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When a λ-lysogenic recombination-deficient (rec−) mutant of Escherichia coli K 12, AB2463(λ), was superinfected with ultraviolet-irradiated λ in the presence of 6-azauracil, there was found among surviving cells a doubly lysogenic form, AB2463 (λ)(ind), with rec + phenotype. The double lysogen obtained by superinfection of AB2463(λ) with λcI857 ind − displayed a rec − phenotype at 30° and rec + characteristics at 40°. Superinfection of AB2463(λ) with phage 434hy2c+h, which lacks the c region of λ, yielded rec − double lysogens. Since heat inactivation of the repressor in AB2463(λ) (λcI857 ind −) produced a rec + form one of the prophages in AB 2463(λ)(ind) may have been incompletely repressed, hence the expression of its recombination function. For its host?converting effect the superinfecting phage appeared to require the c region of its genome. The results of recombination analysis suggested that in AB2463 (λ)(ind) the first prophage was integrated normally while the second prophage occupied a tandem position more distant than usual, and/or existed in a special relationship conferring on it immunity from or the capacity to overcome the repressor function of the cI product in respect of gene recombination function.
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