Host Conversion by Prophage Lambda in a Recombination-deficient Mutant of Escherichia coli

SUMMARY A recombination-deficient (rec −) strain, AB2463, of Escherichia coli K 12 yielded λ lysogens of two types. A rare form, AB2463 (λ)(ind), differed from the usual type, AB2463(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 rec + strain, AB1157. Superinfection of the inducible form with phage 21b2 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 rec − condition as a result of the curing treatment. The phage released from AB2463(λ)(ind) was normal λ. It is concluded that the rec + phenotype of AB2463 (λ)(ind) derives from the presence of the prophage and represents a case of host conversion. It is proposed that AB2463 (λ)(ind) is doubly lysogenic for λ and that partial curing yields the rec − single lysogen AB2463(λ).