Summary: Properties of a transducing system with a phage able to transduce a kanamycin resistance marker of the T compatibility group R factor R394 at high frequency are described. The phage was detected in strain 5006 transduced to kanamycin resistance by phage 34 grown on strain 13 carrying the R factor. The 5006 transductants, specially selected at the lowest multiplicities of infection (m.o.i.) of the high frequency transducing (h.f.t.) phage, are normal lysogens. They liberate h.f.t. phage spontaneously and high-titre phage may be obtained by u.v. induction. The phage is serologically identical to phage 34 and is defective in that, at low m.o.i., transduction frequency is increased by the simultaneous presence of homologous non-transducing phage. Ultraviolet irradiation of the h.f.t. lysate produces an exponential fall in transduction frequency. It is concluded that the defective phage transduces by lysogenization. Phage present in h.f.t. lysates can also transduce various chromosomal markers of 5006 at low frequencies. These low-frequency transductants are usually not kanamycin resistant. Transductants do not transfer the kanamycin resistance marker conjugally and produce kanamycin-sensitive normal lysogenic segregants at a high rate. Strain 5006 is cryptically lysogenic for a phage which is morphologically and serologically identical to phage 34 and many of the particular features of this transduction system are explicable in terms of complementation or recombination between genes of the phages involved.


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