Role of pyrimidine dimer excision in loss of potential streptomycin resistance mutations of ultraviolet-irradiated Escherichia coli on phosphate-buffered agar.

The frequency of ultraviolet (UV)-induced mutations to streptomycin resistance dropped rapidly when starved Escherichia coli strains WP-2 B/r and B/r T- were incubated on phosphate-buffered agar (PBA), but was reduced only slightly in a WP-2 hcr- mutant. During postirradiation, incubation viability remained approximately constant. Cells given an optimal recovery treatment with photo-reactivating light showed no further recovery if subsequently incubated on PBA. At least 70% of the mutations induced to streptomycin resistance by UV could be repaired. The loss of potential streptomycin-resistant mutants was markedly reduced in strain B/r T- when 5 mug of acriflavin or 700 mug of caffeine per ml was added to PBA. The excision of UV-induced thymine-containing dimers from E. coli tb/r T- was investigated. Dimer excision progressed more slowly when the cells were incubated on PBA containing acriflavin or caffeine. There was no congruity between the kinetics of dimer excision and the kinetics of mutant loss. Our results indicate that removal of potential streptomycin-resistant mutants is considerably faster than the excision of pyrimidine dimers.