Generalized Transduction

130 INTRODUCTION Genetic transduction is the transfer, from one cell to another, of nonviral genetic material within a viral coat. Transduction was discovered by Zinder and Lederberg in 1952 (217, 218) during a search for genetic recombination in salmonellae. Expecting to find conjugative transfer, they grew two mutant strains together. Recombinants were indeed produced, but to the authors' surprise, cell-to-cell contact was not required. Rather, recombination proved to be mediated by a DNase-resistant filterable agent which was later demonstrated to be identical to the bacteriophage P22. In 1955, Lennox (103) reported that, similarly, the temperate coliphage P1 could also act as a vector to carry out what has come to be termed generalized transduction. Transducing bacteriophage particles are formed in donor bacterial cells during phage development. They are of one of two types, distinguished by the nature of the DNA molecule which they carry. Generalized transducing particles, competent to mediate generalized transduction, carry a fragment of host DNA; specialized transducing particles, able to carry out specialized transduction, contain both host and viral DNA sequences as part of a single molecule. Such hybrid molecules are formed, in vivo, by aberrant excision of a prophage (or other recombinational event) to yield DNA molecules which can be both replicated and packaged. Specialized transducing particles arise at low frequencies, although, once identified, they can be propagated, with the aid of a helper phage if required, to yield high-frequency transducing lysates. Specialized transductants are likely to be lysogens which are diploid for the transduced host markers. The transduced DNA will have been added to the recipient genome and the transductants are able to produce transducing particles in their turn. The range of host markers which can be transduced in this fashion is limited to those near prophage insertion sites or other sites of recombination between phage and host sequences unless the transducing particles have been engineered in vitro to contain other DNA. Specialized transduction is treated in detail in chapter 131 of this volume. The products of generalized transduction are quite different from those of specialized transduction. Generalized transducing lysates, although composed primarily of infectious phages, contain, in addition, a small proportion of transducing particles which, on subsequent infection of a recipient culture, deliver a fragment of host DNA to a minority of cells. Generalized transducing particles completely lack DNA originating from the viral vector, containing instead only sequences of host origin. They arise when …