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Production of different mature RNA sequences from the same precursor by changing which splice sites or exons are used. A cassette exon can be included in one transcript isoform and skipped in another.

Back-splicing is an RNA splicing arrangement in which a downstream splice donor site joins an upstream splice acceptor site. In a common circularization event, the retained segment becomes circular RNA with a new junction joining a later part of the precursor to an earlier part. The donor is conventionally called a $5^{\prime }$ splice site and the acceptor a $3^{\prime }$ splice site; those labels describe splice-boundary roles, not the order of their coordinates along the precursor.

For a precursor with ordered exons $1\to 2\to 3\to 4$, a circle containing exons 2 and 3 has the ordinary internal junction $2\to 3$ and a circularizing junction $3\to 2$. A sequence read spanning $3\mid 2$ thus reports a different adjacency from the reference transcript order. The example specifies a possible connectivity, not a prediction that every such precursor forms this circle.

A 2013 study combined RNA sequencing with and without RNase R treatment. Reads crossing back-splice junctions were enriched among the more resistant RNA population, and complementary tests supported circular products.^[ref] A non-colinear junction read alone can also raise questions about mapping, another RNA arrangement or genomic structure. Agreement between junction evidence and a test of molecular connectivity makes the circular interpretation stronger; it does not by itself establish the circle's biological function.

Daniel A. Pomeranz Krummel contributed to determining the architecture of U1 snRNP, a small nuclear ribonucleoprotein involved in recognizing a splice site during RNA splicing. He is a joint first author of the 2009 X-ray crystallography study of its human functional core.^[ref]

The investigators reconstituted a particle from RNA and proteins, collected diffraction at the Swiss Light Source and obtained a 5.5-angstrom electron density map. Heavy-atom labeling supplied positional landmarks for fitting individual proteins, including the seven Sm proteins. The crystallized construct omitted the U1A protein and used engineered or truncated components; the paper separately modeled the more complete particle.^[ref]

An original counting example clarifies that distinction. Seven Sm proteins, two additional U1-specific proteins and one U1 small nuclear RNA make ten molecular subunits, of which nine are proteins. Adding the omitted U1A protein gives ten proteins plus the RNA, rather than ten total subunits. This counts molecular components without asserting that every crystallized polypeptide has its full native sequence.

The scientific value is the spatial organization: a list of component names alone cannot show how RNA and proteins support one another. Conversely, the arrangement of an engineered functional core must be interpreted with its composition and resolution in view, rather than presented as an unmodified whole spliceosome.

Juan Valcárcel coauthored research on alternative splicing.^[ref]

Production of different mature RNA sequences from the same precursor by changing which splice sites or exons are used. A cassette exon can be included in one transcript isoform and skipped in another.

María José Lallena coauthored research on splice site.^[ref]

A precursor-RNA position used as a boundary in RNA splicing. Selecting different donor or acceptor sites changes which sequences remain in the mature product.

Miriam Llorian coauthored research on alternative splicing.^[ref]

Production of different mature RNA sequences from the same precursor by changing which splice sites or exons are used. A cassette exon can be included in one transcript isoform and skipped in another.

Replacing the acceptor's A with C changes the canonical $3^{\prime }$ splice site from AG to CG. The predicted primary effect is impaired recognition of the exon-5 acceptor, reducing normal exon-5 inclusion. If the processing machinery instead selects exon 6, TCF-long would decrease and TCF-short would increase, removing the long-specific corepressor-interaction sequence from more of the translated products. Other outcomes are possible, including a cryptic acceptor, intron retention, unstable transcripts or altered total expression; the nucleotide change does not uniquely determine one mature product. Test allele-specific RNA splicing by junction assays and sequencing, with matched alleles or an edited endogenous locus. Unlike a repeat indel, this is initially a splice-site perturbation rather than necessarily a coding-region frameshift mutation.

Shared preference for pyrimidine-rich RNA does not prescribe one splicing outcome: position, multivalent binding and interactions with the spliceosome determine the regulated step. Sex-lethal acts in the Drosophila sex-determination programme, whereas PTBP1 regulates many mammalian precursor RNAs in a cell-type-dependent manner. Their effects must be interpreted against competing splice sites and the proteins normally recognizing them.

During early spliceosome assembly, U1 snRNP recognizes a 5′ splice site, while factors including U2AF help recognize a polypyrimidine tract and 3′ splice-site region and promote assembly around the branch point. Alternative sites compete within this context. Binding of another RNA-binding protein can occlude an essential interaction, organize a repressive complex or favour use of a competing site; recognition need not be determined solely by an isolated site's consensus strength.

