4. Puromycin blocks translation. Mode of action of drug puromycin is
(1) binds to A site and leads to premature termination
(2) stops peptidyl transferase activity
(3) Binds EF-TU-GTP and prevent initiation
(4) donot allow termination of translation

Translation, the process of synthesizing proteins from mRNA templates, is essential for all living organisms. Antibiotics that disrupt translation are powerful tools in both medicine and research. Among these, puromycin stands out for its unique and well-characterized mode of action—causing premature termination of protein synthesis. This article explores how puromycin works at the molecular level, why it is so effective, and its broad significance in molecular biology.

What is Puromycin?

Puromycin is an aminonucleoside antibiotic derived from Streptomyces alboniger. It is widely used in research as a translation inhibitor and as a selectable marker in cell culture. What makes puromycin unique is its ability to mimic the structure of aminoacyl-tRNA, allowing it to directly interact with the ribosome during protein synthesis.

How Puromycin Blocks Translation

Mimicry of Aminoacyl-tRNA

Puromycin closely resembles the 3’ end of a charged tRNA molecule. This structural similarity allows it to enter the A (aminoacyl) site of the ribosome, the location where new amino acids are normally added to the growing peptide chain1345.

Incorporation and Premature Termination

Once inside the A site, puromycin participates in the peptidyl transferase reaction. The ribosome transfers the nascent peptide chain from the tRNA in the P site to puromycin, forming a peptidyl-puromycin molecule. Unlike normal tRNA, peptidyl-puromycin cannot remain attached to the ribosome. As a result, the newly formed peptide is released prematurely, leading to the production of truncated, nonfunctional proteins and immediate cessation of translation145.

Rapid Release of Nascent Chains

Research shows that puromycin causes almost instantaneous release of nascent polypeptides from the ribosome, even in the presence of elongation inhibitors. This rapid action is a hallmark of puromycin’s effectiveness as a translation blocker1.

Clarifying Common Misconceptions

• Does Not Inhibit Peptidyl Transferase Directly: Puromycin does not stop the peptidyl transferase enzyme; it acts as a substrate for it.

• Does Not Bind EF-TU-GTP: Puromycin does not prevent initiation by binding elongation factors.

• Does Not Block Termination: Instead of preventing termination, puromycin causes premature termination by releasing incomplete peptides.

Summary Table: Puromycin’s Mode of Action

Research and Clinical Applications

• Molecular Biology: Puromycin is used to study protein synthesis, ribosome function, and translation regulation.

• Cell Selection: Its toxicity to cells that are not resistant makes it a useful selectable marker in genetic engineering.

• Antibiotic Action: By halting protein synthesis, puromycin is lethal to bacteria and eukaryotic cells alike.

Correct Answer

(1) Binds to A site and leads to premature termination

Puromycin blocks translation by binding to the ribosomal A site, accepting the nascent peptide chain, and causing premature termination of protein synthesis1345.

Conclusion 

Puromycin’s unique ability to mimic aminoacyl-tRNA and bind the ribosomal A site is central to its function as a translation inhibitor. By causing premature termination and rapid release of incomplete polypeptides, puromycin effectively halts protein synthesis. This mechanism not only explains its antibiotic action but also underpins its widespread use in molecular biology research and biotechnology. Understanding puromycin’s mode of action is essential for anyone studying protein synthesis, antibiotic mechanisms, or cellular engineering.