Post-Transcriptional Modifications

PTMs Intro - RNA's Makeover Magic

• Post-Transcriptional Modifications (PTMs): Covalent chemical alterations of RNA molecules (pre-mRNA, pre-tRNA, pre-rRNA) following transcription.
• Significance: Essential for RNA maturation, stability, transport, and function (e.g., translation, splicing).
• Location:
  • Eukaryotes: Primarily nucleus (mRNA, tRNA, rRNA processing). Some tRNA mods in cytoplasm.
  • Prokaryotes: Cytoplasm.
• Key RNA types undergoing PTMs: mRNA, tRNA, rRNA.

⭐ PTMs, like capping and polyadenylation in mRNA, are critical for its export from the nucleus and protection against exonucleases.

mRNA Capping & Tailing - mRNA's Hats & Tails

• 5' Capping: Addition of $m^7G$ (7-methylguanosine) to mRNA 5' end.
  • Unusual 5'-5' triphosphate linkage.
  • Occurs co-transcriptionally.
  • Functions: Protects from 5' exonucleases, aids nuclear export, promotes translation initiation.
• 3' Polyadenylation (Tailing): Addition of ~50-250 adenine residues (poly-A tail) to mRNA 3' end.
  • Signal: AAUAAA sequence.
  • Enzyme: Poly(A) polymerase (template-independent).
  • Functions: Protects from 3' exonucleases, aids nuclear export, enhances translation.
  • 📌 Mnemonic: "Hats (caps) and Tails keep mRNA stable and sailing!"

⭐ Histone mRNAs are a notable exception; they lack poly-A tails and instead have a 3' stem-loop structure.

mRNA Splicing - Splicing's Snip & Stitch

• Core Function: Nuclear process removing introns (non-coding) and ligating exons (coding) from pre-mRNA.
• Spliceosome: Large RNA-protein machine with snRNPs (U1, U2, U4, U5, U6).
• Consensus Sequences:
  • 5' Splice Site (Donor): GU
  • 3' Splice Site (Acceptor): AG
  • Branch Point: Adenine (A) in intron near 3' site.
  • 📌 Mnemonic: "Goo-Agro" for GU-AG intron boundary rule.
• Mechanism: Two transesterification steps:
  • 1st: 2'-OH of branch point A attacks 5' splice site → lariat.
  • 2nd: Free 3'-OH of 5' exon attacks 3' splice site → exons joined, lariat released.
• Alternative Splicing: One gene → multiple mRNA/protein isoforms; ↑ proteomic diversity.

⭐ Splicing errors contribute to ~15% of all human genetic diseases, like some β-thalassemias.

tRNA & rRNA Mods - RNA's Other Tweaks

• tRNA: Extensively modified (~25% bases) for structure, stability, codon recognition.
  • Key Modified Bases:

    Base	Sym	Significance
    Dihydrouridine	$D$	D-loop, ↑ flexibility
    Pseudouridine	$\Psi$	T$\Psi$C loop, stabilizes structure
    Ribothymidine	$T$	T$\Psi$C loop, ribosome binding
    Inosine	$I$	Wobble pairing (anticodon)

  • CCA Addition: Post-transcriptional at 3' end (tRNA nucleotidyltransferase); for amino acid attachment. 📌 "Can Carry Amino acids".
  • Splicing of introns (some eukaryotic tRNAs).
• rRNA: Processed from pre-rRNA (e.g., 45S → 18S, 5.8S, 28S in eukaryotes; 5S rRNA separate).
  • Key Mods: Methylation, pseudouridylation; guide folding, nuclease protection.
  • Significance: Crucial for ribosome assembly, structural integrity & catalytic activity.

⭐ The CCA sequence at the 3'-end of tRNA is added post-transcriptionally (not DNA-encoded) and is vital for aminoacylation.

High-Yield Points - ⚡ Biggest Takeaways

• 5' Capping: 7-methylguanosine (7-mG) cap addition; vital for mRNA stability & translation initiation.
• 3' Polyadenylation: Poly(A) tail addition by Poly(A) polymerase; boosts mRNA stability & translation.
• Splicing: Intron removal, exon joining by spliceosomes (snRNPs); a key nuclear process.
• Alternative Splicing: Generates protein isoforms from a single gene, expanding proteome diversity.
• RNA Editing: Post-transcriptional nucleotide changes; e.g., A-to-I (ADARs), C-to-U (ApoB mRNA).
• tRNA Modifications: Extensive base changes (e.g., pseudouridine) essential for tRNA structure and function.