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

Medical Research Council Microbial Genetics Research Unit, Hammersmith Hospital, Ducane Road, London, W.12 A recombination-deficient ( rec - ) strain, AB 2463, of Escherichia coli K 12 yielded lysogens of two types. A rare form, AB 2463 ( )( ind ), differed from the usual type, AB 2463 ( ), 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, AB 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 rec - condition as a result of the curing treatment. The phage released from AB 2463 ( )( ind ) was normal . It is concluded that the rec + phenotype of AB 2463 ( )( ind ) derives from the presence of the prophage and represents a case of host conversion. It is proposed that AB 2463 ( )( ind ) is doubly lysogenic for and that partial curing yields the rec - single lysogen AB 2463 ( ). * Present address: New Zealand Dairy Research Institute, P.O. Box 1204, Palmerston North New Zealand. Received 5 June 1968; accepted 27 February 1969.