Describe RNA processing (5' cap, poly-A tail, splicing)

Overview

In eukaryotes, the primary transcript (pre-mRNA) synthesized by RNA Polymerase II must undergo extensive post-transcriptional modifications before becoming mature mRNA ready for translation. These modifications—addition of a 5' cap, addition of a 3' poly-A tail, and splicing to remove introns—are essential for mRNA stability, nuclear export, and efficient translation.

Why RNA Processing Exists

Three strategic advantages:

1. Stability: Naked RNA ends are vulnerable to exonucleases. The 5' cap and poly-A tail protect against degradation.
2. Regulation: Alternative splicing allows one gene to produce multiple proteins—massive expansion of proteomic diversity without genome expansion.
3. Quality control: Only properly processed mRNA exits the nucleus. Defective transcripts are retained and degraded.

Prokaryotes skip this because:

• No nucleus → transcription and translation are coupled
• No introns in most bacterial genes → no splicing needed
• mRNA lifespan is intentionally short (minutes) for rapid response

The Three Core Modifications

1. 5' Capping

Structure: m7G(5')ppp(5')N1m-N2m-...

Derivation: How the Cap is Added

Step 1 — Triphosphate removal The pre-mRNA's 5' end initially has a triphosphate: 5'-ppN-... An enzyme (RNA triphosphatase) removes one phosphate:

$\text{5'-pppN-...} \xrightarrow{\text{triphosphatase}} \text{5'-ppN-...} + \text{P}_i$

Step 2 — GTP addition Guanyl transferase adds GMP from GTP in reverse orientation:

$\text{5'-ppN-...} + \text{GTP} \xrightarrow{\text{guanylyl transferase}} \text{5'-Gpp-N-...} + \text{PP}_i$

Why reverse? This creates a 5'-5' bond (not the usual 3'-5'), making it resistant to normal exonucleases.

Step 3 — Methylation

• Guanine-7-methyltransferase: adds CH₃ to N-7 of guanine → m7G
• 2'-O-methyltransferases: add CH₃ to 2'-OH of ribose on first nucleotides

$\text{5'-Gppp-N-...} \xrightarrow{\text{SAM (methyl donor)}} \text{5'-m7Gpp-N1m-...}$

Why these steps?

• The 5'-' linkage is chemically unusual → exonucleases can't cleave it
• Methylation provides recognition signal for cap-binding proteins

$\Delta G_{\text{binding}} \approx -10 \text{ to } -12 \text{ kcal/mol}$

This is sufficient for specific recognition but reversible for downstream handoff to eIF4E during translation.

Functions of the 5' Cap

1. Protection from degradation: 5'-3' exonucleases cannot degrade the unusual 5'-5' bond
2. Translation initiation: Ribosome recruitment via eIF4E (eukaryotic initiation factor 4E) binding
3. Nuclear export: Cap-binding complex (CBC) required for mRNA export through nuclear pore
4. RNA stability signaling: Cells recognize capped mRNA as "self" vs uncapped viral RNA

Step-by-step:

1. eIF4E binds m7G cap (Kd ≈ 10⁻⁸ M)
2. eIF4G scaffolds eIF4E to the40S ribosomal subunit
3. The complex scans 5'→3' to find AUG start codon
4. Translation initiates

Why this step? The cap provides the entry point. Without it, ribosomes cannot efficiently locate the start codon in the sea of 5' UTR nucleotides.

Quantitative: Cap increases translation efficiency ~10-100 fold compared to uncapped mRNA.

2. Polyadenylation (Poly-A Tail)

Derivation: How Polyadenylation Occurs

The signal sequence: Pre-mRNA contains AAUAAA hexanucleotide ~10-30 nt upstream of cleavage site, plus a downstream U-rich or GU-rich element.

Step 1 — Recognition

• CPSF (Cleavage and Polyadenylation Specificity Factor) binds AAUAAA
• CstF (Cleavage stimulation Factor) binds downstream element
• These recruit cleavage factors

Step 2 — Cleavage The endonuclease complex cleaves pre-mRNA 10-30 nt downstream of AAUAAA:

$\text{...AAUAAA----(cleavage site)-----GU-rich...} \xrightarrow{\text{endonuclease}} \text{...AAUAA-----3'-OH} + \text{5'-P-GU-rich...}$

The downstream product is degraded.

