Studies with the bacteriophages of Escherichia coli
Erskine, John Malcolm
In this Thesis a study is made of a number of episome-host relationships, with emphasis placed on use of the recombination-deficient bacterial cell as a new tool for examining the phenomena of transduction, lysogenization and related aspects of phage biology. Each of the relationships considered requires a recombination event, that may be mediated by the host or the phage, for full expression. The introduction deals initially with the details of recombination; current concepts relating to the mechanism of recombination in both bacteriophages and-bacteria are summarized and the earlier work leading to their formulation is reviewed. Important features of the establishment and maintenance of lysogeny are then considered and the mechanism of specialized transduction is examined. The first part of the experimental section describes the use of specialized transduction with defective lambda phage ( λdg) as a means of isolating a recombination-deficient (rec⁻) mutant, which differs from those isolated previously in respect of its normal radiation-resistance, and details experiments designed to evaluate the range of action of the rec mutation in this and other mutants isolated by different methods in other laboratories. It is shown that the rec⁻ mutants differ from their wild-type parents in their response to nitrosoguanidine treatment, the rec⁻ bacteria being more sensitive to inactivation and, for a given dose, less prone to mutate. In addition, UV reactivation of phage λ does not occur in rec⁻ bacteria, they plate phage Pl less efficiently and they exhibit a slower growth rate. The transducing function of λdg, both before and after UV irradiation of the phage, is much reduced in the rec⁻ mutants. Finally, a description is given of conjugation experiments which show that the rec⁻ mutation in three different mutants examined involves a gene (or genes) located in a region of the Kl2 chromosome delimited by the arc B and .thy markers. Part II of the experimental section deals specifically with the function of bacteriophages as cellular genetic elements, their influence on bacterial recombination and the implications of their regulation of bacterial gene action. It is shown that the presence of the rec⁻ mutation in one of the rec⁻ mutants can be used as an aid to elucidating various aspects of the prophage-host relationship and to demonstrate a hitherto undiscovered aspect of this relationship. Evidence is presented to support the hypothesis that a rare form of double lysogeny confers on the rec⁻ host the phenotypic characteristics of a rec⁺ bacterium.