RNA Transcription and Translation

TRANSCRIPTION

• Initiation: RNA polymerase II (in eukaryotes) recognizes promoter sequences (TATA box at -25bp, CAAT box, GC box) and transcription factors (TFIID binding TBP) form the pre-initiation complex; promoter methylation or chromatin remodeling regulates accessibility
• Elongation: RNA polymerase unwinds DNA double helix and synthesizes mRNA in the 5' → 3' direction using ribonucleotides; histone acetylation and chromatin remodeling (SWI/SNF complexes) facilitate passage through nucleosomes
• Termination: In eukaryotes, polyadenylation signals (AAUAAA sequence) and cleavage factors cause transcript release; in prokaryotes, rho-dependent or intrinsic terminators halt transcription

POST-TRANSCRIPTIONAL MODIFICATION

• 5' capping: Addition of 7-methylguanosine cap protects from degradation and facilitates ribosome binding
• 3' polyadenylation: Addition of ~200 adenine nucleotides increases mRNA stability and translation efficiency
• Splicing: Removal of introns and joining of exons by the spliceosome (snRNPs U1, U2, U4, U5, U6) increases protein diversity through alternative splicing patterns

TRANSLATION

• Initiation: The 40S ribosomal subunit recognizes the 5' cap and scans for the start codon (AUG) in optimal Kozak context (GCCRCCAUGG); eukaryotic initiator tRNA (fMet-tRNA) enters P site; 60S subunit joins forming 80S initiation complex
• Elongation: Aminoacyl-tRNA enters A site (via EF1A-GTP), peptide bond forms between P-site and A-site tRNAs (catalyzed by 28S rRNA peptidyl transferase activity), ribosome translocates using EF2-GTP moving tRNA to P and E sites; cycle repeats at rate of ~3-8 amino acids/second
• Termination: Stop codons (UAA, UAG, UGA) are recognized by release factors (RF1, RF2 in prokaryotes; eRF1, eRF3 in eukaryotes), deacylated tRNA is released, mRNA and ribosomal subunits dissociate

GENETIC CODE AND WOBBLE BASE PAIRING

• 64 codons code for 20 standard amino acids plus stop signals (redundant code)
• Wobble position: Third codon position allows non-Watson-Crick pairing (inosine can pair with U, C, or A); explains why one tRNA can recognize multiple codons
• Universal code: Nearly identical across organisms, allowing recombinant protein production in bacteria

• Loss-of-function mutations in transcription factors or RNA polymerase: Present with developmental abnormalities, intellectual disability, or multi-system involvement (e.g., TFIIH mutations in xeroderma pigmentosum with UV hypersensitivity and neurodegeneration)
• β-thalassemia from splicing mutations: Present with hemolytic anemia, jaundice, splenomegaly, and growth retardation; β-globin intron mutations abolish proper splicing, reducing β-globin chain production
• Cystic fibrosis from CFTR mutations: Nonsense mutations triggering premature stop codons cause loss of full-length CFTR protein; manifests as thick secretions, recurrent infections, pancreatic insufficiency
• Cancer from dysregulated transcription: Oncogenic transcription factors (MYC, TP53) or epigenetic silencing cause uncontrolled cell proliferation and are hallmark features of malignancy

Identifying transcription/translation disorders

• Molecular sequencing: DNA sequencing identifies point mutations, insertions, deletions affecting coding or regulatory regions; identifies splice site mutations (GT-AG rule violations at intron boundaries)
• mRNA analysis: RT-PCR and northern blotting detect abnormal transcript sizes from splicing defects; RNA-seq provides genome-wide expression profiles
• Protein-level confirmation: Western blotting or mass spectrometry confirms presence/absence of full-length vs. truncated proteins; immunofluorescence localizes protein products
• Functional assays: Luciferase reporter assays measure promoter activity; translation efficiency assessed by polysome profiling; cell viability assays determine functional impact of mutations
• Chromosomal/epigenetic analysis: Bisulfite sequencing detects abnormal DNA methylation silencing transcription; ChIP-seq maps histone modifications indicating active vs. repressed chromatin

Important diagnostic pearls

• Nonsense-mediated decay (NMD): Stop codons >50-55 nucleotides upstream of exon junction trigger mRNA degradation, explaining absent transcripts in some genetic diseases
• Genetic counseling essential: Autosomal recessive inheritance (cystic fibrosis) vs. X-linked (hemophilia) vs. dominant-negative effects (osteogenesis imperfecta) require different counseling approaches

