Translation and it's Steps in Prokaryotes

Translation:

The interpretation of genetic information contains within the codons of RNA into the amino acid sequences of protein is called translation. It requires MRNA, tRNA, ribosome and aminoacyl-tRNA synthetase.

tRNA charging:

Attachment of amino acid to the 3’end of tRNA is called tRNA charging. The aminoacyl tRNA synthetase is specific and three are 20 different aminoacyl-tRNA synthetase for 20 different amino acids.

Steps in translation:

1.Initiation:

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Three important steps in translation initiation includes:

a.Recruitment of 30s ribosomal sub- unit to the mRNA: First, the 30s sub- unit of ribosome binds to the mRNA, so that the start codon will be in the P-site (peptidyl-site). Binding occurs between 5’-GGAGG-3’ of RBS and 3’-CCUCC-5’ of 165 rRNA.

b.placement of the initiator tRNA into P-site of ribosome:Initiator tRNA is charged with N-formylmethionine (tRNA^fmet)

. Initiator tRNA binds with the start codon (AUG) in the P-site of the ribosome.

Three initiation factors (IF1, IF2,and IF3) are involved in initiation phase with specific functions:

_ IF1 prevents the binding of tRNA^fmet to the A-site of the ribosome (aminoacyl-site).

_ IF2 facilitates the binding of tRNA^fmet to the 30s sub- unit of the ribosome.

_ IF3 prevents the binding of 50s sub- unit to the 30s sub- unit of the ribosome.

c.Positioning of 50s sub- unit to the 30s sub- unit: When the start codon and initiator tRNA^fmet base pairs,30s sub- unit undergoes a conformational change. This results in the release of IF3, which facilitates binding of 50s sub- unit to 30s sub- unit. Binding of 50s sub- unit stimulates GTP_ase activity of initiation factor 2 (IF2)-GTP, causing it to hydrolyse GTP, with the release of IF2-GDP and IF1 from the ribosome. This results in the formation of 70s initiation complex.

2.Elongation:

Chain elongation involves 3 steps:

a.Binding of new aminoacyl-tRNA at the A-site: The elongation factor EF-TU (GTP) binds with the charged tRNA and brings it to the ribosome. Once the charged tRNA is positioned in the A-site, GTP is hydrolysed and EF-TU (GDP) dissociates from the ribosome.

b.Formation of new peptide bond (transpeptidation):

The peptide bond is formed between C-end of 1 amino acid (present in the P-site) and N-end of the other amino acid (present in the A-site). This reaction is catalyzed by transpeptidase enzyme.

c.Translocation of the ribosome:

Finally, the ribosome translocates along the mRNA thereby moving peptidyl-tRNA to the P-site and uncharged tRNA to the E-site (exit site). This step requires Elongation factor, EF-G (GTP). Once the translocation complete, the GTP is hydrolysed to remove elongation factor, EF-G (GDP) because it prevents the binding of charged tRNA and release factors to the A-site.

3.Termination:

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Three important steps in termination include:

a.Binding of release factor:

Translation stops when to stop codons appear in the A-site because there are no tRNA_s that recognized stop codons. These stop codons are recognized by class released factor (RF1 recognizes UAA and UAG and RF2 recognizes UUA and UGA) class release factor RF3 stimulates the binding of RF1/2 at the A-site.

b.Hydrolysis of the peptidyl-tRNA:

Binding of the release factor alters the peptidyl transferase activity resulting in the hydrolysis of peptidyl-tRNA and release of the completed polypeptide chain. RF3-GTP removes RF1/2 from the ribosome. At the same time, RF3-GTP associated with the factor binding center of the large sub- unit. In the absence of RF1/2, RF3-GDP has also a low affinity for the ribosome and is released.

c.Dissociation:

The ribosome recycling factor RRF along with the EF (elongation factor)-G (RRF-EF-G-GTP) binds to the empty A-site. EF-G stimulates the release of uncharged tRNA from P and E-site. Elongation factor G-GDP and RRF are then released from the ribosome along with the mRNA. At last IF3 also participate in the release of mRNA and separates two ribosomal sub- units from each other.

References:

(DL and MM)

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