Termination & Ribosome Recycling

1. Class-I Release Factors (RF1/2 in bacteria, eRF1 in eukaryotes)

Bacterial RF1 recognises UAA + UAG; RF2 recognises UAA + UGA. Each contains a tripeptide “anticodon-like” motif that reads the stop codon via direct protein-RNA contacts in the A site. Eukaryotes use a single universal release factor, eRF1, that reads all three stop codons.

Release factor binding places a universally conserved GGQ motif into the peptidyl transferase centre. The GGQ glutamine positions a water molecule that performs nucleophilic attack on the peptidyl-tRNA ester bond, releasing the completed peptide chain into the exit tunnel. Release is catalysed by the same ribozymic machinery as peptide bond formation, with water rather than an amino group as the nucleophile.

2. Class-II Release Factors (RF3, eRF3)

RF3 (bacteria) and eRF3 (eukaryotes, also GSPT1) are GTPases that accelerate dissociation of class-I release factors from the post-release ribosome. eRF3 forms a complex with eRF1 and shares the GTPase/proofreading architecture of elongation factors; upon stop-codon recognition, eRF1-eRF3 dock on the A site, eRF3 hydrolyses GTP, and release proceeds. Selective targeting of eRF3/GSPT1 by molecular glues (CC-885, CC-90009) is in clinical development for AML — the first pharmacological targeting of translation termination in oncology.

3. Ribosome Recycling

After peptide release, the ribosome is still assembled around the mRNA with deacylated tRNA in the P site. ABCE1(eukaryotic) / RRF + EF-G (bacterial) catalyse subunit dissociation:

• ABCE1: ABC-family ATPase with two FeS clusters, the only essential ribosome factor using metal cofactors. Splits 60S from 40S after eRF1 binding. ATP hydrolysis provides the work.
• Released 40S with P-site tRNA is processed by eIF6 (anti-association factor), eIF3, and factors that remove the tRNA and mRNA, returning the 40S to the initiation pool.

Recycling is coupled to initiation through “ribosome reinitiation” on closed-loop mRNAs — terminated ribosomes can reinitiate on the same message without fully dissociating, especially in secretory cells with heavy translation demand.

4. mRNA Surveillance: NMD / NSD / NGD

Three coupled surveillance pathways detect translation problems and degrade the offending mRNA:

• NMD (nonsense-mediated decay): detects premature termination codons (PTCs), typically those upstream of a downstream exon-junction complex (EJC). Core machinery: UPF1 (SMG-2 helicase), UPF2, UPF3, SMG1 (UPF1 kinase). NMD clears ~10–30% of normal alternative-splicing products with aberrant ORFs and ensures that nonsense-mutant mRNAs are not translated into truncated proteins. Critical in mitigating severity of nonsense-mutation genetic disease.
• NSD (non-stop decay): detects mRNAs lacking a stop codon (due to premature polyadenylation or 3′-end cleavage). The ribosome translates into the poly-A tail and stalls. Ski7 (yeast) / HBS1L (mammals) triggers mRNA degradation from the 3′ end via the exosome.
• NGD (no-go decay): detects stalled ribosomes at damaged bases, pseudo-knots, rare-codon clusters. Dom34 (yeast) / Pelota (mammals) + ABCE1 dissociate the stalled ribosome and target the mRNA for degradation.

5. Ribosome-Associated Quality Control (RQC)

Stalled ribosomes retain a nascent chain that cannot be released conventionally. The RQC pathway handles these: after subunit dissociation by Pelota/Hbs1L, the 60S retains the tRNA-linked nascent chain. NEMF and Rqc2/NEMF add a C-terminal Ala-Thr tail (CAT tail) via non-templated elongation — a degron that marks the protein for proteasomal degradation. Listerin/LTN1 E3 ligase ubiquitinates the nascent chain for proteasomal degradation (Bengtson & Joazeiro 2010; Brandman 2012).

RQC failure causes neurodegeneration: mutations in NEMF and LTN1 produce motor-neuron disease in mice, and are associated with ALS-like phenotypes in humans. Stalled ribosomes whose nascent chains are not cleaned aggregate as cytotoxic protein species in neurons.

6. Read-Through & Stop-Codon Suppression

Stop codons can be “read through” by near-cognate tRNAs at low efficiency (~0.1%). This is normally suppressed but can be therapeutically boosted: ataluren (Translarna) promotes read-through of premature stop codons in Duchenne muscular dystrophy — EMA approval for nonsense-mutation DMD patients, though clinical benefit has been contested. Aminoglycosides (gentamicin) also induce read-through but with severe ototoxicity. Designer tRNAs that specifically read PTC stop codons without affecting normal termination are in preclinical development for nonsense-mutation cystic fibrosis and other genetic diseases.