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

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.