Bacteriophage P22-mediated specialized transduction in Salmonella typhimurium: high frequency of aberrant prophage excision.

The temperate bacteriophage P22 mediates both generalized and specialized transduction in Salmonella typhimurium. Specialized transduction by phage P22 is different from, and less restricted than, the well characterized specialized transduction by phage lambda, due to differences in the phage DNA packaging mechanisms. Based on the properties of the DNA packaging mechanism of phage P22 a model for the generation of various types of specialized transducing particles is presented that suggests generation of substantial numbers of specialized transducing genomes which are heterogeneous but only some of which have terminally redundant ends. The primary attachment site, ataA, for phage P22 in S. typhimurium is located between the genes proA,B and supQ newD. (The newD gene is a substitute gene for the leuD gene, restoring leucine prototrophy of leuD mutant strains.) The proA,B and supQ newD genes are very closely linked and thus cotransducible by generalized transducing particles. Specialized transducing particles can carry either proA,B or supQ newD but not both simultaneously, and thus cannot give rise to cotransduction of the proA,B and supQ newD genes. This difference is used to calculate the frequency of generalized and specialized transducing particles from the observed cotransduction frequency in phage lysates. By this method, very high frequencies of supQ newD (10(-2)/PFU)- and proA,B (10(-3)/PFU)-specialized transducing particles were detected in lysates produced by induction of lysogenic strains. These transducing particles most of which would have been produced by independent aberrant excision events (which include in situ packaging), were of various types.