UV mutagenesis in E. coli with excision repair initiated by uvrABC or denV gene products

Mutation frequency responses produced by ultraviolet light are compared in 4 closely related strains of E. coli B/r having the same tyr(Oc) allele and different excision-repair capabilities: uvr⁺ (excision repair initiated by wild-type UvrABC activity), uvrA (excision repair defective), uvrA/pdenV-7 (excision repair initiated by endonuclease V of bacteriophage T4, DenV activity), and uvr⁺/pdenV-7 (excision repair initiated by UvrABC and DenV activities). The production of Tyr⁺ prototrophic mutants is classified into back-mutations and de novo or converted glutamine tRNA suppressor mutations to indicate different mutation events. Cells transformed with the plasmid pdenV-7 require larger exposures than the parent strains to produce comparible mutation frequency responses, indicating that DenV activity can repair mutagenic photoproducts. When damage reduction by UvrABC or DenV is compared for each of the specific categories of mutation, the results are consistent with the idea that pyrimidine dimers infrequently or never target back-mutations of this allele, frequently target the de novo suppressor mutations, and extensively or exclusively target the converted suppressor mutations. This analysis is based on the distinction that UvrABC-initiated excision repair recognizes dimer and non-dimer (pyrimidine (4–6) pyrimidone) photoproducts but that DenV-initiated repair recognizes only pyrimidine dimers.