High frequency transducing (HFT) phages 5006MHFT: and 5006MHFT for kanamycin or ampicillin plus kanamycin resistance, derived from strains PM5006(R394) and PM5006(R394) respectively, transduced (at low multiplicities of infection, m.o.i.) antibiotic resistance and prototrophy to PM5006 at high frequency. Simultaneous transduction of these markers occurred at very much lower frequencies. The latter result was correlated with the proportion of multiply-infected bacteria which, due to the great transducing potential of the phage, could register as transductants. Each HFT lysate was thus heterogenous with regard to high frequency transducing phage. Apart from the additional antibiotic resistance marker carried by one phage, no other difference between the two lysates was detected. High segregation frequencies of antibiotic-resistant or proto-trophic transductants indicated transduction by lysogenization. Although antibiotic-sensitive segregants of antibiotic-resistant prototrophic transductants occurred at high frequency, no auxotrophic segregants of these transductants were found. This suggests transduction by a double cross-over event in the leucine region. Most transductants, even at low m.o.i., were lysogenically converted to homologous phage non-adsorption as a result of interaction between the transducing phage genome and the resident cryptic prophage. They could, however, be retransduced by appropriate phage lysates; thus, lysogenic conversion to non-adsorption was not absolute. Some prototrophic transductants were non-lysogenic although their segregants liberated low-titre phage. The latter anomaly, and the fact that the leucine marker and antibiotic resistance were not cotransduced, are explained by the mode of integration of the phage into the host chromosome in relation to the resident cryptic prophage and the leucine region.


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