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A radiosensitive mutant E. coli B,-, was discovered by Hill. 1 2 It exhibits the same plating efficiency as the parental strain E. coli B for normal T1 bacteriophage, but a greatly reduced efficiency with ultraviolet (UV) irradiated T1 phage.A The sensitive mutant apparently lacks some factor capable of reactivating certain photoproducts in the UV-irradiated DNA of T1 phage. Sensitive mutants of E. coli K12 which show the same low efficiency for scoring UV-irradiated T1 phage were induced by nitrous acid treatment.4 This paper reports experiments on genetic crosses between a sensitive F-strain and several resistant Hfr strains of K12. The inheritance of resistance to UV-irradiation among the progeny from the crosses was analyzed following methods similar to those previously reported,5 and a locus on the male chromosome was identified that can transmit ITV resistance to the zygote. This locus will be referred to as UVR or UVs ac

Strains of Escherichia coli that carry the mutation uvrA6 show no measurable excision of pyrimidine dimers and are easily killed by ultraviolet (UV) light, whereas strains that carry recA13 are defective in genetic recombination and are also UVsensitive. An Hfr strain carrying uvrA6 was crossed with an F-strain carrying recA13. Among the recombinants identified, one carrying uvrA recA proved to be of exceptional sensitivity to UV light. It is estimated from the UV dose (0.2 erg/ mm2 at 253.7 nm) required to reduce the number of colony-forming cells by one natural logarithm that about 1.3 pyrimidine dimers were formed in a genome of 5 X 106 base pairs for each lethal event. This double mutant is 40 times more UVsensitive than the excision-defective strain carrying uvrA6. The replication of one pyrimidine dimer is generally a lethal event in strains carrying recA13. Spontaneous breakdown and UV-induced breakdown of the deoxyribonucleic acid (DNA) of cells of the various genotypes were estimated by growing the cells in medium containing 3H-thymidine and measuring both acid-precipitable and acid-soluble radioactivity. The UV-induced degradation in strains with recA13 did not require the uvr+ genes and hence appears to depend upon a mechanism other than dimer excision. The greater level of survival after irradiation in Rec+ as compared to Rec-bacteria may be due to a recovery mechanism involving the reconstruction of the bacterial chromosome through genetic exchanges which occur between the newly replicated sister duplexes and which effectively circumvent the damaged bases remaining in the DNA.

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