To improve our understanding of the role of DNA replication fidelity i
n mutagenesis, we undertook a search for Escherichia coli antimutator
strains with increased fidelity of DNA replication. The region between
4 and 5 min of the E. coli chromosome was mutagenized using localized
mutagenesis mediated by bacteriophage P1. This region contains the dn
aE and dnaQ genes, which encode, respectively, the DNA polymerase (alp
ha subunit) and 3' exonucleolytic proofreading activity (epsilon subun
it) of DNA polymerase III holoenzyme, the enzyme primarily responsible
for replicating the bacterial chromosome. The mutated bacteria were s
creened for antimutator phenotype in a strain defective in DNA mismatc
h repair (mutL), using a papillation assay based on the reversion of t
he galK2 mutation. In a mutL strain, mutations result primarily from D
NA replication errors. Among 10,000 colonies, seven mutants were obtai
ned whose level of papillation was reduced 5-30-fold. These mutants al
so displayed decreased mutation frequencies for rifampicin or nalidixi
c acid resistance as well as for other markers. Mapping by PI transduc
tion and complementation showed each to reside in dnaE. These observat
ions support the idea that the mutants represent antimutators which re
plicate their DNA with increased fidelity. Mutation rates were reduced
in both mutL and mutT backgrounds, but mutagenesis by ultraviolet lig
ht was not significantly affected, suggesting that the antimutator eff
ect may be largely restricted to normal DNA replication.