6.An antibiotic that resembles the 3′ end of a charged tRNA molecule is:
(1) Streptomycin(2) Sporsomycin
(3) Puromycin     (4) Tetracycline

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Antibiotics have revolutionized medicine and molecular biology by targeting critical cellular processes. One of the most fascinating antibiotics in this context is puromycin, renowned for its unique ability to mimic a fundamental component of protein synthesis machinery—the 3′ end of a charged tRNA molecule. This article explores the structural and functional aspects of puromycin, its mechanism of action, and why its resemblance to aminoacyl-tRNA is so significant.

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Understanding the 3′ End of Charged tRNA

In every cell, protein synthesis occurs on ribosomes, where aminoacyl-tRNAs deliver specific amino acids to the growing polypeptide chain. The 3′ end of a charged tRNA (also known as aminoacyl-tRNA) is where the amino acid is covalently attached, forming a critical linkage for peptide bond formation. This 3′ end, with its characteristic CCA sequence and attached amino acid, enters the ribosome’s A site during translation.

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Puromycin: The Structural Mimic

Puromycin is an aminonucleoside antibiotic produced by Streptomyces alboniger. What sets puromycin apart is its remarkable structural similarity to the 3′ end of aminoacylated tRNA. Specifically, puromycin consists of a modified adenosine base covalently linked to a tyrosine amino acid, closely resembling the natural configuration found at the tRNA’s 3′ terminus1235.

This mimicry is not superficial—it is functionally significant. Puromycin can enter the ribosomal A site just like a real aminoacyl-tRNA, allowing it to participate in the peptidyl transferase reaction. The ribosome then transfers the growing peptide chain to puromycin, but unlike normal tRNA, the resulting peptidyl-puromycin cannot support further elongation. This leads to premature chain termination and release of an incomplete polypeptide1457.

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Mechanism of Action: How Puromycin Stops Protein Synthesis

• Entry into the Ribosome: Puromycin enters the A site of the ribosome, mimicking the 3′ end of a charged tRNA.

• Peptide Transfer: The ribosome’s peptidyl transferase center transfers the nascent polypeptide from the P-site tRNA to the amino group of puromycin.

• Premature Termination: The peptidyl-puromycin product is released from the ribosome, halting protein synthesis and producing a truncated, nonfunctional protein1457.

This mechanism is so effective that puromycin is widely used in molecular biology to study translation and ribosome function, as well as in mRNA display technologies.

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Why Not the Other Antibiotics?

• Streptomycin: This antibiotic binds to the 30S subunit of the ribosome, causing misreading of mRNA but does not structurally resemble tRNA.

• Sporsomycin: There is no widely recognized antibiotic by this name affecting translation via tRNA mimicry.

• Tetracycline: Tetracycline inhibits protein synthesis by blocking the A site on the ribosome, preventing aminoacyl-tRNA entry, but does not mimic tRNA structure.

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Summary Table: Antibiotics and tRNA Mimicry

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Correct Answer

(3) Puromycin

Puromycin is the antibiotic that structurally and functionally resembles the 3′ end of a charged tRNA molecule, allowing it to enter the ribosomal A site and cause premature termination of protein synthesis123457.

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Conclusion

Puromycin’s ability to mimic the 3′ end of aminoacyl-tRNA is a classic example of molecular mimicry in antibiotic action. This unique property not only underlies its effectiveness as a translation inhibitor but also makes it an invaluable tool in molecular biology research. By understanding how puromycin operates, scientists can better explore the intricacies of protein synthesis and develop new strategies for antibiotic intervention and translational research.