In molecular biology, which catalytic activities are present as distinct active sites within the single polypeptide of E. coli DNA polymerase I?

Introduction / Context:
DNA polymerase I from Escherichia coli is a single polypeptide enzyme that performs multiple catalytic functions during DNA repair and processing. This question tests recognition of the distinct active sites carried on that one polypeptide chain and how they contribute to fidelity and repair.

Given Data / Assumptions:

• Organism: E. coli.
• Enzyme: DNA polymerase I (Pol I).
• Activities to evaluate: 5' → 3' polymerase, 3' → 5' exonuclease, 5' → 3' exonuclease.

Concept / Approach:
Pol I possesses three separable catalytic activities located at distinct active sites on the same polypeptide. The polymerase adds nucleotides. The 3' → 5' exonuclease proofreads newly added bases. The 5' → 3' exonuclease removes RNA primers or damaged DNA ahead of a nick (nick translation).

Step-by-Step Solution:
1) Identify polymerase site: adds dNTPs in the 5' → 3' direction.2) Identify proofreading site: 3' → 5' exonuclease removes misincorporated nucleotides, increasing fidelity.3) Identify primer-removal site: 5' → 3' exonuclease cleaves nucleotides ahead of a nick, enabling primer removal and gap filling.4) All three sites are present on the single Pol I polypeptide; thus the inclusive option applies.

Verification / Alternative check:
Biochemical domain mapping shows the Klenow fragment (polymerase + 3' → 5' exonuclease) after limited proteolysis, while the small fragment contains the 5' → 3' exonuclease. This confirms distinct active sites on one polypeptide.

Why Other Options Are Wrong:
5' → 3' polymerase alone: incomplete; Pol I also has both exonucleases.3' → 5' exonuclease alone: incomplete; lacks polymerase and 5' → 3' exonuclease.5' → 3' exonuclease alone: incomplete; lacks other functions.

Common Pitfalls:
Confusing 5' → 3' exonuclease with 3' → 5' proofreading; assuming separate subunits rather than distinct sites within one polypeptide; mixing up Pol I with Pol III core enzyme.

Final Answer:
All of the above