8. Which one Of the following RNAs possesses the peptidyl transferase activity?
   (1) tRNA             (2) 2.5S rRNA
   (3) 16S rRNA      (4) 23S rRNA

Introduction

The ribosome is the molecular machine responsible for translating genetic information from messenger RNA (mRNA) into proteins. One of the most remarkable features of the ribosome is its ability to catalyze the formation of peptide bonds, a process known as peptidyl transferase activity. But which RNA component of the ribosome actually carries out this vital enzymatic function? This article explores the options and explains why the correct answer is essential for understanding the core of protein synthesis.

Understanding the Players: tRNA vs. rRNA

To answer the question, it’s important to distinguish between the main types of RNA involved in translation:

• tRNA (Transfer RNA):

  • Role: Delivers amino acids to the ribosome in response to specific codons on the mRNA.

  • Function: Acts as an adapter molecule, ensuring that the correct amino acid is incorporated into the growing polypeptide chain.

  • Peptidyl transferase activity: tRNA does not possess peptidyl transferase activity.

• rRNA (Ribosomal RNA):

  • Role: Forms the structural and catalytic core of the ribosome.

  • Function: Provides the scaffold for ribosome assembly and catalyzes key reactions, most notably peptide bond formation.

  • Peptidyl transferase activity: rRNA is responsible for peptidyl transferase activity.

The Peptidyl Transferase Center

The peptidyl transferase center (PTC) is the catalytic site within the large ribosomal subunit where peptide bonds are formed between amino acids during translation. This center is not composed of protein but is made entirely of ribosomal RNA (rRNA)

Key Facts About the Peptidyl Transferase Center

• Location: The PTC is found in the large subunit of the ribosome.

• Catalytic RNA: The activity is carried out by a specific region of rRNA, not by any ribosomal proteins or tRNA.

• Evolutionary Significance: The fact that RNA catalyzes this reaction is strong evidence for the RNA World hypothesis, which suggests that life may have begun with RNA-based catalysis.

Which rRNA Possesses Peptidyl Transferase Activity?

Different organisms have ribosomes composed of slightly different rRNA molecules. Here’s how it breaks down:

• Prokaryotes (e.g., bacteria):

  • Large ribosomal subunit: 50S

  • rRNA component: 23S rRNA

  • Peptidyl transferase activity: Resides in domain V of the 23S rRNA.

• Eukaryotes (e.g., humans):

  • Large ribosomal subunit: 60S

  • rRNA component: 28S rRNA (with some regions contributed by 5.8S rRNA)

  • Peptidyl transferase activity: Resides in the 28S rRNA.

• Other rRNAs:

  • 16S rRNA: Found in the small ribosomal subunit (30S in prokaryotes, 40S in eukaryotes). It does not possess peptidyl transferase activity.

  • 2.5S rRNA: Not a standard rRNA size; likely a typographical error or confusion with other rRNA fragments (e.g., 5S rRNA, which is present but not catalytic).

Mechanism of Peptide Bond Formation

The peptidyl transferase reaction is a central step in protein synthesis. Here’s how it works:

1. Substrates: Two tRNAs are present in the ribosome—one in the peptidyl (P) site (carrying the growing peptide chain) and one in the aminoacyl (A) site (carrying the next amino acid to be added).

2. Catalysis: The amino group of the aminoacyl-tRNA attacks the carbonyl carbon of the peptidyl-tRNA, forming a peptide bond.

3. Role of rRNA: The 23S (or 28S) rRNA aligns the tRNAs and facilitates the reaction, acting as a ribozyme.

Why Not tRNA or Other rRNAs?

• tRNA:

  • Function: Delivers amino acids.

  • Limitation: Does not catalyze peptide bond formation.

• 16S rRNA:

  • Function: Involved in decoding mRNA and ensuring the correct tRNA enters the ribosome.

  • Limitation: Does not possess peptidyl transferase activity.

• 2.5S rRNA:

  • Not a recognized rRNA in standard ribosomes; likely a misprint or confusion with 5S rRNA, which is present but not catalytic.

Antibiotics and the Peptidyl Transferase Center

The peptidyl transferase center is a common target for antibiotics that inhibit bacterial protein synthesis. For example, chloramphenicol and macrolides bind to the 23S rRNA, blocking peptide bond formation and thus stopping bacterial growth.

Evolutionary Insights

The peptidyl transferase center is considered one of the most ancient parts of the ribosome. Its structure and function have been highly conserved throughout evolution, underscoring its critical role in life.

Summary Table

Conclusion

The RNA that possesses peptidyl transferase activity is the 23S rRNA in prokaryotes and the 28S rRNA in eukaryotes. Among the options provided in your question, the correct answer is 23S rRNA (option 4), assuming the context is prokaryotic translation, which is standard in many educational settings.

Key Takeaway:
Peptidyl transferase activity is a catalytic function of the large subunit rRNA (23S in prokaryotes, 28S in eukaryotes), not tRNA or other rRNAs. This highlights the unique role of rRNA as a ribozyme at the heart of protein synthesis.

Additional Insights

• Ribozyme Nature: The discovery that rRNA catalyzes peptide bond formation was a landmark in molecular biology, demonstrating that RNA can act as an enzyme (ribozyme).

• Conservation: The peptidyl transferase center is highly conserved across all life forms, reflecting its fundamental importance.

• Antibiotic Targets: Understanding the structure and function of the PTC has led to the development of antibiotics that specifically target bacterial ribosomes.

Final Answer

Which one of the following RNAs possesses the peptidyl transferase activity?

(4) 23S rRNA

This is the correct answer for prokaryotic systems. In eukaryotes, the equivalent function is carried out by 28S rRNA. Neither tRNA nor 16S rRNA nor any “2.5S rRNA” possesses this catalytic activity.