Q.30 Which one of the following statements about the DNA polymerase III of E. coli is
NOT correct?
(A) It catalyzes nick translation.
(B) Its absence is lethal to E. coli.
(C) It synthesizes a complementary DNA strand using a single-stranded template.
(D) It possesses 3′ → 5′ exonuclease activity.
DNA Polymerase III of E. coli does not catalyze nick translation. This is the incorrect statement among the options, as confirmed by detailed analysis of its enzymatic activities and roles in replication.
Option Analysis
DNA Polymerase III (Pol III) holoenzyme serves as the primary replicative polymerase in E. coli, featuring a core with α (polymerase), ε (3′→5′ exonuclease), and θ subunits, plus accessory subunits for processivity.
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(A) It catalyzes nick translation: Incorrect. Nick translation requires simultaneous 5′→3′ exonuclease and polymerase activities to displace and replace nucleotides at a nick, a function of DNA Polymerase I (Pol I), not Pol III. Pol III lacks 5′→3′ exonuclease activity and primarily elongates primers on primed templates during replication.
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(B) Its absence is lethal to E. coli: Correct. Mutations in dnaE (encoding α subunit) are conditionally lethal at non-permissive temperatures, halting chromosomal replication as Pol III synthesizes most of the genome.
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(C) It synthesizes a complementary DNA strand using a single-stranded template: Correct. Pol III uses single-stranded DNA templates coated with SSB protein, synthesizing new strands processively with a sliding clamp (β subunit).
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(D) It possesses 3′ → 5′ exonuclease activity: Correct. The ε subunit provides proofreading via 3′→5′ exonuclease activity, enhancing replication fidelity.
DNA polymerase III E. coli nick translation is a common CSIR NET confusion—Pol III does not perform this function. As the main replicative enzyme, Pol III excels in high-fidelity chromosomal synthesis but relies on Pol I for nick processing.
Core Functions
Pol III holoenzyme drives E. coli DNA replication with exceptional processivity (>500 kb per binding event), thanks to the β-clamp and loader. The α subunit polymerizes DNA 5′→3′, while ε handles proofreading.
Why No Nick Translation?
Nick translation involves coordinated 5′→3′ exonuclease removal of ahead nucleotides while polymerizing—a Pol I hallmark using its Klenow-like domains. Pol III, optimized for elongation on primed single-stranded templates with SSB, cannot efficiently perform this.
Exam-Relevant Comparisons
| Feature | DNA Pol III | DNA Pol I |
|---|---|---|
| Primary Role | Genome replication | Okazaki maturation, nick translation |
| Exonuclease Activities | 3′→5′ (proofreading) | 3′→5′ + 5′→3′ |
| Lethality | dnaE mutants lethal | polA viable (backup roles) |
| Template Use | ssDNA with primer/SSB | Nicked dsDNA |
This distinction is critical for CSIR NET questions on prokaryotic replication fidelity and enzyme specificity.
