CONTRIBUTION OF 3'-]5' EXONUCLEASES FROM RAT-LIVER NUCLEI TO THE FIDELITY OF DNA-SYNTHESIS CATALYZED BY MAMMALIAN DNA-POLYMERASES-ALPHA

Mammalian nuclear DNA polymerases a and B are known to lack proofreadi ng 3'-->5' exonuclease activity, which may be carried out by exonuclea ses not associated covalently with DNA polymerases. It was shown that two 3'-->5' exonucleases of 40 and 50 kDa (exe-40 and exo-50) isolated from rat liver nuclei and purified to near homogeneity excise the mis matched nucleotides from the poly [d(A-T)] template 10- and 2-fold fas ter, respectively, than the complementary ones. Each of these exonucle ases upon addition to DNA polymerase a from rat liver or calf thymus i ncreased five- to tenfold the fidelity of bacteriophage phi X174amber3 -primed DNA replication, if the exonuclease and polymerase activities were approximately the same. Extrapolation of the replication fidelity to the intracellular activities of exonucleases and polymerase a sugg ests that exonuclease proofreading increases DNA synthesis accuracy at least by three orders of magnitude.