Exonuclease activities of E. coli DNA polymerases II and III Which activity do Escherichia coli DNA polymerases II and III lack among the listed choices?

Difficulty: Medium

Correct Answer: 5' to 3' exonuclease activity

Explanation:


Introduction:
Different DNA polymerases in bacteria specialize in distinct tasks and possess characteristic nuclease activities. Identifying which enzyme has which exonuclease capability clarifies roles in replication versus primer removal and repair.


Given Data / Assumptions:

  • Pol I, Pol II, and Pol III are the classic E. coli DNA polymerases.
  • Pol I performs primer removal using a 5 to 3 exonuclease activity.
  • Pol II and Pol III are involved in repair and replication with strong proofreading but without primer removal.


Concept / Approach:

Pol II and Pol III share a 3 to 5 exonuclease for proofreading misincorporations. Only Pol I has the 5 to 3 exonuclease that degrades RNA primers or damaged DNA ahead of synthesis, a key function during lagging strand maturation.


Step-by-Step Solution:

1) Assign replication synthesis to Pol III holoenzyme with 3 to 5 proofreading.2) Assign repair synthesis roles to Pol II with 3 to 5 proofreading as well.3) Assign primer removal to Pol I with 5 to 3 exonuclease.4) Conclude that Pol II and Pol III lack 5 to 3 exonuclease activity.


Verification / Alternative check:

Genetic and biochemical studies of mutants confirm that loss of 5 to 3 exonuclease maps to Pol I, not Pol II or Pol III.


Why Other Options Are Wrong:

Pol II and Pol III do have 3 to 5 exonuclease for proofreading, contradicting option E. Endonuclease activities listed are not defining features of these polymerases.


Common Pitfalls:

Confusing exonuclease directionality or assuming all polymerases remove primers. Remember the division of labor at the replication fork.


Final Answer:

5' to 3' exonuclease activity

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