Transducing fragments in generalized transduction by phage P1: I. Molecular origin of the fragments

Abstract

Bromouracil-labelled Escherichia coli thy− was infected with a virulent mutant of phage P1kc and incubated in a medium containing thymine until lysis occurred. The analysis of the phage yield by CsCl density-gradient centrifugation showed that the transducing particles carrying various chromosomal genes have a density and band profile similar to those prepared on bromouracil-labelled bacteria in a medium containing bromouracil. When the phage particles were prepared on the bromouracil-labelled bacteria in a medium containing thymine and 32PO4, radioactivity was not found in fractions which contained transducing particles. These reults indicate that most of the transducing particles lack phage genome and carry only fragments of the bacterial chromosome existing at the time of infection. The results show interruption of the replication of bacterial chromosome by phage infection. The replication of λ prophage and F′ lac was also interrupted by infection. On the other hand, the replication of an R factor was not arrested. In contrast to the behaviour of the virulent mutant, the original P1kc phage did not arrest the replication of bacterial chromosome. When the virulent mutant was grown on [3H]thymidine-labelled bacteria in a medium containing 32PO4, infective particles and transducing particles could be differentially labelled with 32P and 3H, respectively. The results of the analysis of the phage particles showed that the transducing particles comprised 0·3% of the total phage particles. Physical properties of DNA's of infective and transducing particles were studied. The density of P1 DNA is 1·706 g cm−3, which corresponds to a guanine–cytosine content of 46%. The molecular weights of DNA's of infective and transducing particles are the same, namely, 6 × 107 daltons. Homology between PI DNA and E. coli DNA is rare.