DNA Proof-Reading by a Eukaryotic DNA Polymerase

Abstract

Replication of DNA, as reflected in the low mutation frequency of the cell must occur with a high degree of fidelity. In agreement is the observation that when directed by synthetic template-primers, many prokaryotic and eukaryotic DNA polymerases misincorporate nucleotides not complementary to the template at an extremely low frequency (1–3). This fidelity may, in fact, be greater than can be accounted for by the measure difference in free energy between complementary and noncomplementary base pairs (4). It is likely, therefore, that DNA polymerases and/or other proteins increase fidelity above that determined by base specificity alone. The 3’ → 5’ exonuclease activity associated with prokaryotic polymerases (5) is a prime candidate to accomplish this by excising a noncomplementary base misincorporated at the 3’-terminus of a growing DNA chain by the polymerase activity. Indeed, where specifically tested, this exonuclease will excise such a preformed primer nucleotide before primer extension can occur (6). In addition, there is a correlation between the polymerase to exonuclease specific activity ratios of DNA polymerases from phage T4 strains harbouring mutations in their structural polymerase genes and the mutation frequencies of these strains (7,8).

A.S.I. Participant