Properties of four mutants of Escherichia coli defective in genetic recombination

• Various properties of four mutants of Escherichia coli CR34, defective in genetic recombination (Rec−) were studied. • The mutants are more sensitive to UV- and X-irradiation, in comparison with the wild-type strain, but are able to propagate irradiated bacteriophage λ like the wild type. • The mutants are able to conjugate with F′ strains and to maintain the F factor. In crosses with Hfr strains the appearance of stable recombinants is delayed until the entrance of the allelic rec+ marker of the donor in the Rec− recipient. Therefore the rec+ allele must be dominant over the rec− allele. This allows the determination of the approximate position of the rec− mutations on the chromosome. Three different sites appear to be involved; two mutations (rec35−, rec38−) are most probably located near thr, one (rec36−) between Sr and his and one (rec34−) probably between try and his. • Derivatives of the Rec− mutants, lysogenic for λ, show different UV prophage induction patterns. • Recombination among genetically-marked λ phages is not restricted in any of the Rec− strains.