Q.6 Peptidyl transferase activity resides in

• (A) 16S rRNA
• (B) 23S rRNA
• (C) 5S rRNA
• (D) 28S rRNA

  Peptidyl transferase catalyzes peptide bond formation during protein synthesis on ribosomes. This ribozyme activity drives translation elongation. For biotech students, this MCQ clarifies its precise RNA location: (B) 23S rRNA.

  Ribosome Structure and Peptidyl Transferase

  Ribosomes consist of small and large subunits with rRNAs and proteins. In prokaryotes, the large 50S subunit houses the peptidyl transferase center (PTC) in domain V of 23S rRNA. This RNA forms the active site, positioning A- and P-site tRNAs for nucleophilic attack by the amino group on the peptidyl ester bond.

  Correct Answer: (B) 23S rRNA
  The PTC resides exclusively in 23S rRNA of the prokaryotic large subunit (analogous to eukaryotic 28S). Key nucleotides like A2451 catalyze bond formation without proteins, confirming the ribosome as a ribozyme.

  Explanation of All Options

  Each rRNA’s role in translation:

  • (A) 16S rRNA
    Incorrect. Found in the prokaryotic small 30S subunit, it decodes mRNA via anticodon-codon matching and initiates assembly—no peptidyl transferase function.

  • (B) 23S rRNA
    Correct. Forms the PTC in the 50S large subunit, enabling peptide bond formation at 15-50 bonds/second. Antibiotics like chloramphenicol target it.​

  • (C) 5S rRNA
    Incorrect. Part of the large subunit, it stabilizes ribosome structure and aids tRNA binding but lacks catalytic peptidyl transferase activity.

  • (D) 28S rRNA
    Incorrect for prokaryotic context (common in MCQs). Eukaryotic large subunit equivalent to 23S; not standard in bacterial systems where 23S predominates.

  Option	Ribosome Subunit	Primary Role	Peptidyl Transferase?
  (A) 16S rRNA	Small (30S)	mRNA decoding	No ​
  (B) 23S rRNA	Large (50S)	Peptide bonding	Yes ​
  (C) 5S rRNA	Large (50S)	Structural stability	No
  (D) 28S rRNA	Eukaryotic large	Equivalent to 23S	No (prokaryote MCQ)

  Relevance in Molecular Biology

  Understanding 23S rRNA’s role aids antibiotic design and ribosome engineering. In bioinformatics, model PTC with tools like BLAST for rRNA sequences or RasMol for structures—key for your biotech studies.