Step 3 — Poly-A addition Poly-A polymerase (PAP) adds ~250 adenines without template:

$\text{3'-OH} + n \cdot \text{ATP} \xrightarrow{\text{PAP}} \text{3'-(A)}_n + n \cdot \text{PP}_i$

Why no template? PAP has a unique active site that specifically adds only AMP, regardless of template. The length (~250 nt) is determined by poly-A binding proteins (PABPs) coating the growing tail.

Termination: When ~250 PABPs are bound, they block further PAP activity.

$\frac{dL}{dt} = k - \lambda L$

At steady state: $L_{\text{ss}} = \frac{k}{\lambda}$

Typical values: k ≈ 20nt/s during synthesis, λ ≈ 0.001-0.01 s⁻¹ in cytoplasm.

Result: Newly exported mRNA has ~250 A's. Over hours, deadenylases shorten it. When L< 30, mRNA is targeted for decay.

Functions of Poly-A Tail

1. Stability: Protects 3' end from '-5' exonucleases
2. Translation enhancement: PABP binds poly-A tail, interacts with eIF4G at 5' cap → circularizes mRNA
3. mRNA localization: Some mRNAs are localized via poly-A-binding protein interactions
4. Decay timer: Progressive deadenylation serves as molecular clock for mRNA lifespan

Mechanism:

1. eIF4E binds 5' cap
2. eIF4G (scaffold protein) binds eIF4E
3. PABP binds poly-A tail
4. PABP binds eIF4G → the mRNA forms a closed loop

Why? This topology:

• Allows ribosome recycling:bosome finishing at 3' end can immediately re-initiate at 5' cap
• Increases translation efficiency 2-3 fold
• Coordinates 5' and 3' end surveillance (if one end is damaged, both degradation pathways are activated)

Formula for loop probability: For mRNA of length L, cap-tail distance r:

$r_{\text{eff}} = \sqrt{\frac{L \cdot l_p}{3}}$

where $l_p \approx 2$ nm is persistence length. Circularization is favorable when protein bridges (eIF4G-PABP) span < 50 nm.

3. Splicing (Intron Removal)

The Chemistry of Splicing: A Two-Step Transesterification

Key sequences:

• 5' splice site (donor): consensus AG|GUAUGU (| = exon-intron boundary)
• 3' splice site (acceptor): CAG|G
• Branch point: An adenine ~20-50 nt upstream of 3' splice site, consensus YNYURAY (Y=pyrimidine, R=purine)

Step 1 — Branch point attack

The 2'-OH of the branch point adenine attacks the phosphodiester bond at the 5' splice site:

$\text{Exon1-3'-O-}\overset{\text{O}}{\text{||}}{\text{-P-O-5'-Intron}} + \text{A(2'-OH)} \rightarrow \text{Exon1-3'-OH} + \text{A(2'-O-5'-Intron)}$

Result:

• Exon 1 has free 3'-OH
• Intron forms lariat structure (2'-5' phosphodiester bond creates a loop)

Step 2 — Exon ligation

The 3'-OH of exon 1 attacks the phosphodiester bond at 3' splice site:

$\text{Exon1-3'-OH} + \text{Intron-3'-O-}\overset{\text{O}}{\text{||}}{\text{-P-O-5'-Exon2}} \rightarrow \text{Exon1-Exon2} + \text{Lariat-Intron}$

Result: Exons are joined; intron is released as lariat and degraded.

$\Delta G^{\circ} \approx 0 \text{ kcal/mol per reaction}$

But: The spliceosome hydrolyzes multiple ATPs to drive conformational changes that ensure:

• Correct splice site pairing
• Proofreading
• Directionality

Total energy cost: ~10-15 ATP per splicing event.

The Spliceosome: How It Works

Solution: A dynamic, self-assembling molecular machine.

