms from a single gene. This phenomenon is called alternative splicing, and it is a major mechanism for generating proteomic diversity in eukaryotes.
It is estimated that over 95% of human multi-exon genes undergo alternative splicing. This means that the ~20,000 protein-coding genes in the human genome can produce hundreds of thousands of distinct proteins. Alternative splicing is particularly prevalent in the nervous system, where it contributes to the enormous complexity of neuronal function.
Types of Alternative Splicing
There are several distinct patterns of alternative splicing:
- Exon skipping: An entire exon is excluded from the mature mRNA. This is the most common type of alternative splicing in humans.
- Alternative 5' splice site selection: Two or more different 5' splice sites are used, leading to extension or truncation of the upstream exon.
- Alternative 3' splice site selection: Two or more different 3' splice sites are used, leading to extension or truncation of the downstream exon.
- Intron retention: An intron is not removed and remains in the mature mRNA. This is the most common type in plants, fungi, and protozoa.
- Mutually exclusive exons: Two or more exons are present, but only one can be included in any given mRNA.
These patterns can be combined in complex ways, allowing a single gene to produce dozens or even hundreds of distinct mRNA isoforms.
Regulation of Splice Site Choice
The choice of splice sites is regulated by a combination of cis-acting elements and trans-acting factors. Cis-acting elements are sequences in the pre-mRNA that either enhance or silence splicing. These include:
- Exonic splicing enhancers (ESEs): Sequences within exons that promote exon inclusion.
- Exonic splicing silencers (ESSs): Sequences within exons that promote exon skipping.
- Intronic splicing enhancers (ISEs): Sequences within introns that promote splicing.
- Intronic splicing silencers (ISSs): Sequences within introns that inhibit splicing.
These elements are recognized by trans-acting proteins. The serine/arginine-rich (SR) proteins are a family of splicing factors that generally bind to ESEs and promote splicing by recruiting the spliceosome. The heterogeneous nuclear ribonucleoproteins (hnRNPs) generally bind to silencers and inhibit splicing by blocking access of the spliceosome.
The balance between SR proteins and hnRNPs, as well as their phosphorylation states, determines whether a particular splice site is used. This regulation is cell-type-specific and developmentally controlled, allowing different tissues to produce different protein isoforms from the same gene. The Spliceosome Splicing machinery is thus a central hub for integrating regulatory signals that control gene expression.
Methods to Study the Spliceosome
Understanding the spliceosome has required the development of sophisticated experimental techniques. These methods have revealed the composition, structure, and dynamics of the splicing machinery.
In Vitro Splicing Assays
The development of cell-free splicing systems in the early 1980s was a breakthrough. Nuclear extracts from HeLa cells (a human cell line) or yeast can catalyze splicing of exogenously added pre-mRNA substrates. In a typical assay, a radiolabeled pre-mRNA is incubated with nuclear extract, ATP, and a buffer containing 20 mM HEPES (pH 7.9), 60 mM KCl, 3 mM MgCl₂, and 2 mM ATP at 30°C for 60–90 minutes. The products are then separated by denaturing polyacrylamide gel electrophoresis and visualized by autoradiography.
This system allowed researchers to identify the intermediates and products of splicing, to determine the order of snRNP assembly, and to test the function of individual components by depletion and add-back experiments. In vitro splicing assays remain a cornerstone of spliceosome research.
Cryo-EM and Structural Biology
The spliceosome is a large, dynamic complex that has resisted high-resolution structural analysis for decades. However, the advent of cryo-electron microscopy (cryo-EM) has revolutionized the field. Cryo-EM allows visualization of macromolecular complexes in their native, frozen-hydrated state without the need for crystallization.
Since 2015, cryo-EM structures of the yeast and human spliceosome at various stages of the splicing cycle have been determined at resolutions of 3–6 Å. These structures have revealed the molecular architecture of the catalytic core, the arrangement of the snRNAs, and the positions of key protein factors. For example, the structure of the C complex showed how U2 and U6 snRNAs form the catalytic center, with the two metal ions that catalyze the transesterification reactions coordinated by conserved phosphate groups. The Spliceosome Structure is now known in exquisite detail, providing a framework for understanding the mechanism of splicing.
High-Throughput Sequencing of Splicing Events
RNA sequencing (RNA-seq) has transformed the study of splicing on a genome-wide scale. By sequencing the RNA content of cells, researchers can identify all splicing events, quantify isoform abundance, and compare splicing patterns between different conditions.
