Error-free uvr+-dependent inducible DNA repair in Escherichia coli B/r Hcr+ cells

The frequency of suppressor (tryptophan reversions) and of true (streptomycin-resistant and dependent) mutations has been followed in E. coli cells irradiated with a single dose or two separate doses of ultraviolet (U.V.) radiation. Under these conditions dimers were efficiently excised after a single dose, while about 40 per cent of the dimers remained unexcised after two doses. Although the level of unexcised dimers in the latter case increased proportionally with the second U.V. dose, the mutation frequency increased by 1.5-2-fold, but did not continue to increase with the level of unexcised dimers. A comparison of excision-proficient and excision-deficient cells containing similar amounts of persisting dimers has shown that proficient cells can tolerate a high level of dimers without an adequate increase in mutation frequency. Our results suggest the existence of an error-free uvr+-dependent inducible repair in E. coli B/r Hcr+ cells.