Overproduction of the epsilon subunit of DNA polymerase III counteracts the SOS mutagenic response of Escherichia coli.

It has been found that the mutator phenotype of the recA441 and recA730 strains that express the SOS response constitutively is suppressed by pIP1, a high-copy plasmid carrying the dnaQ gene encoding the 3'----5' exonuclease subunit (epsilon) of DNA polymerase III. We have constructed plasmid pIP11, in which the dnaQ gene is fused to the strong tac (trp-lac) promoter. Enhanced synthesis of the epsilon subunit stimulated by isopropyl beta-D-thiogalactopyranoside, the inducer of tac, prevents expression of the mutator phenotype of recA441 and markedly decreases the frequency of UV-induced mutations. These results strongly suggest that a loss of editing capacity by the epsilon subunit of DNA polymerase III holoenzyme plays a crucial role in generation of mutations during the SOS response.