Which additional nucleolytic activities are exhibited by E. coli DNA polymerase I besides its polymerase function?

Introduction / Context:
Beyond catalyzing DNA synthesis, DNA polymerase I participates in repair by excising nucleotides and correcting errors. Recognizing its exonuclease repertoire explains primer removal and proofreading.

Given Data / Assumptions:

• Enzyme: E. coli DNA polymerase I.
• We evaluate exonuclease and endonuclease activities.

Concept / Approach:
Pol I has two exonuclease activities: a 3' → 5' exonuclease for proofreading and a 5' → 3' exonuclease that removes nucleotides ahead of a nick, including RNA primers. It does not have a 5' → 3' endonuclease; the activity is exonucleolytic and nick-dependent.

Step-by-Step Solution:
1) Identify proofreading: 3' → 5' exonuclease maintains fidelity by removing incorrect bases.2) Identify nick translation: 5' → 3' exonuclease removes primers/damage ahead of a nick while polymerase fills the gap.3) Conclude both exonucleases are present; endonuclease is not.

Verification / Alternative check:
Proteolysis yields the Klenow fragment lacking the 5' → 3' exonuclease but retaining polymerase and 3' → 5' exonuclease; the small fragment contains the 5' → 3' exonuclease, confirming dual activities.

Why Other Options Are Wrong:
Only 5' → 3': incomplete; ignores proofreading.Only 3' → 5': incomplete; ignores primer removal.5' → 3' endonuclease: incorrect enzyme class.

Common Pitfalls:
Confusing endonuclease with exonuclease, or attributing 5' → 3' exonuclease to Pol III instead of Pol I.

Final Answer:
Both (a) and (b)