Components:

• snRNPs (small nuclear ribonucleoproteins): U1, U2, U4, U5, U6
  • Each contains snRNA (small nuclear RNA) + proteins
  • snRNAs base-pair with splice sites and catalyze chemistry
• Protein factors: SF1, U2AF, other regulatory proteins

Assembly pathway:

1. E complex (early): U1 snRNP binds 5' splice site via base pairing; SF1 binds branch point; U2AF binds 3' splice site
2. A complex: U2 snRNP displaces SF1, base-pairs with branch point (bulging out the reactive adenine)
3. B complex: U4/U6•U5 tri-snRNP joins
4. *B (activated)**: U4 leaves; U6 replaces U1 at 5' splice site; active site forms
5. C complex: Catalysis of step 1 → lariat formation
6. C complex*: Catalysis of step 2 → exon ligation

Why this complexity? Each transition is a checkpoint. If splice sites are mismatched or regulatory signals block assembly, splicing aborts.

Splicing of intron 1:

Step 1:

• U2 snRNP identifies branch A at position105 (130-25)
• The 2'-OH of A₁₀₅ attacks the bond between exon 1 (nt 142) and intron ()
• Products: Exon 1 (142 nt, free 3'-OH) + Lariat-intron 1-exon 2-intron 2-exon 3

Step 2:

• 3'-OH of exon 1 (nt 142) attacks bond between intron 1 (nt 130) and exon 2 (nt 1)
• Products: Exon 1-exon 2 (364 nt) + Lariat-intron 1 (130 nt)

Why these steps? The lariat intermediate is obligatory—it's the only way the branch point 2'-OH can attack in trans. Direct attack of exon 1 on exon 2 is geometrically impossible without this intermediate.

Verification: The spliced junction should read ...exon1-CAG|exon2... Check: last 3nt of exon 1 + first nt of exon 2 should maintain reading frame. For β-globin, exon 1 ends ...GAG, exon 2 starts GUG → ...GAGGUG... ✓ Lys-Val codons maintained.

Alternative Splicing: One Gene, Many Proteins

Mechanisms:

1. Exon skipping: An exon is included in some mRNA isoforms, excluded in others
2. Alternative 5' or 3' splice sites: Shifts the boundary → longer/shorter exons
3. Intron retention: Intron is not removed → often introduces stop codon
4. Mutually exclusive exons: Only one of two exons is included

Regulation:

• SR proteins (serine-arginine rich) bind exonic splicing enhancers (ESE) → promote inclusion
• hnRNPs bind exonic splicing silencers (ES) → promote skipping

Why? Allows:

• Tissue-specific protein isoforms (e.g., muscle vs brain)
• Developmental stage-specific variants
• Response to signaling (e.g., neuronal activity changes splicing patterns)

Calculation: Potential protein isoforms = 12 × 48 × 33 × 2 = 38,016 different proteins from one gene.

Why? Dscam is a cell-surface protein in neurons. Each neuron expresses a unique combination → molecular barcodes for self-recognition (prevents self-synapsing).

Formula for diversity: $N_{\text{isoforms}} = \prod_{i=1}^{k} n_i$ where $n_i$ is the number of alternatives in cluster i.

Common Mistakes

Steel-man: It's reasonable to think template-based synthesis applies throughout. After all, transcription follows a template.

The truth: The poly-A tail is added post-transcriptionally without a template. The DNA has the AUAAA signal and cleavage site, but poly-A polymerase adds ~250 A's independent of template.

Evidence: If you sequence the genomic DNA at the 3' end of a gene, you'll find the polyadenylation signal but NOT a poly-A stretch. Only the mRNA has poly-A.

Fix: Remember poly-A tail = "added by PAP enzyme after cleavage."

Steel-man: Linguistic confusion is real. "Exon" sounds like it should be removed.

The truth:

• Exons = expressed sequences (kept in mature mRNA)
• Introns = intragenic regions (intervening, removed)

Mnemonic trick: EXons are EXported from nucleus (they're expressed). INTRons stay INTRanuclear (degraded there).

Fix: Exons → exit nucleus. Introns → stay in nucleus (as lariat debris).

Steel-man: Textbooks often present them in sequence, implying sequential independence.

The truth: They are co-transcriptional and functionally coupled.

