SUPPRESSORS OF ESCHERICHIA-COLI MUTT - ANTIMUTATORS FOR DNA-REPLICATION ERRORS

Previous studies in our laboratory used a papillation assay to identif y a set of mutations in the E. coli dnaE gene that confer increased ac curacy of DNA replication (antimutators). These antimutators were isol ated as suppressors of the high mutability of a mismatch-repair-defect ive mutL strain, in which the majority of mutations represent uncorrec ted replication errors (mainly A . T --> G . C and G . C --> A . T tra nsitions). In the present study, we have sought suppressors of the hig h mutability of a mutT mutator strain. mutT strains produce a high fre quency of A . T --> C . G transversions due to their lack of the mutT- encoded 8-oxo-dGTPase, leading to a high frequency of A (8-oxoG) mispa iring errors. Following localized mutagenesis of the dnaE-dnaQ region of the chromosome, two strong suppressors of mutT mutability were obta ined, both residing in the dnaE gene (dnaE940 and dnaE941). When subse quently tested in a mutL strain, these two alleles also proved antimut ators in this background, dnaE941 being significantly stronger than th e previously isolated antimutators. The results suggest that the DNA p olymerase may use similar mechanisms to discriminate against A .(8-oxo G) transversion mispairs and A . C or T . G transition mispairs. The f indings may also have significance for the interpretation of the antim utator effect conferred by these dnaE alleles in a wild-type (mut(+)) background.