Phage P22 can integrate as prophage into a recombination-deficient (Rec−) strain of Salmonella typhimurium. At 37 C, the integration efficiency is only 10% that in Rec+ infection, but at 25 C the efficiencies in Rec− and Rec+ hosts are similar. Rec− lysogens cannot be induced by ultraviolet irradiation or by treatments with the chemical inducing agents streptonigrin or mitomycin C. Heat induction of Rec− cells lysogenic for a temperature-sensitive c2 mutant (ts c2) is normal, showing that the Rec− cell has the machinery necessary for prophage excision. Ultraviolet irradiation of Rec− (ts c2) lysogens prior to heat induction does not prevent the formation of infective centers after temperature shift. Thus, the noninducibility of Rec− lysogens is not due to destruction of the prophage as a result of ultraviolet irradiation. Deoxyribonucleic acid-ribonucleic acid (RNA) hybridization experiments demonstrate that no increase in phage-specific RNA synthesis occurs after ultraviolet irradiation of a Rec− (c+) lysogen. The Rec− mutant appears to lack part of the mechanism required to destroy the phage repressor and allow the initiation of early phage functions such as messenger RNA synthesis. A similar conclusion was reached previously for an Escherichia coli Rec− strain.
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