Transcription and RNA Processing

Core Explanation

The Three Eukaryotic RNA Polymerases

All three polymerases are multi-subunit enzymes. Pol II has 12 subunits in yeast (Rpb1–Rpb12). Rpb1 contains the C-terminal domain (CTD) — a unique, unstructured tail of heptapeptide repeats (consensus YSPTSPS; 52 repeats in human). The CTD phosphorylation state orchestrates the transcription cycle and couples transcription to RNA processing.

The Pol II Transcription Cycle

1. Promoter Recognition and Preinitiation Complex (PIC) Assembly

Core promoter elements include the TATA box (TATAAA, ~−30 bp from transcription start site, recognized by TBP), the Initiator (Inr) element surrounding the TSS, and downstream promoter elements (DPE).

PIC assembly follows an ordered pathway:

1. TFIID binds the promoter. TFIID contains TBP (TATA-binding protein), which sharply bends DNA (~80°), and TAFs (TBP-associated factors) that recognize Inr and DPE.
2. TFIIA stabilizes TBP–DNA binding.
3. TFIIB binds TBP and DNA on both sides of TATA; positions the active site relative to the TSS.
4. Pol II–TFIIF complex is recruited.
5. TFIIE and TFIIH complete the PIC. TFIIH contains XPB (ATP-dependent DNA translocase that opens the promoter) and CDK7 (kinase subunit that phosphorylates Pol II CTD at Ser5).

2. Initiation and Promoter Escape

TFIIH's XPB subunit feeds ~11–15 bp of template DNA into the Pol II active site, forming the transcription bubble. The first phosphodiester bonds are formed. Early transcription is abortive — Pol II synthesizes and releases short RNAs (2–8 nt) before escaping the promoter upon stable CTD phosphorylation (Ser5-P by CDK7) and elongation factor recruitment.

3. Elongation

Productive elongation begins after Pol II clears the promoter. Key factors:

• P-TEFb (CDK9/cyclin T): phosphorylates CTD at Ser2, overcoming promoter-proximal pausing caused by NELF and DSF (DRB sensitivity-inducing factor). This is a major rate-limiting step — many genes have poised Pol II paused ~20–60 bp downstream of the TSS.
• SPT4/SPT5 (DSIF): converts from a negative to positive elongation factor after phosphorylation.
• FACT complex: facilitates nucleosome disassembly and reassembly ahead of and behind Pol II, maintaining chromatin integrity during elongation.
• Elongation rate: ~2–4 kb/min in mammals. This is not uniform — pausing, secondary structure, and nucleosome density modulate rate.

4. Termination

Termination is coupled to 3′ end processing:

1. CPSF (cleavage and polyadenylation specificity factor) recognizes the AAUAAA poly(A) signal in the nascent RNA.
2. CstF binds the GU-rich downstream sequence element (DSE).
3. Endonucleolytic cleavage by CPSF-73 releases the pre-mRNA from Pol II.
4. Xrn2 (5′→3′ exonuclease, the "torpedo") degrades the downstream nascent RNA still tethered to Pol II, catching up to and dislodging Pol II (the torpedo model of termination).

Enhancers, Activators, and Mediator

Transcriptional regulation integrates signals from activators bound to enhancers — distal regulatory elements that can be located tens to hundreds of kilobases from the promoter.

• Activators contain DNA-binding domains (DBDs) that recognize specific enhancer sequences and activation domains that recruit coactivators and chromatin remodelers. Classic examples: GAL4 (yeast), p53, NF-κB, nuclear hormone receptors.
• Mediator is a ~30-subunit complex that bridges activators bound at enhancers to Pol II and GTFs at the promoter. Different Mediator subunits interact with different classes of activators, providing regulatory specificity. CDK8 (Mediator kinase module) can phosphorylate and regulate transcription factor stability.
• Enhancer–promoter looping: Cohesin-mediated loop extrusion and CTCF boundary elements bring enhancers into physical proximity with their target promoters. This looping and the A/B compartmentalization observed by Hi-C provide the 3D framework for enhancer–promoter communication.

Chromatin Context

Transcription occurs in the context of chromatin. Nucleosomes are barriers to transcription. Gene activation involves:

1. Chromatin remodeling: SWI/SNF and related complexes slide or eject promoter nucleosomes to expose the TATA box and TSS.
2. Histone acetylation: HATs (CBP/p300, GCN5) acetylate histone tails, neutralizing positive charges and relaxing chromatin; acetylated lysines also recruit bromodomain-containing factors (TFIID, BRD4).
3. Histone methylation: H3K4me3 at promoters (set by MLL/COMPASS complexes) recruits TFIID and other initiation factors.

RNA Processing

Three major processing events occur co-transcriptionally, physically and kinetically coupled to the Pol II CTD phosphorylation cycle.

