50. During translation which one of the following aminoacyl-tRNA binds first to the P site of bacterial
ribosomes?
(a) fMet-tRNAfMet
(b) fMet-tRNAMet
(c) Met-tRNAfMet
(d) Met-tRNAMet

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Article:

Introduction

In the process of protein synthesis, or translation, ribosomes decode messenger RNA (mRNA) sequences into polypeptide chains. This highly orchestrated process begins with the correct binding of an aminoacyl-tRNA to the P site of the bacterial ribosome. But which tRNA gets there first, and why?

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The Role of tRNA in Translation

Transfer RNAs (tRNAs) are adaptor molecules that carry amino acids to the ribosome. Each tRNA has:

• An anticodon that pairs with a codon on the mRNA

• An attached amino acid that will be added to the growing peptide chain

In bacteria, translation initiation requires a special form of methionine: N-formylmethionine (fMet).

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What is fMet-tRNAfMet?

• fMet stands for N-formylmethionine, a chemically modified version of methionine.

• tRNAfMet is a special initiator tRNA that is recognized by the bacterial initiation complex.

• Together, they form fMet-tRNAfMet, the first aminoacyl-tRNA to bind the ribosome.

This complex is specifically recognized by the initiation factors (IFs) and directly binds to the P site of the 30S ribosomal subunit, unlike elongator tRNAs which enter at the A site.

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Why fMet-tRNAfMet and Not the Others?

Let’s break down the options:

• (a) fMet-tRNAfMet ✅
  ✔ This is the correct initiator tRNA used in bacteria. It binds directly to the P site and starts translation.

• (b) fMet-tRNAMet ❌
  ✘ Incorrect pairing — tRNAMet is not used with formylmethionine in bacteria.

• (c) Met-tRNAfMet ❌
  ✘ Met is not formylated here, which is necessary in bacterial initiation.

• (d) Met-tRNAMet ❌
  ✘ This is the standard elongator tRNA used in eukaryotes, not for initiation in bacteria.

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How Does fMet-tRNAfMet Function?

1. Initiation Factors (IF1, IF2, IF3) help assemble the 30S subunit with mRNA.

2. fMet-tRNAfMet is delivered to the P site by IF2 in a GTP-dependent manner.

3. Once base-pairing occurs with the start codon (AUG), the 50S ribosomal subunit joins.

4. Translation elongation then proceeds, with the next aminoacyl-tRNA entering the A site.

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Conclusion

In bacterial translation, the first aminoacyl-tRNA to bind the P site is fMet-tRNAfMet, a specialized tRNA that recognizes the start codon and initiates protein synthesis.

✅ Correct Answer: (a) fMet-tRNAfMet

Understanding this process is essential in molecular biology and helps explain how prokaryotic organisms initiate translation differently than eukaryotes — a fact that is even exploited in antibiotic design.