Question

Explain the process of translation from RBS (ribosome binding site) recognition to hitting the stop codon.

Answer 1

Ribosome binding site is an RNA sequence found in mRNA that promotes efficient and accurate translation of mRNA and is known as Shine-Dalgarno sequence in prokaryotes.

When the IF-2 (initiation factor) which is a small GTPase binds to a tRNA and facilitate its binding with the small ribosomal subunit. Now the 16s rRNA (3'end) of the 30s ribosomal subunit recognizes the ribosomal binding site of the Shine-Dalgarno sequence through its anti- Shine-Dalgarno sequence which is present 5-10 base pairs upstream of the start codon. This facilitate the ribosome to position correctly on the mRNA that P-site directs on the AUG initiation codon.

Now IF-3 facilitates the interaction of tRNA with the mRNA initiation codon AUG via base pairing. Now the smaller subunit binds between its subunits and ribosomal binding site and after this binding N-formyl methionine tRNA recognizes and binds to the initiator codon. An initiation complex is formed when the large subunit binds to it and this facilitate the fMet-tRNA to occupy the P-site and emptying of A-site. This ends the initiation stage of translation.

Elongation- The entering of fmet-tRNA to the P-site initiate the elonagation stage by causing a confirmational change in order to bind Aminoacyl tRNA to the A-site. Now a peptide bond is formed between the last amino acid and the one that is still attached to the tRNA at the A-site, when a polypeptide connected to a tRNA at the P-site detached from the tRNA and this process is facilitated by an enzyme called peptidyltransferase. Now the A-site has a newly formed aminoacid whereas the tRNA at the P-site is empty or with no aminoacid. This cause the shifting of ribosome towards the 3' end of RNA by 3 nucleotides and this is called as translocation. This facilitates the movement of uncharged tRNA to the E-site or exit site. All these steps will be continued to take place to form a polypeptide chain at the P-site of ribosome until the ribosome reaches to the stop codon(UAA, UAG, UGA) on mRNA.

Termination- It takes place when one of the initiation codon reaches to the A-site of the ribosome. As these stop codons are not recognized by the tRNA but by some proteins called release factors (RF-1 or 2) and this facilitate the hydrolyis of ester bond of peptidyl-tRNA. Now the bond is broken and the newly synthesied polypeptide releases from the ribosome.