5′ Capping

The 5′ cap (7-methylguanosine linked via a 5′→5′ triphosphate bridge) is added when the nascent RNA is ~20–30 nt:

1. RNA triphosphatase: removes the γ-phosphate from the 5′ triphosphate.
2. Guanylyltransferase: adds GMP in a 5′→5′ linkage.
3. Methyltransferase: methylates the guanine at N7 position (m⁷G cap).

The cap protects mRNA from 5′→3′ exonucleases, promotes splicing, export, and translation initiation (recognized by eIF4E). Capping enzymes are recruited to the Ser5-phosphorylated CTD.

Splicing

Most human genes contain introns (average ~8 per gene, often much larger than exons). Splicing removes introns and joins exons precisely. The spliceosome — a dynamic ~5 MDa ribonucleoprotein machine comprising five snRNPs (U1, U2, U4, U5, U6) and ~150 associated proteins — catalyzes splicing through two transesterification reactions.

Mechanism:

1. U1 snRNP base-pairs with the 5′ splice site (consensus: AG|GURAGU).
2. U2AF (U2 auxiliary factor) binds the polypyrimidine tract and 3′ splice site (consensus: YAG|); SF1/BBP binds the branch-point adenosine.
3. U2 snRNP displaces SF1 and base-pairs with the branch point, extruding the branch-point adenosine (bulged A).
4. U4/U6–U5 tri-snRNP joins, displacing U1 at the 5′ splice site. U6 replaces U1, forming the catalytic core.
5. First transesterification: The branch-point A 2′-OH attacks the 5′ splice site phosphate, forming a lariat intermediate.
6. Second transesterification: The freed 5′ exon 3′-OH attacks the 3′ splice site phosphate, joining the exons and releasing the intron lariat.
7. Lariat is debranched and degraded; snRNPs are recycled.

Splicing is extraordinarily precise — a one-nucleotide shift in splice site choice (frameshift) produces a nonfunctional protein. The spliceosome achieves this through multiple proofreading steps (DExD/H-box ATPases like Prp16, Prp22) that discard misassembled intermediates.

3′ Polyadenylation

The poly(A) tail (~200–250 adenosines in mammals) is added post-cleavage by poly(A) polymerase (PAP):

1. CPSF and CstF bind the AAUAAA and DSE elements, respectively.
2. Endonucleolytic cleavage 10–30 nt downstream of AAUAAA.
3. PAP adds ~12 adenosines slowly (distributive), then PABPN1 (nuclear poly(A)-binding protein) binds, stimulating rapid, processive addition.
4. The poly(A) tail promotes mRNA stability, nuclear export, and translation (PABPC1 in the cytoplasm circularizes mRNA via eIF4G interaction).

Alternative Splicing and Proteome Expansion

Alternative splicing allows a single pre-mRNA to produce multiple mRNA isoforms through differential exon inclusion/exclusion, yielding distinct protein products. Major modes:

Splicing is regulated by splicing regulatory elements (SREs) and RNA-binding proteins:

• SR proteins (serine/arginine-rich) bind exonic splicing enhancers (ESEs) and promote exon inclusion by recruiting U1 and U2.
• hnRNP proteins often bind exonic/intronic splicing silencers (ESSs/ISSs) and repress exon inclusion.
• Tissue-specific splicing factors (Nova, Fox, CELF, MBNL, RBFOX) create tissue-specific isoform patterns — e.g., the Dscam gene in Drosophila can produce >38,000 isoforms through alternative splicing.

Alternative splicing is a major source of proteome diversity. Single genes like Bcl-x produce anti-apoptotic (Bcl-xL) and pro-apoptotic (Bcl-xS) isoforms from the same pre-mRNA. Splicing mutations cause ~15% of human genetic diseases, with β-thalassemia being the most common — point mutations that create cryptic splice sites or disrupt normal ones.

Experimental Evidence

• α-Amanitin sensitivity: The differential sensitivity of Pol I, Pol II, and Pol III to α-amanitin was a key biochemical tool in assigning cellular RNA synthesis products to specific polymerases.
• Run-on transcription assays (nuclear run-on): Nuclei are isolated, and nascent transcripts are extended in vitro with labeled UTP. This measures transcriptionally engaged Pol II density, distinguishing transcriptional regulation from RNA stability effects.
• In vitro splicing: HeLa cell nuclear extracts can splice synthetic pre-mRNA substrates, enabling biochemical dissection of spliceosome assembly and identification of snRNPs. This system revealed the two-step transesterification mechanism.
• Splice site mutations in β-thalassemia: Naturally occurring human mutations that create new splice sites or destroy existing ones, combined with mapping of β-globin mRNA isoforms, provided early evidence for splice site consensus sequences and the functional importance of accurate splicing.
• Chromatin immunoprecipitation (ChIP-seq): Genome-wide mapping of Pol II, histone modifications, and transcription factors has revealed promoter-proximal pausing, enhancer landscapes, and the relationship between chromatin state and transcription.

