Proof-reading 3'-->5' exonucleases isolated from rat liver nuclei

Eur J Biochem. 1993 Oct 15;217(2):493-500. doi: 10.1111/j.1432-1033.1993.tb18269.x.


Mammalian nuclear DNA polymerases alpha and beta 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-5) 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 alpha from rat liver or calf thymus, the fidelity of in-vitro reproduction of the primed DNA from bacteriophage phi X174 amber 3 is increased 5-10-fold, levels of exonuclease and DNA-polymerase activities being similar. Extrapolation of in-vitro DNA-replication fidelity to the cellular levels of activities of the exonucleases and the alpha-polymerase suggests that exonucleolytic proof-reading augments the accuracy of DNA synthesis by 2-3 orders of magnitude.

MeSH terms

  • Animals
  • Base Sequence
  • Binding Sites
  • Cell Nucleus / enzymology
  • DNA / biosynthesis*
  • DNA Polymerase II / metabolism*
  • Exonucleases / isolation & purification
  • Exonucleases / metabolism*
  • Liver / enzymology*
  • Male
  • Molecular Sequence Data
  • Molecular Weight
  • Poly dA-dT / metabolism
  • Rats


  • Poly dA-dT
  • DNA
  • DNA Polymerase II
  • Exonucleases