A lysogenic, recombination-deficient bacterial mutant (rec−) can be induced to produce λ ind− C587 by a temperature shift, although it does not produce phage after u.v. irradiation when lysogenic for wild λ. When this mutant (rec−) was made colicinogenic for colicins E2 and I, colicin production was found not to be inducible.

Superinfection of irradiated lysogenic rec− (λ) bacteria, at doses of irradiation that did not reduce their capacity to produce phage λ, was not productive. It seems therefore that a certain product(s) that usually arises following u.v. irradiation in rec+ cells, and triggers the induction of lysogenic and colicinogenic cells by causing loss of immunity or repression, is not produced in the rec− mutant. However, immunity is lost and the prophage is induced after a temperature shift in rec− cells which are lysogenic for λ ind− C857.