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Prokaryotic Translation Elongation & Termination
Prokaryotic Translation Elongation & Termination
Dr.V.Malathi
Elongation
- Elongation includes repetitive cycles of decoding the m RNA , peptide bond formation, and translocation.
- Elongation begins as soon as the second codon of the ORF becomes accessible for reading by the next amino acylated tRNAs
- Elongation is facilitated by translation factors EF-Tu, EF-G ,EF-Ts EF-P and EF-4.
| Bacterial Elongation Factors | Function |
| EF-TU | mediates the entry of the aminoacyl t RNA into a free site of the ribosome |
| EF-TS | serves as the guanine nucleotide exchange factor for EF-Tu and catalyzes the release of GDP from EF-Tu. |
| EF-G | catalyzes the translocation of the tRNA and mRNA down the ribosome. |
| EF-P | possibly stimulates formation of peptide bonds and resolves stalls. |
| EF-4 | Proofreading |
- The second codon in the P site is recognized by aminoacylated -tRNAs. A ternary complex of aa-tRANA ,EF-Tu and GTP is attached to the P site .This interaction triggers GTP hydrolysis by EF-Tu. After Pi release, EF-Tu rearranges into the guanosine diphosphate (GDP)-bound form and releases the aa-tRNA.
- The amino group of the amino acid attached to the A-site tRNA makes a peptide bond with the carboxyl group of the amino acid attached to the P-site tRNA. This reaction is catalyzed by peptidyl transferase, an RNA-based ribozyme that is integrated into the 50S ribosomal subunit. Now the A site carries a peptide comprising of two amino acids and leaves the uncharged t RNA in the p site .
- During elongation the ribosomes move one codon along the mRNA in a process called Translocation. Translocation is promoted by EF-G at the cost of GTP hydrolysis .During each translocation event, a new charged tRNAs enter at the A site, then shift to the P site, and then finally to the E site for removal. Ribosomal movements are induced by conformational changes that advance the ribosome by three bases in the 3′ direction.
- This process of elongation continues until the ribosome arrives at the stop codon.
- Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200 amino-acid protein can be translated in just 10 seconds.
Termination
- Termination occurs when the ribosome encounters a stop codon UAG/UAA and UGA/UAA, respectively in the mRNA.
- These stop codons are recognized by the termination (or release) factors RF1 and RF2.
- Another termination factor, RF3, facilitates turnover of RF1 and RF2 but is not required for peptidyl-tRNA hydrolysis.
- The mechanism of termination encompasses three steps:
- recognition of the stop codon,
- hydrolysis of the ester bond of the peptidyl-tRNA (these two steps are accomplished by RF1 or RF2) and
- dissociation of RF1/RF2 with the help of RF3.
- The releasing factors instruct peptidyl transferase to add a water molecule to the carboxyl end of the P-site amino acid. This reaction forces the P-site amino acid to de