A particularly clear SXL mechanism operates on precursor RNA encoding Transformer protein. In the absence of SXL, U2AF recognizes a strong default 3′ site, producing an unproductive transcript. In female biological cells, SXL binds the pyrimidine-rich region at that site and competes with U2AF. The weaker alternative female site can then be used, producing functional Transformer protein. Reconstituted splicing experiments showed the antagonism, and adding a U2AF effector domain to SXL converted its activity from repression toward activation.^[ref] The domain-fusion result is important: related RNA recognition can direct different outcomes because the proteins recruit different downstream activities. SXL need not directly activate the female acceptor to shift usage toward it; repression of the stronger competitor is sufficient in this substrate.

SXL also maintains its own productive expression through exclusion of a male-specific exon containing a premature stop. This provides positive feedback in female biological cells. It is not adequately described as another instance of simple U2AF displacement. Work on the SXL autoregulatory substrate identified a specialized arrangement of AG dinucleotides and a pyrimidine tract: U2AF and SPF45 recognize different features, and interaction with SXL inhibits SPF45-dependent progression at the second catalytic step.^[ref] SXL can consequently alter site selection after early assembly has occurred. Accumulation of a first-step intermediate in an appropriate biochemical assay distinguishes such a block from failure to recruit the spliceosome initially. The same regulator can use different mechanisms on different RNAs.

PTBP1 has four RNA recognition motifs and commonly binds CU-rich sequences at more than one position. Structural work showed distinct RNA-recognition arrangements and an organization of its third and fourth domains compatible with bridging separated RNA segments.^[ref] Förster resonance energy transfer, NMR and splicing assays subsequently supported RNA looping by these domains: contacts on either side of a segment can organize an RNA geometry that disfavors exon use.^[ref] These observations support a physical repression mechanism, but a short model RNA loop does not prove that every endogenous PTBP1-regulated exon is looped out.

Other PTBP1 arrangements can interfere with early assembly or communication between sites without directly replacing U2AF at the same tract. Binding near or within an alternative exon can impair its definition, whereas in another arrangement PTBP1 can favour exon inclusion, including by suppressing a competing event. Genome-wide perturbation and positional analyses revealed both activation and repression associated with different binding contexts.^[ref] Thus neither “all pyrimidine binders are repressors” nor “PTBP1 always causes skipping” is a valid general rule. protein abundance, RNA architecture, neighbouring factors and related PTBP family members change the effective regulatory complex.

To discriminate mechanisms, map direct contacts by crosslinking and immunoprecipitation, measure exon-junction usage after acute depletion, and rescue with wild-type or selected RNA-binding/interface mutants. Use a minigene splicing reporter to mutate individual tracts, change their spacing or move them relative to a regulated exon, then confirm important effects at the endogenous locus. Assays of U2AF or U1 snRNP association and native spliceosomal assembly can reveal an early block; time-resolved splicing-intermediate analysis can reveal a late one. Control reporter transcription and precursor abundance, and account for nonsense-mediated decay, because an unstable mature isoform can conceal the splice choice that generated it. Mutants must retain expression and the intended remaining contacts. SXL supplies examples of both competition and late catalytic control; PTBP1 illustrates multivalent, context-dependent organization of splice-site recognition. RNA affinity is only one part of specificity.

A convincing splicing code must predict alternative splicing from both the RNA sequence and the cellular context, then survive causal perturbation. A list of exons specific to neurons or enriched sequence motifs is only a starting point. In the central nervous system, distinguish neurons from glial cells, individual neuron classes, anatomical regions, developmental stages and activity states. Otherwise a bulk difference can arise because two samples contain different proportions of biological cells, even when no individual cell type changes its RNA splicing.

Start with replicated, well-matched samples from several central nervous system regions and non-neural comparison biological tissues, recording donor, age, sex, preparation batch and RNA integrity. Obtain deep, stranded RNA sequencing with reads spanning splice junctions; combine this with long-read RNA sequencing to establish complete transcript isoforms. Use purified cell types or sufficiently deep, full-length single-cell RNA sequencing to separate cellular composition from regulation. Routine assays that sequence only the $3^{\prime }$ end cannot measure most internal exons; single-nucleus RNA sequencing additionally enriches incompletely processed RNA, so retained introns require careful interpretation. A matched bulk or purified-cell dataset supplies coverage that sparse individual-cell observations lack.

Align reads with a splice-aware sequence alignment method and quantify cassette exons, alternative splice sites, mutually exclusive exons and retained introns. Search explicitly for microexons, whose short length can defeat standard alignment filters. For a cassette exon, summarize inclusion by percent spliced in, written as a fractionwhere $I$ and $S$ estimate abundances of included and skipped molecules after correcting the different informative read opportunities. Adding the two inclusion-junction counts and comparing their sum directly with a single skipping-junction count would bias this estimate. Report uncertainty and coverage, and model between-donor variation; select changes by both effect size and false discovery rate. Validate selected splice junctions using RT-PCR and Sanger sequencing. Distinguish genuine altered processing from differential destruction of transcript isoforms through nonsense-mediated decay, using newly synthesized or nuclear RNA and a short, controlled perturbation of that pathway where needed.

