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#proofreading activity

3 public questions tagged with this topic.

Replication in Prokaryotes

Proofreading activity of Pol III is due to

High-fidelity replication by Pol III relies critically on epsilon subunit, potent 3' to 5' exonuclease belonging to DnaQ family. When alpha polymerase misincorporates base causing mismatch and frayed terminus, primer moves from polymerase site to epsilon active site where phosphodiester bond hydrolyzed releasing monophosphate. Epsilon requires two Mg2+ ions and prefers single-stranded mismatched ends over duplex. Mutator dnaQ mutants lacking epsilon exhibit 1000-fold increased spontaneous mutation rates illustrating proofreading contribution. Theta subunit wraps around epsilon protecting from heat denaturation enhancing activity.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Epsilon subunit proofreading in Pol III fidelity

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PCR

Taq polymerase lacks

Taq polymerase retains 5' to 3' polymerase activity necessary for nucleotide incorporation and 5' to 3' exonuclease activity useful for nick translation, but lacks 3' to 5' exonuclease function that mediates proofreading. This absence means polymerase cannot reverse and remove incorrectly paired bases at the growing 3' terminus, leading to fixed mutations in amplicons. The structural basis is absence of exonuclease domain. Knowledge of domain architecture explains error spectrum and rationale for blending Taq with proofreading enzymes for long PCR.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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PCR

Which DNA Pol has proofreading activity?

DNA polymerases vary in fidelity due to presence of 3' to 5' exonuclease domain. Tli polymerase, also called Vent polymerase from Thermococcus litoralis, possesses intrinsic proofreading activity, excising mispaired nucleotides and increasing accuracy approximately 5-10 fold over Taq. Taq from Thermus aquaticus and Tth from Thermus thermophilus lack proofreading, exhibiting higher error rates. For high-fidelity PCR, cloning, mutagenesis, and sequencing applications requiring accurate replication, proofreading enzymes like Tli, Pfu, and Q5 are preferred despite slower extension rates and higher cost.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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