Antibiotics are widely known for their ability to inhibit bacterial protein synthesis by targeting the prokaryotic 70S ribosome. However, some antibiotics specifically inhibit the eukaryotic 80S ribosome, which is structurally and functionally distinct. Among the options listed—tetracycline, streptomycin, cycloheximide, and chloramphenicol—only one is a well-established inhibitor of the eukaryotic 80S ribosome.

Understanding Ribosomes: 70S vs. 80S

• Prokaryotic Ribosomes (70S): Composed of 50S large and 30S small subunits.

• Eukaryotic Ribosomes (80S): Composed of 60S large and 40S small subunits.

Most antibiotics are designed to target bacterial 70S ribosomes, exploiting structural differences to avoid toxicity to eukaryotic cells. However, some compounds like cycloheximide specifically inhibit the eukaryotic 80S ribosome, making them useful tools in research and potential anticancer agents.

Cycloheximide: The Eukaryotic 80S Ribosome Inhibitor

Cycloheximide is a glutarimide antibiotic that selectively binds to the eukaryotic 80S ribosome. It inhibits protein synthesis by blocking the elongation step of translation. Specifically, cycloheximide interferes with the translocation process, preventing the ribosome from moving along the mRNA and thus halting the addition of new amino acids to the growing polypeptide chain.

Mechanism of Action

• Binds to the 60S large subunit of the eukaryotic ribosome.

• Interacts near the peptidyl transferase center, disrupting elongation.

• Prevents the ribosome from translocating, effectively freezing translation.

• Does not affect bacterial 70S ribosomes, which is why it is toxic to eukaryotic cells but not bacteria.

Why Not the Other Antibiotics?

• Tetracycline: Targets the 30S subunit of bacterial 70S ribosomes, blocking aminoacyl-tRNA binding.

• Streptomycin: Binds to the 30S bacterial ribosomal subunit, causing misreading of mRNA.

• Chloramphenicol: Binds to the 50S subunit of bacterial ribosomes, inhibiting peptidyl transferase activity.

None of these inhibit the eukaryotic 80S ribosome effectively.

Summary Table: Antibiotics and Ribosome Targets

Correct Answer

(3) Cycloheximide

Cycloheximide is the antibiotic that inhibits the eukaryotic 80S ribosome by blocking the elongation phase of protein synthesis.

Biological and Research Significance

Cycloheximide is widely used in molecular biology to study eukaryotic translation and protein turnover. By halting protein synthesis in eukaryotic cells, it allows researchers to investigate mRNA stability, protein degradation rates, and the effects of translation inhibition on cellular processes.

However, due to its toxicity to eukaryotic cells, cycloheximide is not used clinically as an antibiotic but remains a vital research tool.

Conclusion

Among the antibiotics listed, cycloheximide uniquely and specifically inhibits the eukaryotic 80S ribosome. Its ability to block translation elongation in eukaryotic cells distinguishes it from other antibiotics that target bacterial ribosomes. Understanding these differences is crucial for both clinical antibiotic use and experimental design in molecular biology.