Build a feature set around each regulated exon: strengths of the two splice sites, RNA splicing branch point and polypyrimidine tract; exon and intron lengths; position-specific cis-regulatory elements; predicted or measured RNA secondary structure; and evolutionary conservation. Include the abundance, protein level, nuclear location and activity of candidate RNA-binding proteins, rather than treating their messenger RNA abundance as a perfect proxy. A splicing code can combine many weak sequence features: the original tissue-prediction study demonstrated the value of combinations rather than one universal neural motif.^[ref]

Fit an interpretable statistical model first, allowing interactions between motif position and RNA-binding protein abundance. Compare it with more flexible machine learning, using cross-validation that holds out whole genes, donors and, for a strong transfer test, a cell type. Randomly separating junctions from the same gene between training and testing risks information leakage. Compare against sequence-only, context-only and expression-matched baseline predictors. Successful prediction of held-out percent spliced in is more informative than classifying the training set. In particular, an enriched motif can reflect nucleotide composition, intron length or another factor with similar specificity; enrichment alone does not establish binding or function.

Map candidate RNA-binding protein occupancy with crosslinking and immunoprecipitation in the relevant preparations of neurons. Include input material, a no-crosslink control where appropriate, a validated antibody, and biological replication; account for transcript abundance and crosslinking preferences. Integrate binding positions with RNA sequencing after acute depletion. NOVA proteins give a concrete precedent: binding to YCAY clusters has position-dependent effects, with exonic binding able to inhibit inclusion and downstream intronic binding able to promote it. Here Y denotes a pyrimidine nucleobase. Thus the same motif can support opposite predictions at different positions.^[ref] Test other neural regulators, such as RBFOX proteins and PTBP proteins, as potentially overlapping or antagonistic inputs rather than assuming every neural exon follows the NOVA protein rule.

Finally, make the predictions experimentally vulnerable. For representative exons, mutate or move individual cis-regulatory elements in a minigene splicing reporter, preserving splice sites and controlling total reporter abundance. Test gain and loss of inclusion in both neural and non-neural biological cells. Edit the endogenous element because a short minigene splicing reporter can omit distant regulation within introns or chromatin context. Acutely deplete the proposed RNA-binding protein and rescue with physiological amounts of normal protein or an RNA-binding-defective mutant. Combined depletion tests redundancy; motif mutation plus factor depletion tests whether both act through the same element. A particularly strong result is the predicted reversal after moving a binding site from a repressive to an activating position. Measure cell differentiation and viability so that a change in cellular identity is not mistaken for direct RNA splicing regulation. The resulting code is a tested, context-dependent set of regulatory rules, with an explicit domain of validity.

A pyrimidine-rich precursor-RNA segment involved in recognition near many spliceosomal $3^{\prime }$ splice sites.

A splice acceptor site is the splice site at the $3^{\prime }$ end of an intron, at its boundary with the downstream exon, in a precursor RNA. Selecting it determines where the downstream exon begins in the joined product. It is distinct from both the upstream splice donor site and the RNA splicing branch point.

For a toy precursor A|intron|B, the acceptor is the boundary intron|B. If an alternative acceptor lies farther into B, the intervening part of B is removed along with the intron. Thus alternative splicing can change mature sequence without changing the underlying DNA. Coordinates must specify whether they refer to precursor RNA, mature RNA or a genomic reference.

Acceptor recognition can involve more than one stage. In an experimental splice substrate containing two AG sequences separated by a polypyrimidine tract, U2AF recognized the tract and downstream AG, whereas SPF45 recognized the upstream AG used for the second catalytic step. Regulation could therefore block the second step after earlier assembly events.^[ref]

This observation explains why simply spotting an AG sequence is insufficient to identify which acceptor is used. Sequence context, interacting RNA splicing factors and measurements of the resulting splice junctions distinguish a candidate boundary from an experimentally observed one.

A splice donor site is the splice site at the $5^{\prime }$ end of an intron in a precursor RNA. It marks the boundary between the upstream exon and the intron. Its name is oriented along the precursor RNA, not along whichever direction a DNA reference sequence happens to be displayed.

In the toy precursor A|intron|B, with letters representing whole exons, the first boundary is the splice donor site and the second is the splice acceptor site. Removing the intron and joining A to B gives the mature splice junction. A change in donor choice can change the end of A even when acceptor choice remains fixed; this is one route to alternative splicing.

Recognition involves sequence and molecular context. The RNA component of U1 snRNP can base-pair with the donor region. A particularly informative experiment changed a donor sequence and then changed U1 to restore complementarity. Rescue of one donor mutation supported direct pairing; weak rescue of another showed that complementarity alone did not guarantee productive RNA splicing.^[ref]

This is a useful causal test: perturbing either partner can disrupt an interaction, while a compensating change in the other partner can distinguish a specific pairing model from a generic loss of function. A candidate sequence bound by U1 snRNP still has to participate in the rest of the spliceosome's reaction.