Specialized transduction of tetracycline resistance by phage P22 in Salmonella typhimurium. II. Properties of a high-frequency-transducing lysate.

A high-frequency-transducing (HFT) lysate for tetracycline resistance (Lets) was obtained by inducing an unusual tets P22 lysogen which had been made by transducing Salmonella typhimurium LT.2 to letR with P22 phage grown on an LT-2 strain carrying the R factor 222. The HFT lysate contains defective P22 particles, called P22 Tc-10, which cannot grow or lysogenize (i.e., transduce tetn) upon single infection. However, two defective P22 Tc-10 particles can cooperate with each other for growth or for lysogeny (i.e., transduction of tetR). “Wild-type” phages which can grow on single infection and which no longer transduce tetR can be found at a frequency of 5 X 10-j PFUjparticle in the HFT lysate. Five percent of these “wild-type” phages are also integration deficient. At high multiplicities of infection the teta transductants made with an HFT lysate appear to be similar to a P22 Tc-10 lysogen (i.e., they are immune to P22 and yield HFT Iysates upon induction) ; at low multiplicities of infection, many of the tetR transductants are tetR P22 prophage deletions. To account for these observat’ions, a model is proposed in which the genes for tet” are inserted into the intact P22 genome in the P22 Tc-10 prophage. The consequences of such an insert,ion are discussed with regard to the structure of phage P22 DNA and the t,opological requirements for surcessful DNA replication, phage growth, and lysogenization.