Ultraviolet mutagenesis in repair-deficient derivatives of Escherichia coli B/r: uvrA uvrB and uvrA recC strains

Abstract

UV mutability of uvrA derivatives of E. coli B/r carrying recB or recC mutations was compared with that of otherwise isogenic Rec + strains. The frequency of UV-induced mutations per survivor from Try − to Try + and from Str-s to Str-r, as well as the absolute yields of these mutations, was significantly lower in the recB and recC strains thatn in Rec + at the same UV doses. The results support the hypothesis that UV-induced mutations in these strains originate as errors associated with the occurrence of recA +-dependent post-replication repair of gaps in DNA opposite pyrimidine dimers, and that maximal UV mutability requires participation of the recB- recC nuclease in this repair process. The reduced mutability of recB and recC strains may reflect a change in the accuracy of post-replication repair, such that fewer errors are made per gap repaired than in Rec + strains. Alternatively, the accuracy of post-replication repair may be unchanged, and the reduced UV mutability may result from the reduced tolerance of recB and recC strains for exercised pyrimidine dimers, with consequent selective survival of a fraction of the irradiated poplation containing fewer dimers per cell, compared to the surviving fraction in Rec + at the same UV dose. Data obtained at very low doses of UV indicate that reduced dimer tolerance alone cannot account for the observed reduction in UV mutability of recB and recC strains, unless it is assumed that dimer tolerance is not equally reduced in different regions of the genome.