Management of transcription/translation disorders

First-line approaches

• Gene therapy: Directly replace defective gene using viral vectors (e.g., AAV for spinal muscular atrophy caused by SMN1 mutations) or CRISPR-Cas9 genome editing; addresses root cause but currently limited availability
• Supportive care and symptom management: Respiratory support (CF), transfusions (thalassemia), pancreatic enzyme replacement (CF), physical therapy
• Dietary/environmental modification: Avoiding UV exposure (xeroderma pigmentosum), high-calorie nutrition (CF with pancreatic insufficiency)

Second-line and emerging therapies

• Nonsense suppression therapy: Aminoglycosides (gentamicin, amikacin) cause ribosomal read-through of premature stop codons, allowing translation of full-length protein; used in some CF patients with nonsense mutations (cystic fibrosis transmembrane conductance regulator—CFTR-G542X)
• Splicing modulation: Antisense oligonucleotides (e.g., nusinersen for spinal muscular atrophy) restore correct splicing patterns of SMN2 gene; exon-skipping therapy (eteplirsen in Duchenne muscular dystrophy) allows translation of internally deleted but partially functional protein
• Chaperone assistance: Ivacaftor potentiates residual CFTR-G551D function by improving protein trafficking; lumacaftor rescues ΔF508-CFTR from ER retention
• mRNA replacement: Direct mRNA delivery or mRNA vaccines (e.g., investigational approaches for genetic disorders) provide functional mRNA directly

Special populations

• Pediatric patients: Gene therapy (SMA) offers best outcomes if administered before irreversible neuronal loss; genetic counseling critical for family planning
• Pregnant women: Teratogenic medications contraindicated; counsel on medication risks for genetic disorders
• Pharmacogenetic considerations: Some antibiotics (aminoglycosides) can cause permanent hearing loss—weigh risks/benefits in nonsense suppression therapy

Dysregulation and associated complications

• Premature protein termination and loss-of-function: Nonsense mutations creating stop codons produce truncated, non-functional proteins often degraded by proteasome, resulting in complete loss of protein activity (β-thalassemia major with no β-globin chains); NMD-mediated mRNA decay exacerb

Polymerases and their poisons

• RNA polymerase I/II/III: Pol I makes rRNA (nucleolus), Pol II makes mRNA (plus snRNA/miRNA), Pol III makes tRNA and 5S rRNA. "I, II, III → rRNA, mRNA, tRNA" — alphabetical by product size in that order.
• α-Amanitin: from Amanita phalloides (death cap); inhibits RNA polymerase II → halts mRNA synthesis → fulminant hepatic necrosis. Classic stem: mushroom forager with vomiting then delayed liver failure.
• Rifampin inhibits bacterial DNA-dependent RNA polymerase (no eukaryotic effect); actinomycin D (dactinomycin) intercalates DNA and blocks elongation in both. A distractor is fluoroquinolones — those hit DNA gyrase/topoisomerase IV, not transcription.

Translation machinery

• Peptidyl transferase is a ribozyme: catalytic activity resides in rRNA (23S in prokaryotes, 28S in eukaryotes), not in ribosomal protein. Energy cost is 4 high-energy phosphate bonds per peptide bond (2 for tRNA charging, 2 GTP for elongation).
• Initiator tRNA: bacteria (and mitochondria) use *formyl*-methionine; the eukaryotic cytoplasmic initiator is unformylated Met-tRNAi. fMet peptides are a neutrophil chemoattractant — the reason mitochondrial trauma can mimic sepsis.
• Toxins to memorize: diphtheria toxin and Pseudomonas exotoxin A ADP-ribosylate EF-2 (blocks translocation); Shiga and Shiga-like toxin (and ricin) depurinate the 60S/28S rRNA. Antibiotic sieve: 30S = aminoglycosides and tetracyclines; 50S = macrolides, clindamycin, chloramphenicol, linezolid.

Processing and modification

• Anti-Smith antibodies target snRNP core proteins of the spliceosome — highly specific for SLE. This is the single most tested clinical link to splicing.
• I-cell disease (mucolipidosis II): defective N-acetylglucosaminyl-1-phosphotransferase → no mannose-6-phosphate tag → lysosomal enzymes secreted into serum. Coarse facies, corneal clouding, high plasma lysosomal enzyme levels.
• Cofactor-dependent modifications: vitamin C for prolyl/lysyl hydroxylation of collagen (scurvy); vitamin K for γ-carboxylation of factors II, VII, IX, X (warfarin's target). C-peptide distinguishes endogenous insulin from exogenous injection.