• Capping occurs when transcript is only 20-30 nt long (RNA Pol II pauses)
• The C-terminal domain (CTD) of RNA Pol II recruits capping enzymes, splicing factors, and polyadenylation machinery
• Cap-binding complex (CBC) enhances splicing of first intron by recruiting U1 snRNP

Evidence: Mutations disrupting the 5' cap reduce splicing efficiency of nearby introns.

Formula for coupling: If cap deposition has probability p_cap and splicing has probability p_splice:

• Independent: P(both) = p_cap × p_splice≈ 0.9 × 0.95 = 0.855
• Observed (coupled): P(both) ≈ 0.98

Fix: Think of RNA processing as a coordinated assembly line on RNA Pol II's CTD, not isolated events.

Integration: The Full Processing Pathway

Timeline (all co-transcriptional):

1. 0-30 nt transcribed: 5' cap added
2. Elongation: Splicing factors recruited to CTD → spliceosomes assemble on introns
3. Polyadenylation signal transcribed: CPSF/CstF bind, trigger cleavage
4. Post-cleavage: Poly-A tail added (~1 minute)
5. Quality control: Exon junction complexes (EJC) deposited 20-24 nt upstream of each splice junction
6. Export: CBC (cap) + PABP (poly-A) + EJCs recognized by export machinery
7. Nuclear pore transit: Only fully processed mRNA passes
8. Cytoplasm: CBC replaced by eIF4E, translation begins

Why this order? Cap protects early. Splicing must finish before polyadenylation because cleavage terminates transcription. Export is the final checkpoint.

$\eta = p_{\text{cap}} \cdot p_{\text{splice}}^{n} \cdot p_{\text{polyA}}$

where n = number of introns.

Typical values: p_cap ≈ 0.99, p_splice ≈ 0.98, p_polyA ≈ 0.95

For a gene with 5 introns: η = 0.99 × 0.98⁵ × 0.95 ≈ 0.85

Result: ~15% of transcripts fail processing and are degraded. This is intentional—quality control over quantity.

Before the cell can use this recipe to make a protein, it has to:

1. Put a special cap on the front (5' cap) - Think of it like a protective cover on a book. This cap protects the RNA from being chewed up by enzymes, and it's also like a "START HERE" flag for the protein-making machines later.

2. Add a long tail at the end (poly-A tail) - This is like adding 200-250 copies of the letter'A' at the end. Why? It's another protective layer, like laminating the last page. It also helps the recipe last longer and work better.

3. Cut out all the nonsense (splicing) - This is the coolest part! Special molecular scissors (the spliceosome) cut out every intron and glue the exons back together. It's like cutting out all the random sentences someone scribbled in the margins of your recipe and taping the real steps into one clean instruction.

After all three steps, you have mature mRNA - a clean, protected recipe ready to be read by ribosomes to make proteins. Without these steps, the cell would try to use the mesy rough draft and make broken proteins!

Another: "CLAPS" for mature mRNA:

• Cap (5' end)
• Ligate exons (splicing)
• Add poly-A
• Process quality check
• Ship to cytoplasm

Connections

• RNA Polymerase II Transcription — RNA Pol II's CTD coordinates all processing events
• Eukaryotic Translation Initiation — 5' cap and poly-A tail enable ribosome recruitment
• Nonsense-Mediated Decay (NMD) — Exon junction complexes mark proper splicing; retained introns trigger NMD
• Alternative Splicing Regulation — SR proteins and hnRNPs control exon inclusion
• Gene Expression Regulation — Processing provides multiple control points beyond transcription
• RNA Interference and microRNAs — Competes with splicing machinery for pre-mRNA access
• Nuclear Export Mechanisms — Only properly capped, spliced, and polyadenylated mRNA exits
• mRNA Stability and Decay Pathways — Deadenylation triggers decapping and degradation
• Intron Evolution and Function — Why do introns exist? Exon shuffling, regulation

#flashcards/biology

What are the three major RNA processing events in eukaryotes? :: 5' capping (addition of m7G cap), splicing (intron removal and exon ligation), and 3' polyadenylation (addition of ~200-250 adenines).

What are three functions of the 5' cap? :: (1) Protection from 5'-3' exonuclease degradation, (2) Ribosome recruitment via eIF4E for translation initiation, (3) Nuclear export signal via cap-binding complex.

Concept Map

Hinglish (regional understanding)