J. MO,?. Bid. (1972) 71, 433448 Generalized Transduction by Phage P22 in Salmonella typhimurium I. Molecular Origin of Transducing DNA JUDITH EBEL-Tsmst, DAVID BOTST~ AND MATJRIUE S. Fox Department of Biology Mamxhumtts In&we of Technolqy Cambridge,Ma-se. 02139, U.S.A. (Received 13 March 1972) P22, a temperate phage which grows on Salmoneua typhimurium, is capable of carrying out general&d transduction in this host. The general&d transducing particles are the same size ss P22 phage particles but instead of phage DNA they contain bacterial DNA which was synthesized before phage infection. Further, DNA molecules isolated from the transducing particles have the same molecular weight as P22 DNA, 27 x IO6 daltons. Examination of the formation of transducing particles under conditions in which the AS’. typhimurium recombination system (WC) end/or the P22 recombi- nation system (erf) are defective indicates that neither of these general recombi- nation systems is necessary for the formation of transducing particles. 1. Introduction P22, a temperate phage which grows on I%?~.% typhimurium, is capable of carrying out both generalized and specialized transduction in this host (Zinder &I Lederberg, 1952; Smith-Keary, 1966; Wing, 1968). These two processes, both involving the transfer of genetic information from one bacterium to another by means of a phage vector, differ in several respects. Whereas specialized transduction is restricted to the region of the bacterial chromosome adjacent to the prophage attachment sits, generalized transduction involves the transfer of any region of the bacterial ohromo- some. Specialized transducing particles are formed only after induction of a lysogen (Morse, Lederberg & Lederberg, 1956; Wing, 1968), while generalized transducing particles seem to be formed in all lytio infections (Zinder, 1955). Inaddition, specialized transducing particles contain segments of both phage and bacterial DNA oovalently joined to one another (Campbell, 1962; Smith, 1968), while the experiments of Ikeda & Tomizawa (1965) demonstrated that in the case of oohphage Pl, generalized trans- ducing particles contain bacterial DNA with little or no phage DNA. We shall describe observations showing that the generalized transducing particles of phage P22 contain primarily bacterial DNA synthesized before infection, and little or no phage DNA. These transducing particles have the same sedimentation properties t Pmsent address: Department of Miorobiology cmd Molecular Gene&s, Harvard Medical Sohool, Boston, Mass., U.S.A. 2s 433