Age dependent decline in the 3 '-> 5 ' exonuclease activity involved in proofreading during DNA synthesis

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). Consequentl y, 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 approximate to 30% lower than the activity found in 4-month-old rats. Furt hermore, non-complementary nucleotide incorporations by DNA polymerases fro m aged rats are observed during DNA synthesis on poly dA-dT(10). The number of misincorporations decreases in the presence of the 3' --> 5' exonucleas e, 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. T he data suggest that declines in both the fidelity of DNA polymerase and th e 3' --> 5' exonuclease activity related to proofreading during the aging p rocess lead to a higher frequency of base misincorporations during DNA repl ication. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.