The TREX2 3′→ 5′ Exonuclease Physically Interacts with DNA Polymerase δ and Increases Its Accuracy

Abstract

Proofreading function by the 3′→ 5′ exonuclease of DNA polymerase δ (pol δ) is consistent with the observation that deficiency of the associated exonuclease can lead to a strong mutation phenotype, high error rates during DNA replication, and ultimately cancer. We have isolated pol δ_d from isotonic (pol δ_i) and detergent (pol δ_d) calf thymus extracts. Pol δ_d had a 20-fold higher ratio of exonuclease to DNA polymerase than pol δ_i. This was due to the physical association of the TREX2 exonuclease to pol δ_d, which was missing from pol δ_i. Pol δ_d was fivefold more accurate than pol δ_i under error-prone conditions (1 μM dGTP and 20 dATP, dCTP, and dTTP) in a M13mp2 DNA forward mutation assay, and fourfold more accurate in an M13mp2T90 reversion assay. Under error-free conditions (20 μM each of the four dNTPs), however, both polymerases showed equal fidelity. Our data suggested that autonomous 3′→ 5′ exonucleases, such as TREX2, through its association with pol I can guarantee high fidelity under difficult conditions in the cell (e.g., imbalance of dNTPs) and can add to the accuracy of the DNA replication machinery, thus preventing mutagenesis.