RNA-seq involves converting RNA to complementary DNA (cDNA), fragmenting it, and sequencing the fragments. The resulting reads are aligned to the genome, and junctions between exons can be detected. This allows identification of novel splice sites, quantification of alternative splicing events, and discovery of splicing changes associated with disease.
More recently, techniques such as CLIP-seq (crosslinking and immunoprecipitation followed by sequencing) have been used to map the binding sites of splicing factors on RNA genome-wide. These methods have revealed the regulatory networks that control splicing and have identified disease-associated mutations that disrupt splicing factor binding.
Spliceosome Dysfunction and Human Disease
Given the central role of splicing in gene expression, it is not surprising that defects in splicing cause a wide range of human diseases. These can arise from mutations in the spliceosome components themselves or from mutations in the cis-acting elements that regulate splicing.
Spliceosomopathies
The term "spliceosomopathy" refers to diseases caused by mutations in core spliceosomal components. These are often rare, multisystem disorders with distinct clinical features.
- Spinal muscular atrophy (SMA): This is one of the best-studied spliceosomopathies. SMA is caused by mutations in the SMN1 gene, which encodes the survival of motor neuron protein. SMN is not a core spliceosomal protein but is essential for the assembly of the Sm core on snRNAs. Without SMN, snRNPs are not properly assembled, leading to global splicing defects that particularly affect motor neurons. SMA is characterized by progressive muscle weakness and atrophy. The related gene SMN2 can partially compensate, and therapies that increase SMN2 expression or modify its splicing have been developed.
- Retinitis pigmentosa: This is a group of inherited retinal degenerative diseases. Mutations in several splicing factors, including PRPF3, PRPF8, PRPF31, and RP9, cause autosomal dominant retinitis pigmentosa. These proteins are components of the U4/U6.U5 tri-snRNP, and their mutations lead to defects in splicing that are particularly toxic to photoreceptor cells.
- Cerebellar ataxia: Mutations in the splicing factor SPT5 and other components have been linked to neurodegenerative disorders characterized by progressive loss of motor coordination.
- Mosaic variegated aneuploidy syndrome: Mutations in the splicing factor BUB1B cause this rare disorder, which is characterized by chromosomal instability and a predisposition to cancer.
Splicing Defects in Cancer
Cancer cells frequently exhibit widespread changes in splicing. These changes can be caused by:
- Mutations in splicing factors: Recurrent mutations in the splicing factors SF3B1, U2AF1, and SRSF2 are found in many cancers, particularly in myelodysplastic syndromes, chronic lymphocytic leukemia, and uveal melanoma. These mutations alter the splicing preferences of the factors, leading to aberrant splicing of specific target genes.
- Altered expression of splicing factors: Many cancers overexpress SR proteins or downregulate hnRNPs, shifting the balance of splicing regulation.
- Mutations in splicing regulatory elements: Mutations in ESEs or ESSs can disrupt the normal splicing of tumor suppressor genes or oncogenes. For example, mutations in the splicing regulatory elements of the tumor suppressor TP53 can lead to exon skipping and loss of function.
The splicing changes in cancer can generate protein isoforms that promote cell proliferation, survival, and metastasis. This has led to interest in developing drugs that target the spliceosome as anticancer therapies. For example, the compound pladienolide B and its derivatives bind to the SF3B1 subunit of U2 snRNP and inhibit splicing, showing antitumor activity in preclinical models.
Common Misconceptions and Pitfalls
Several misconceptions about the spliceosome are common among students and even some researchers. Clarifying these can prevent confusion.
Misconception 1: The spliceosome is a static structure. The spliceosome is often depicted in textbooks as a pre-formed complex that binds to pre-mRNA and performs splicing. In reality, the spliceosome assembles de novo on each intron and disassembles after each round of splicing. It is a highly dynamic machine that undergoes major conformational rearrangements, with different snRNPs joining and leaving at different stages.
Misconception 2: Splicing is the same as capping or polyadenylation. These are all RNA processing events, but they are distinct. Capping (addition of a 7-methylguanosine cap to the 5' end) and polyadenylation (addition of a poly(A) tail to the 3' end) are modifications of the ends of the RNA, while splicing is the removal of internal intron sequences. Capping and polyadenylation are catalyzed by different enzymes, not by the spliceosome.
Misconception 3: All introns are removed by the same mechanism. The major spliceosome described in this article removes the vast majority of introns, which have GU at the 5' end and AG at the 3' end. However, there is also a minor spliceosome that removes a rare class of introns with AU at the 5' end and AC at the 3' end. The minor spliceosome uses different snRNPs (U11, U12, U4atac, U6atac) but a shared U5 snRNP. Additionally, some introns are self-splicing, meaning they can catalyze their own excision without the spliceosome, as in the case of group I and group II introns.
