As you well know, DNA Polymerase III Holoenzyme (Pol III HE) is the major polymerase involved in DNA replication. During DNA replication, new strand is synthesized by adding nucleotides to the 3′ terminus of the primer that are complementary to the original DNA template strand. DNA synthesis by Pol III HE is very high fidelity with the error rate is low as 10^–6 to 10^–8. So, which factor contributes to this accuracy of Pol III HE?

Pol III HE is a big molecule that consists of 10 different subunits. Beside the 5′-3′ polymerase activity, the enzyme also has a 3′ to 5′ exonuclease or proofreading activity. By the 5′-3′ polymerase activity, DNA synthesis occurs in 5′-3′ direction. Therefore, during chain elongation, if an incorrect nucleotide is incorporated, Pol III HE will change the mode from polymerase activity to 3′-5′ exonuclease activity to excise the wrong incorporated nucleotide. The 3′ to 5′ exonuclease activity of Pol III HE is by epsilon subunit and stimulated by theta subunit. This characteristic of Pol III HE helps the enzyme can synthesize new strand with high fidelity.