Mechanisms of Translation: Elongation and Termination in Prokaryotes and Eukaryotes

Translation: Elongation and Termination

Overview of Translation

Translation is the process by which ribosomes synthesize proteins using messenger RNA (mRNA) as a template. This process involves three main stages: initiation, elongation, and termination. The notes focus on the molecular mechanisms of elongation and termination, highlighting the roles of transfer RNA (tRNA) and various protein factors in both prokaryotes and eukaryotes.

Elongation Phase of Translation

During elongation, amino acids are sequentially added to the growing polypeptide chain. This process is highly conserved between prokaryotes and eukaryotes and involves several key steps and factors.

• Entry of Aminoacyl-tRNA: An aminoacyl-tRNA enters the empty A (aminoacyl) site of the ribosome, guided by elongation factors (EF-Tu in prokaryotes, eEF1A in eukaryotes).

• Peptide Bond Formation: The amino acid attached to the tRNA in the P (peptidyl) site is transferred to the amino acid on the tRNA in the A site, forming a peptide bond. This reaction is catalyzed by the peptidyl transferase activity of the ribosome's large subunit (50S in prokaryotes, 60S in eukaryotes).

• Translocation: The ribosome moves one codon forward along the mRNA, shifting the tRNAs from the A and P sites to the P and E (exit) sites, respectively. This step is facilitated by elongation factors (EF-G in prokaryotes, eEF2 in eukaryotes).

• Cycle Repeats: The process repeats as new aminoacyl-tRNAs enter the A site, and the polypeptide chain elongates.

Key Terms:

• Aminoacyl-tRNA: tRNA molecule charged with its corresponding amino acid.

• Peptidyl-tRNA: tRNA carrying the growing polypeptide chain.

• Elongation Factors: Proteins that facilitate the entry of tRNA and translocation of the ribosome (EF-Tu, EF-G).

Example: In prokaryotes, EF-Tu-GTP binds aminoacyl-tRNA and delivers it to the A site. Upon correct codon-anticodon pairing, GTP is hydrolyzed, and EF-Tu-GDP is released.

Mechanism of Elongation: Stepwise Events

• Step 1: Aminoacyl-tRNA enters the A site.

• Step 2: Peptidyl transferase catalyzes peptide bond formation between the polypeptide (on the P site tRNA) and the new amino acid (on the A site tRNA).

• Step 3: Ribosome translocates, moving the tRNAs to the next sites (A → P, P → E).

• Step 4: Uncharged tRNA exits from the E site; the cycle repeats.

Role of Elongation Factors

• EF-Tu (prokaryotes): Delivers aminoacyl-tRNA to the A site in a GTP-dependent manner.

• EF-G (prokaryotes): Promotes translocation of the ribosome along the mRNA.

• eEF1A and eEF2 (eukaryotes): Functional analogs of EF-Tu and EF-G, respectively.

• GTP Hydrolysis: Provides energy for conformational changes and factor release.

Additional info: EF-Tu and EF-G mimic tRNA structure to interact with the ribosome, ensuring accurate and efficient translation.

Termination of Translation

Termination occurs when a stop codon is encountered in the mRNA. This process involves release factors that recognize stop codons and promote the release of the newly synthesized polypeptide.

• Release Factors (RFs): Proteins such as RF1, RF2, and RF3 in prokaryotes, and eRF1/eRF3 in eukaryotes, bind to the A site when a stop codon is present.

• Polypeptide Release: The peptidyl-tRNA bond is hydrolyzed, releasing the polypeptide from the ribosome.

• Ribosome Disassembly: Ribosomal subunits, mRNA, and tRNA dissociate, allowing the components to be recycled.

Example: RF1 recognizes UAA and UAG stop codons in prokaryotes, triggering termination.

Summary Table: tRNA States During Translation

Site	A	P	E
Entry	Aminoacyl-tRNA	Initiator aminoacyl-tRNA / Peptidyl-tRNA	Uncharged tRNA
After Peptide Transfer	Peptidyl-tRNA	Uncharged tRNA	Uncharged tRNA (exiting)

Additional info: The ribosome has three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit). The movement of tRNAs through these sites is essential for accurate protein synthesis.

Antibiotic Inhibition of Translation

Certain antibiotics can inhibit translation by interfering with tRNA binding or ribosome function.

• Puromycin: Mimics aminoacyl-tRNA and enters the A site, causing premature chain termination.

• Mechanism: Puromycin accepts the growing polypeptide but cannot anchor to the ribosome, leading to early release.

Example: Puromycin is used experimentally to study translation mechanisms.

Conclusion

Translation elongation and termination are complex, highly regulated processes involving coordinated action of tRNAs, ribosomal subunits, and protein factors. Understanding these mechanisms is fundamental to molecular genetics and the study of gene expression.