Misconception 4: The spliceosome is a protein enzyme. While the spliceosome contains many proteins, the catalytic center is formed by RNA—specifically the U2 and U6 snRNAs. The spliceosome is therefore a ribozyme, a catalytic RNA-protein complex. The Spliceosome a Ribozyme designation reflects the evolutionary relationship between the spliceosome and self-splicing introns.
Misconception 5: Splicing always occurs after transcription is complete. In vivo, splicing is often co-transcriptional, meaning it occurs while the pre-mRNA is still being synthesized by RNA polymerase II. The spliceosome can assemble on the nascent RNA as soon as the relevant splice sites are transcribed. This coupling of transcription and splicing is important for efficient processing and for the regulation of alternative splicing.
Misconception 6: The spliceosome recognizes introns by their sequence alone. While the consensus sequences at the 5' splice site, branch point, and 3' splice site are important, they are not sufficient for accurate splicing. Many introns have weak consensus sequences that require additional regulatory elements and factors for recognition. The Spliceosome Complex integrates information from multiple sequence elements to achieve specificity.
Frequently Asked Questions
What is the simplest definition of a spliceosome?
The spliceosome is a large molecular machine in the cell nucleus that removes non-coding sequences (introns) from pre-mRNA and joins the coding sequences (exons) together to produce mature mRNA. It is composed of RNA and protein and is essential for gene expression in all eukaryotes.
What does the spliceosome do in biology?
The spliceosome performs pre-mRNA splicing, which is the removal of introns and the ligation of exons. This is a required step in the expression of most protein-coding genes. The spliceosome also enables alternative splicing, allowing a single gene to produce multiple different proteins.
Is the spliceosome an enzyme?
The spliceosome is a ribonucleoprotein complex that catalyzes two transesterification reactions. The catalytic center is formed by RNA (U2 and U6 snRNAs), making the spliceosome a ribozyme. However, it also contains many protein components that stabilize the structure and regulate the reaction.
Where is the spliceosome found in the cell?
The spliceosome is found in the nucleus of eukaryotic cells. It assembles on pre-mRNA either during transcription (co-transcriptionally) or shortly after transcription is complete. The mature mRNA is then exported to the cytoplasm for translation.
What are the main components of the spliceosome?
The main components are the five small nuclear ribonucleoproteins (snRNPs): U1, U2, U4/U6, and U5. Each snRNP consists of a small nuclear RNA and associated proteins. The spliceosome also contains numerous non-snRNP proteins, including RNA helicases, scaffolding proteins, and regulatory factors.
How does the spliceosome recognize intron boundaries?
The spliceosome recognizes intron boundaries through base-pairing between snRNAs and conserved sequences in the pre-mRNA. U1 snRNA base-pairs with the 5' splice site, U2 snRNA base-pairs with the branch point, and U2AF binds to the polypyrimidine tract and 3' splice site. Additional regulatory elements and proteins help refine this recognition.
What is alternative splicing and how does the spliceosome achieve it?
Alternative splicing is the process by which different combinations of exons are joined together to produce multiple mRNA isoforms from a single gene. The spliceosome achieves this by using different splice sites, skipping exons, or retaining introns. This choice is regulated by splicing enhancers and silencers that bind SR proteins and hnRNPs, which either promote or inhibit spliceosome assembly at specific sites.
Key Takeaways
- The spliceosome is a large, dynamic ribonucleoprotein complex that removes introns from pre-mRNA and joins exons to produce mature mRNA.
- It is composed of five snRNPs (U1, U2, U4/U6, U5) and over 150 proteins, with the catalytic center formed by U2 and U6 snRNAs, making it a ribozyme.
- Splicing occurs through a stepwise cycle of assembly, two transesterification reactions, and disassembly, driven by ATP-dependent RNA helicases.
- Alternative splicing allows a single gene to produce multiple protein isoforms, greatly expanding the proteome, and is regulated by cis-acting elements and trans-acting factors.
- The spliceosome was discovered in 1977 through experiments showing that genes are split into exons and introns, a finding recognized with the 1993 Nobel Prize.
- Mutations in spliceosomal components or splicing regulatory elements cause diseases including spinal muscular atrophy, retinitis pigmentosa, and many cancers.
- The spliceosome is a highly dynamic machine that assembles anew on each intron, and its study has been revolutionized by cryo-EM and high-throughput sequencing.