Question

Among the list of events happen during the initiation of translation in prokaryotes, find out the exact order in which the initiation factor 3(IF3) is involved.
(A) Promote the dissociation of inactive 70S ribosomal complex.
(B) Facilitate the binding of 50S ribosome with the 30S initiation complex.
(C) Assist the binding of 30S ribosome with the shine-dalgarno sequence of mRNA.
Your response

• A. (A), (B), (C)
• B. (A), (C), (B)
• C. (B), (C), (A)
• D. (C), (A), (B)

Hint:

To remember the role of IF3:
- It acts as an anti-association factor.
- It must bind first to separate the subunits (A), help the small subunit find the mRNA (C), and finally leave so the large subunit can join (B).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Translation initiation in prokaryotes is a multi-step process regulated by three protein initiation factors: IF1, IF2, and IF3.
These factors coordinate the assembly of the functional 70S ribosome on the mRNA.

Step 2: Detailed Explanation:
Let us trace the precise chronological involvement of Initiation Factor 3 (IF3) during translation initiation:
1. First Event (A): After a previous round of translation, the ribosome exists as an inactive 70S complex.
IF3 binds specifically to the 30S ribosomal subunit.
This binding prevents the 50S subunit from reassociating with it, thereby promoting the dissociation of the inactive 70S complex into free 30S and 50S subunits.
This ensures a pool of free 30S subunits is available for new initiation events.
2. Second Event (C): Once the free 30S subunit is stabilized by IF3 and IF1, it can bind to the mRNA.
IF3 directly assists the 30S subunit in identifying and binding to the ribosome-binding site, which is the Shine-Dalgarno sequence on the mRNA.
The initiator tRNA (fMet-tRNA\(^{\text{fMet}}\)) then binds to the P-site with the help of IF2-GTP, completing the 30S initiation complex.
3. Third Event (B): Once the 30S initiation complex is correctly assembled and verified, IF3 must dissociate from the 30S subunit.
The release of IF3 removes the steric hindrance that was preventing subunit reassociation.
This allows and facilitates the rapid binding of the large 50S ribosomal subunit to the 30S initiation complex.
This forms the active 70S ribosome ready for elongation.
Thus, the correct chronological order of these events is (A) \(\rightarrow\) (C) \(\rightarrow\) (B), which corresponds to Option (B).

Step 3: Final Answer:
The exact order of events involving IF3 is (A), (C), (B).