When a λ-lysogenic recombination-deficient (rec) mutant of 12, 2463 (λ), was superinfected with ultraviolet-irradiated λ in the presence of 6-azauracil, there was found among surviving cells a doubly lysogenic form, 2463 (λ)(), with phenotype. The double lysogen obtained by superinfection of 2463 (λ) with λcI 857 displayed a phenotype at 30° and characteristics at 40°. Superinfection of 2463 (λ) with phage 434hy 2ch, which lacks the c region of λ, yielded double lysogens. Since heat inactivation of the repressor in AB 2463 (λ) (λcI 857 ) produced a form one of the prophages in 2463 (λ)() 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 2463 (λ)() 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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