Effect of a 3′→5′ exonuclease with a proofreading function on the fidelity of error-prone DNA polymerase α from regenerating liver of aged rats

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A nuclease that releases noncomplementary nucleotides from the 3′-end of DNA was isolated and highly purified from rat liver extract. The d(T 9-C) priming activities for DNA synthesis in vitro by DNA polymerases α and β were recovered by the addition of this enzyme, which itself does not contain a DNA polymerase activity. This nuclease hydrolysed nucleotides from the 3′-end, but did not remove [ 32P]-labeled nucleotides from the 5′-terminus of specifically labeled DNA. Also, the reaction products released from the 3′-end of DNA were all mononucleotides. These results indicate that the exonuclease is a 3′→5′ exonuclease with properties the same as those of DNase VII from human placenta. Rat DNase VII requires 4 mM MgCl 2 or 0.125 mM MnCl 2 for maximum activity, and shows a pH optimum of 7.5. These optimal conditions are similar to those of DNA polymerases, and indicate that both rat DNase VII and DNA polymerases are able to act under same conditions. Non-complementary nucleotide incorporation by DNA polymerase α from aged rat has been observed during in vitro DNA synthesis on poly dA–dT 10. The amount of this mis-incorporation is decreased by the coexistence of the 3′→5′ exonuclease, but not all errors are edited out. Thus, this rat DNase VII is suggested to play an important role in proofreading during DNA synthesis.