Disease and Clinical Connections

High-Yield Summary

• Pol I: rRNA (nucleolus); Pol II: mRNA, snRNAs, miRNAs; Pol III: tRNA, 5S rRNA.
• Pol II CTD heptapeptide repeats (YSPTSPS); phosphorylation state orchestrates initiation → elongation → processing.
• PIC assembly: TFIID (TBP + TAFs) → TFIIA → TFIIB → Pol II–TFIIF → TFIIE → TFIIH (XPB helicase, CDK7 kinase).
• P-TEFb (CDK9) phosphorylates Ser2, releasing promoter-proximal pause; key rate-limiting step.
• 5′ capping: m⁷GpppN cap, co-transcriptional at ~20–30 nt; protects from exonucleases, enables translation.
• Splicing: two transesterifications via spliceosome (U1, U2, U4/U6.U5 snRNPs); branch-point A lariat intermediate.
• Polyadenylation: CPSF/CstF recognition → cleavage → PAP adds ~200 A's.
• Alternative splicing: exon skipping most common; SR proteins and hnRNPs regulate; ~95% of multi-exon human genes alternatively spliced.
• Enhancers recruit activators, which interact with Mediator to regulate Pol II at promoters.

Practice Questions

1. The drug flavopiridol inhibits CDK9, the kinase subunit of P-TEFb. What immediate effect does this have on global transcription, and which class of genes would be most affected?

Answer: CDK9 inhibition blocks CTD Ser2 phosphorylation, preventing release of promoter-proximally paused Pol II into productive elongation. Global nascent transcription drops as Pol II accumulates in a paused state ~20–60 bp downstream of TSSs. Genes with high levels of paused Pol II — including many immediate-early response genes (c-FOS, c-MYC), heat shock genes, and developmental regulators — are most affected because their expression depends on rapid pause release. Constitutively active genes with low pausing ratios (e.g., ribosomal protein genes) are relatively spared.

2. A point mutation in the β-globin gene creates a new AG dinucleotide within intron 1, 15 nucleotides upstream of the normal 3′ splice site. The mutation causes β⁺-thalassemia (reduced β-globin). Explain the mechanism at the level of spliceosome assembly.

Answer: Splice site selection follows the AG-rule: U2AF recognizes the 3′ splice site AG as part of the splicing signal. The cryptic AG 15 nt upstream creates an alternative 3′ splice site. When the spliceosome utilizes this cryptic site, the resulting mRNA contains 15 extra nucleotides in the exon (intron sequence now included), causing a frameshift and premature stop codon → nonsense-mediated decay (NMD). If the cryptic site is used some but not all of the time, a fraction of splicing uses the correct site, producing normal β-globin, while the rest is aberrant and degraded — explaining the β⁺ (reduced, not absent) phenotype.

3. Treatment with leptomycin B (a CRM1 inhibitor that blocks nuclear export) causes nuclear accumulation of poly(A)⁺ RNA. Does this mean polyadenylation is required for export? Design a control experiment.

Answer: The observation is correlative: poly(A)⁺ RNA accumulates, but this could be because polyadenylated mRNAs are the major export substrate OR because polyadenylation and export are mechanistically coupled. A proper test: express a reporter mRNA whose poly(A) signal (AAUAAA) is mutated, preventing polyadenylation. If this unadenylated mRNA still exits the nucleus (detected by FISH or subcellular fractionation), polyadenylation is not strictly required for export. In reality, some histone mRNAs (which lack poly(A) tails and instead have a stem-loop structure) are exported, demonstrating that polyadenylation is not absolutely required. However, for most mRNAs, polyadenylation and the recruitment of export factors (e.g., ALY/REF, TAP-p15) are coupled — not because the tail itself is the export signal, but because the 3′ end processing machinery loads export adaptors onto the mRNA.

Common Misconceptions

"Transcription factors just bind DNA and that's it." DNA binding is necessary but far from sufficient. Most activators function by recruiting coactivators (Mediator, CBP/p300, SWI/SNF), which modify chromatin and bridge to the basal transcription machinery. The activation domain — not the DNA-binding domain — does the functional work.

"Splicing happens after transcription is complete." No. Splicing is primarily co-transcriptional. The CTD of elongating Pol II directly recruits splicing factors. This coupling ensures efficiency and may influence splice site choice — the rate of Pol II elongation can affect which splice sites are selected.

"The poly(A) tail is just a degradation timer." The poly(A) tail has multiple roles: it protects mRNA from 3′→5′ exonucleases, promotes nuclear export, enables efficient translation via PABPC1–eIF4G–eIF4E circularization, and its regulated shortening (deadenylation) controls mRNA stability and translational efficiency. It's a regulatory hub, not merely a countdown clock.