Initiation of Translation in Prokaryotes—30S Initiation Complex In bacteria, translation begins with formation of a 30S initiation complex. Which components assemble together at this stage?

Introduction / Context:
Prokaryotic translation initiation positions the ribosome at the correct start codon and recruits the initiator tRNA charged with N-formyl-methionine (fMet). The small 30S subunit, initiation factors (IF1, IF2, IF3), mRNA, and fMet-tRNA form the 30S initiation complex before the 50S subunit joins to create the 70S initiation complex ready for elongation.

Given Data / Assumptions:

• Initiator tRNA in bacteria: fMet-tRNA^fMet.
• Shine–Dalgarno sequence in mRNA base-pairs with 16S rRNA to position the start codon.
• Initiation factors assist subunit assembly and initiator tRNA placement.

Concept / Approach:
Identify all elements required for the 30S initiation complex. The mRNA template must be present, as must the initiator tRNA and initiation factors. The large 50S subunit is not yet part of the complex until after correct positioning and factor-mediated steps are complete.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays show that only after this complex forms does 50S join, GTP is hydrolyzed, and initiation factors are released to form the 70S initiation complex.

Why Other Options Are Wrong:

• No mRNA or no initiator tRNA: cannot establish start codon recognition.
• 30S + mRNA only: lacks factors and initiator tRNA, incomplete.
• Starting with 50S: initiation begins on 30S, not 50S.

Common Pitfalls:
Assuming the large subunit participates before start codon selection; confusing elongator Met-tRNA with initiator fMet-tRNA.

Final Answer:
30S ribosomal subunit, mRNA, initiation factors, and N-formyl-methionyl tRNA (fMet-tRNA)