Q.18 Tetracycline binds to the
(A) 30S subunit and inhibits aminoacyl–tRNA binding
(B) 50S subunit and inhibits aminoacyl–tRNA binding
(C) 30S subunit and prevents codon:anticodon interactions
(D) 50S subunit and blocks exit of growing polypeptide chain
Tetracycline binds to the 30S ribosomal subunit and inhibits aminoacyl-tRNA binding to the A-site, making option (A) correct for this CSIR NET Life Sciences question. This mechanism disrupts bacterial protein synthesis during translation elongation.
Option Analysis
Option (A): Correct. Tetracycline reversibly binds to the 30S subunit near helix 34 of 16S rRNA, sterically blocking aminoacyl-tRNA from entering the A-site. Crystal structures confirm this high-affinity site overlaps the tRNA anticodon path, preventing accommodation without affecting codon-anticodon pairing directly.
Option (B): Incorrect. Tetracycline primarily targets the 30S subunit, not 50S; secondary low-affinity sites on 50S exist but do not mediate main inhibition.
Option (C): Incorrect. While tetracycline occupies space near the decoding center, it blocks tRNA binding before codon-anticodon recognition occurs, not preventing the interaction itself.
Option (D): Incorrect. Polypeptide exit involves the peptide tunnel on the 50S subunit; tetracycline does not block this process.
Introduction to Tetracycline Mechanism
Tetracycline binds to 30S subunit and inhibits aminoacyl-tRNA binding, halting bacterial protein synthesis at the elongation stage. This action targets the A-site on bacterial ribosomes, selective due to structural differences from eukaryotic ones. For CSIR NET aspirants, understanding this distinguishes it from other antibiotics.
Detailed Binding and Inhibition
Tetracycline diffuses into bacteria and attaches to a pocket in the 30S subunit formed by 16S rRNA helices 31 and 34. This position sterically hinders aminoacyl-tRNA entry, even when delivered by EF-Tu, preventing peptide bond formation.
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Primary site: High-affinity on 30S near decoding center.
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Effect: Blocks A-site occupation, stalls elongation.
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Evidence: Cryo-EM and X-ray studies show overlap with tRNA path.
Comparison with Other Antibiotics
| Antibiotic | Ribosome Target | Specific Inhibition |
|---|---|---|
| Tetracycline | 30S subunit | Aminoacyl-tRNA binding to A-site |
| Chloramphenicol | 50S subunit | Peptidyl transferase activity |
| Streptomycin | 30S subunit | Codon misreading |
| Erythromycin | 50S subunit | Translocation |
This table highlights tetracycline’s unique 30S aminoacyl-tRNA focus.
Relevance for CSIR NET Exam
In translation inhibition questions, tetracycline consistently matches “30S subunit and inhibits aminoacyl-tRNA binding.” Resistance via Tet(O) proteins displaces it from 30S, emphasizing clinical importance. Practice similar MCQs reinforces molecular biology concepts.
