Specialized Transducing Phages Derived from Salmonella Phage

Salmonella phage P22 has been used in the construction of three sorts of specialized transducing phage: P22 proAB, P22 proABlac and P22 argF. The bacterial genes carried are derived from E. coli K12. Since E. coli and Salmonella chromosomes recombine very poorly, E. coli genes cannot be transduced into Salmonella recipients by P22‘s generalized transduction mechanism. Therefore, stable inheritance of E. coli material provides a means of detecting specialized transduction. Formation of these phages was possible because the P22 prophage recognizes an attachment site in the E. coli F prolac episome. Salmonella strains carrying the F’ prolac episome can be lysogenized by P22 SO as to leave the prophage inserted into the E. coli material of the F’ factor. Improper prophage excision can then lead to formation of P22 specialized phages carrying E. coli genetic material. ALMONELLA phage P22 has been widely used in genetic analysis as a generalized transducing phage. The process of generalized transduction (reviewed by OZEKI and IKEDA 1968) involves formation of transducing particles during lytic phage growth. These particles appear to arise when bacterial genes, selected more or less at random, are encapsulated in a phage head. Upon subsequent infection of a strain by such a particle the bacterial genes are injected and can recombine with the host chromosome. This sort of transduction requires homology between the transduced material and the recipient chromosome; the host recombination system ( m c ) is also required. The recombinants arising by this transduction mode are stable. Specialized transduction as performed by coliphages A and $30 is a basically different process (reviewed by CAMPBELL 1971). Bacterial genes are incorporated into a phage genome as a consequence of improper prophage excision. Therefore, lysogeny and prophage induction are generally prerequisites for transducing phage formation, Only bacterial genes located near the prophage attachment site are subject to specialized transduction. Since the transduced genes are part of the phage genome, they can be inserted into the recipient chromosome by the phage integration system; this does not require the host rec system. Like a prophage, the transducing phage genome can be lost from the chromosome. This makes the transductant clones unstable in that the inherited genes are subject