R factors improving survival of Escherichia coli K-12 after ultraviolet irradiation.

Two R factors which have the capacity to improve survival of some strains of Escherichia coli K-12 by approximately 60% after ultraviolet light have been identified and characterized. Both are fi(-), but neither produce colicins. The ability to enhance survival can be separated from all other identified R-factor functions. Improved survival does not result from improved excisional capacity, but does require an intact host capacity for genetic recombination. No effects on host cell growth or postirradiation lag were observed. A proposed mechanism of action is described.