Mammalian nuclear DNA polymerases a and p are known to be devoid of the editing 3'-+5' exonucleolytic activity. Presumably this activity could be effected by the exonucleases non-associated covalently with DNA polymerases. Two 3'+5' exonucleases of 40 kDa and 50 kDa (exo-40 and exo-50) have been isolated from rat liver nuclei and purified to near homogeneity. They are shown to excise mismatched nucleotides from poly[d(A-T)] template, respectively, 10-fold and 2-fold faster than the matched ones. Upon addition of either of these exonucleases to the DNA polymerase a from rat liver or calf thymus, the fidelity of in-vitro reproduction of the primed DNA from bacteriophage 6x174 amber 3 is increased 5-lO-fold, levels of exonuclease and DNApolymerase activities being similar. Extrapolation of in-vitro DNA-replication fidelity to the cellular levels of activities of the exonucleases and the a-polymerase suggests that exonucleolytic proofreading augments the accuracy of DNA synthesis by 2-3 orders of magnitude.Fidelity of DNA synthesis depends on the sum of at least two processes : the accuracy of matched nucleotide selection effected by DNA polymerase, and the 3'45' exonucleolytic correction of possible misinsertions. Incorrect bases incorporated during replication and not corrected by a proof-reading function may be recognized by a post-replicative mismatchrepair system which is yet to be characterized biochemically and will not be considered here. Proof-reading enhances the accuracy of DNA synthesis by 2-3 orders in the case of prokaryotic DNA polymerases with 3'+5' exonuclease activity [l]. However, highly purified mammalian nuclear DNA polymerases a and p lack associated 3'+5' exonuclease activity while polymerase 6 and E display rather low levels of this activity, and their accuracy is only 1-2 orders higher than that of DNA polymerase a [1, 21. In addition, the biological ubiquity of DNA polymerases 6 and E is still doubtful. Exonucleases non-covalently associated with DNA polymerases were also suggested to proof-read in mammalian nuclei 131. Eligible 3'45' exonucleases were found in rabbit bone marrow [4], calf spleen 131, rat liver [5, 61, rat hepatoma [7], and human cells [S, 91. However, the real influence of these exonucleases on the fidelity of synthesis with DNA polymerase has not been investigated. Whether such exonucleases play any part in the overall fidelity of mammalian DNA replication is still a matter of debate.

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