The process by which proteins are produced with amino acid sequences specified by the sequence of codons in messenger RNA is called translation. Translation is the first stage of protein biosynthesis.
In prokaryotes (unicellular), translation occurs in the cytosol, where the medium and small subunits of the ribosome bind to the tRNA. Translation process consists of following major phases that are discussed below;
Activation of amino acids:
- The activation of amino acids takes place in cytosol.
- The activation of amino acids is catalyzed by their aminoacyl tRNA synthetases.
- All the 20 amino acids are activated and bound to 3’ end of their specific tRNA in the presence of ATP and Mg++.
- The N-formylated methionine is chain initiating amino acid in bacteria whereas methionine is chain initiating amino acid in eukaryotes.
- Methionine is activated by methionyl-tRNA synthetase. For N-formylmethionine two types of tRNA are used. Similarly, all 2o aminoacids are activated (aminoacyl-AMP enzyme complex) and then bound to their specific tRNA forming Aminoacyl tRNA.
- In the first step, initiation factor-3 (IF-3) binds to the 30S ribosomal unit.
- Then mRNA binds to 30S ribosomal subunit in such a way that AUG codon lies on the peptidyl (P) site and the second codon lies on aminoacyl (A) site.
- The tRNA carrying formylated methionine FMet–tRNA is placed at P-site. This specificity is induced by IF-2 with utilization of GTP.
- Shine Dalgarno sequence in the mRNA guide correct positioning of AUG codon at P-site of 30S ribosome.
Binding of AA-tRNA at A-site:
- The 2nd tRNA carrying the next amino acid comes into A-site and recognizes the codon on mRNA. This binding is facilitated by EF-TU and utilizes GTP.
- After binding, GTP is hydrolyzed and EF-TU-GDP is released
- EF=TU-GDP then enters into EF-TS cycle.
ii. Peptide bond formation:
- The amino acid present in the t-RNA of P-site i.e. met is transferred to t-RNA of A-site forming peptide bond. This reaction is catalyzed by peptidyltransferase.
- Now, the t-RNA at P-site has become uncharged.
iii. Ribosome translocation:
- After peptide bond formation, the ribosome moves one codon ahead along 5’-3’ direction on mRNA, so that dipeptide-tRNA appears on P-site and next codon appears on A-site.
- The uncharged tRNA exits from the ribosome and enters the cytosol.
- The ribosomal translocation requires EF-G-GTP (translocase enzyme) which changes the 3D structure of the ribosome and catalyzes 5’-3’ movement.
- The codon on A-site is now recognized by other aminoacyl-tRNA as in previous.
- The dipeptide on P-site is transferred to A-site forming tripeptide.
- This process continues by giving a long polypeptide chain of amino acids.
- The peptide bond formation and elongation of polypeptide continues until a stop codon appears on A-site.
- If stop codon appears on A-site it is not recognized by t-RNA carrying amino acids because stop codon do not have anticodon on mRNA.
- The stop codons are recognized by the next protein called release factor which hydrolyzes and causes release of all components such as 30s, 50S, mRNA and polypeptide separates.
- RF-1 recognizes UAA and UAg while RF-2 recognizes UAA and UGA while RF-3 dissociates 30S and 50S subunits.