Prokaryote translation is a complex, highly coordinated process that converts genetic information from mRNA into functional proteins. It involves three stages: initiation, elongation, and termination, each facilitated by specific molecular components.
Initiation of Translation
The process begins with the assembly of the ribosomal subunits and initiation factors on the mRNA. In bacteria, the 30S ribosomal subunit recognizes the Shine-Dalgarno sequence in the mRNA, a conserved region upstream of the start codon that aligns the ribosome for translation initiation. This alignment ensures the start codon is positioned correctly in the ribosome's P site. The initiator tRNA, charged with N-formylmethionine (fMet-tRNA), binds to the start codon with the help of initiation factors and GTP. Once the 50S ribosomal subunit joins the complex, the active 70S ribosome is formed, ready for elongation.
Elongation and Peptide Chain Synthesis
During elongation, aminoacyl-tRNAs sequentially deliver amino acids to the ribosome. The tRNA enters the A site, where its anticodon pairs with the complementary mRNA codon. Peptidyl transferase, an enzymatic activity of the 50S subunit, catalyzes the formation of a peptide bond between the growing polypeptide chain on the tRNA in the P site and the incoming amino acid in the A site. This reaction transfers the polypeptide chain to the tRNA in the A site. Elongation factors and GTP facilitate the translocation of the ribosome, moving the tRNA from the A site to the P site and shifting the empty tRNA to the E site for exit. This process exposes a new codon in the A site, allowing the cycle to continue.
Termination of Translation
Translation terminates when the ribosome encounters a mRNA stop codon (UAA, UAG, or UGA). Since no tRNA recognizes these codons, release factors bind to the ribosome, catalyzing the cleavage of the polypeptide from the tRNA in the P site. The ribosomal subunits, mRNA, and tRNA dissociate, completing the process.
Initiation, elongation, and termination are translation steps shared by bacteria and archaea. The initiation begins when the 30S and 50S ribosomal subunits bind to the mRNA.
In bacteria, the 30S subunit recognizes the Shine-Dalgarno sequence on the mRNA. Then, the initiator fMet-tRNA loaded with N-formylmethionine, initiation factors, and GTP binds to the mRNA
The 50S subunit then binds to the initiation complex, forming the active 70S ribosome with three key sites.
While the fMet-RNA is held at the P site, an aminoacyl-tRNA enters the A site, aligning its anticodon with the second codon.
The enzyme peptidyl transferase in the 50S subunit transfers N-formylmethionine from fMet-tRNA in the P site to the amino acid on the tRNA in the A site.
The elongation factors and GTP keep translocating the ribosome, exposing a new codon in the A site and pushing the empty tRNA via the E site.
Translation terminates when the ribosome encounters a stop codon. Release factors cleave the polypeptide from the last tRNA.
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