Dark-recovery processes in Escherichia coli irradiated with ultraviolet light. 3. Effect of rec mutations on recovery of excision-deficient mutants of Escherichia coli K-12.

Mutants of Escherichia coli K-12 unable to excise pyrimidine dimers from their deoxyribonucleic acid (DNA) because of a uvr mutation show a higher survival when plated on a minimal salts medium after exposure to ultraviolet radiation than when plated on a complex medium such as nutrient agar containing yeast extract. This response has been called minimal medium recovery (MMR). Recovery of uvr mutants can take place in liquid as well as on solid medium, but not in buffer or under conditions of amino acid starvation that do not permit cell growth and normal DNA replication. MMR can thus be distinguished from the recovery of recombination-deficient (rec(-)uvr(+)) derivatives of K-12 which can occur under conditions where growth is not possible. Because MMR is characteristic of excision-defective mutants, it evidently reflects a type of repair independent of excision. We have obtained genetic evidence that MMR is determined by the rec genes, which also control recombination in K-12. Cells carrying a uvr mutation together with recA13, recA56, recB21, or recC22 failed to show MMR and were more sensitive to ultraviolet radiation than either their rec(+)uvr(-) or rec(-)uvr(+) parents. The rec(+)uvr(-) derivatives obtained from recA uvr(-) strains by transduction or by reversion regained the capacity for MMR. Our results indicate that inactivation of any one of the three genes, recA, recB, or recC, prevents cells from showing MMR.