A recombination-deficient ( ) strain, 2463, of 12 yielded λ lysogens of two types. A rare form, 2463 (λ)(), differed from the usual type, 2463 (λ), in showing ultraviolet (u.v.) irradiation induction of the prophage, forming normal numbers of conjugational recombinants and displaying a u.v. response curve similar to that of the parental strain, 1157. Superinfection of the inducible form with phage 21 b2 led either to loss of the prophage or to the infrequent production of a form which was still lysogenic but no longer inducible. In both cases the bacteria regained their condition as a result of the curing treatment. The phage released from 2463 (λ)() was normal λ. It is concluded that the phenotype of 2463 (λ)() derives from the presence of the prophage and represents a case of host conversion. It is proposed that 2463 (λ)() is doubly lysogenic for λ and that partial curing yields the single lysogen 2463 (λ).


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