Age dependent decline in the 3′→5′ exonuclease activity involved in proofreading during DNA synthesis

Abstract

A 3′→5′ exonuclease found in rat liver excises mispaired nucleotides at the 3′-hydroxyl end of primer chains such as poly dA-d(T 9-C). Consequently, the priming activity of the chain from which the mispaired base was cut is greatly increased during DNA synthesis. These results suggest that the 3′→5′ exonuclease acts as a proofreading enzyme during DNA synthesis. The activity of this 3′→5′ exonuclease in the liver of 24-month-old rats is ≈30% lower than the activity found in 4-month-old rats. Furthermore, non-complementary nucleotide incorporations by DNA polymerases from aged rats are observed during DNA synthesis on poly dA-dT 10. The number of misincorporations decreases in the presence of the 3′→5′ exonuclease, but not all errors are prevented even when DNA polymerase and 3′→5′ exonuclease are added at an activity ratio similar to that found in vivo. The data suggest that declines in both the fidelity of DNA polymerase and the 3′→5′ exonuclease activity related to proofreading during the aging process lead to a higher frequency of base misincorporations during DNA replication.