Studies on the Mechanism of Transduction by Bacteriophage Phi Gamma. II. Formation of Transducing Elements

The formation of the transducing elements (TE) of bacteriophage phi gamma, analyzed in lysogens of the thermo-inducible derivative phi gamma hyI, has been found to parallel the formation of plaque-forming particles with a frequency of 2 X 10(-2) TE/PFU, but is mor sensitive to temperature and ti UV. Deletion of one of the prophage termini (attR) prevents normal excision and formation of plaque-forming particles, but does not affect the formation of transducing elements, which arise at a rate of nearly 10(-1) TE per induced bacterium. Transducing elements, would be formed by in situ encapsulation of a hybrid segment from a specidic point in the induced prophage, possibly the presumed packaging initiation site of the normal phage genome, before excision of the latter has occurred. Analysis of the mechanism of transduction to partly heterologous lysogens has revealed the participation of a co-infecting genome arranged in a linear fashion and has given evidence for a permutation in the sequence of transducing and nontransducing genomes. The data re consistent with a mechanism of encapsidation distinct from the Ter system even for hybrids inheriting part of the phi 80 genome, but endowed with the property to form transducing elements like those of phi gamma. Upon infection, transducing elements are formed after one cycle of lytic development with the same characteristics as those resulting from induction, but with a frequency 50 to 100 times lower. This process is dependent on the efficiency of Int promoted recombination. Superinfection experiments performed under conditions preventing Int promoted recombination reveal that any superinfecting phi gamma can promote the formation of transducing particles, depending on the presence within the host prophage of a site from which transducing genome packaging initiates.