Ribosome composition in bacteria What are the Svedberg sedimentation values of the two subunits that compose the 70S ribosome of Escherichia coli?

Introduction / Context:Prokaryotic ribosomes are termed 70S, composed of two unequal subunits whose sizes are crucial to antibiotic targeting, translation initiation, and molecular biology techniques. Recognizing the subunit S values helps differentiate bacterial from eukaryotic ribosomes (80S with 60S/40S subunits).

Given Data / Assumptions:

• Bacterial ribosome overall: 70S (sedimentation coefficient, not additive).
• Goal: identify the pair of subunit S values for E. coli.
• Standard growth conditions and wild-type ribosomes assumed.

Concept / Approach:

The prokaryotic ribosome comprises a large 50S subunit (containing 23S and 5S rRNAs with proteins) and a small 30S subunit (16S rRNA with proteins). The combined particle is 70S due to non-linear sedimentation properties; Svedberg units are not strictly additive.

Step-by-Step Solution:

Verification / Alternative check:

Textbook diagrams and antibiotic mechanisms (e.g., macrolides binding 50S, aminoglycosides targeting 30S) corroborate the subunit identities and sizes.

Why Other Options Are Wrong:

• 40S/30S or 40S/20S: Mixed or nonstandard values; 40S is eukaryotic small subunit.
• 50S/20S: No 20S bacterial subunit exists.
• 60S/40S: Eukaryotic ribosome (80S), not bacterial.

Common Pitfalls:

• Assuming Svedberg units add linearly; they do not due to shape/mass/friction effects.

Final Answer:

50S and 30S