Specialized transduction by bacteriophage P22 in Salmonella typhimurium: genetic and physical structure of the transducing genomes and the prophage attachment site

Abstract

P22pro 1 and P22pro 3 are specialized transducing derivatives of phage P22 that carry the proA and proB genes of Salmonella typhimurium. These genes lie immediately adjacent to the prophage attachment site on the bacterial chromosome. By examining DNA heteroduplexes in the electron microscope, we found that DNA molecules from P22pro 1 and P22pro 3 each contain a substitution which adds length to the composite genome making the intracellular replicated genome too long to fit into a single phage particle. In this respect, and in many of their biological properties, the proline transducing phages resemble P22Tc 10, another specialized transducing phage with an oversize, intracellular replicated genome which carries a tetracycline resistance determinant from an R factor. Unlike P22Tc 10, however, P22pro 1 and P22pro 3 fail to integrate normally during lysogenizing infections, even when provided with the proline substitutions have created a defect in the phage attachment site and suggest that the Campbell model for the formation of specialized transducing phages is applicable to phage P22 with the additional feature that oversize genomes can be produced and propagated. A physical and genetic map of the P22 genome near the prophage attachment site was constructed which shows that the insertion from the R factor in P22Tc 10 is not at the attachment site: it is therefore unlikely that P22Tc 10 was formed in an abnormal prophage excision event as envisioned in the Campbell model, but was instead the result of a direct translocation from the R plasmid to P22.