SUMMARY: The expression of resistance to u.v. irradiation mediated by R factor R46 has been studied in strains deficient in excision repair and recombination repair. The R factor protected wild-type bacteria and also wild-type cells in which repair had been inhibited by the substitution of bromouracil for chromosomal thymine. It increased the survival of strains defective in the endonucleolytic (), repolymerizing () and joining () stages of the excision repair process. Recombination deficient bacteria mutant at the or loci were protected by R46, but the R factor had little effect on the survival of a strain or a double mutant. R46 increased the survival of cells that had been treated with chloramphenicol before u.v. irradiation, but did not protect cultures treated with chloramphenicol after irradiation. It is concluded that R46 confers resistance to the lethal effects of u.v. irradiation by a mechanism that is independent of excision repair. Resistance appears to be mediated by an inducible gene product, which is possibly a nuclease and dependent on a functional host gene for expression.


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