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#DNA polymerase III

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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

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

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Replication in Prokaryotes

Core subunits of DNA Pol III are

Pol III core minimal replication unit that co-purifies consists of three subunits tightly associated: alpha encoded by dnaE harbors polymerase active site with palm-thumb-fingers architecture binding Mg2+, epsilon encoded by dnaQ provides 3' to 5' exonuclease proofreading through DEDD motif, and theta encoded by holE stabilizes epsilon and enhances fidelity against thermal denaturation. This alpha-epsilon-theta heterotrimer associates with tau and beta to form holoenzyme capable of high processivity. Early models suggesting alpha-beta-gamma composition confused holoenzyme subunits with core de

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Pol III core subunits alpha, epsilon, theta

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