Vegetative bacteriophage λ DNA: I. Infectiv in a spheroplast assay

Abstract

The conditions for infection of spheroplasts of several strains of Escherichia coli by λ DNA have been studied. An assay has been developed and used to compare the capacity of λ DNA, at various stages of infection, to infect either spheroplasts or helper-phage-infected whole bacteria. In contrast to the results obtained with the helper assay, λ DNA extracted from an immune lysogen after superinfection is able to infect spheroplasts. The component from the infected cell which is responsible for most of the infectivity in the spheroplast assay is the twisted, circular form of λ DNA. This component is non-infective in the helper assay. Similarly, after infection of a sensitive bacterium by λ, there is no loss of DNA infectivity, measured in the spheroplast assay, although the capacity of the extracted DNA to infect helper-infected bacteria decreases 85%. During infection, synthesis of λ DNA which is able to infect spheroplasts precedes the synthesis of λ DNA which is able to infect helper-infected bacteria by at least 20 minutes at 30 °C. The conclusion suggested by these and experiments presented in the following paper is that synthesis of the twisted, circular form of λ DNA occurs until the time of phage maturation, at which time linear phage DNA, containing cohesive ends, is synthesized and encapsulated. The "notched" circular form of λ DNA, prepared in vitro, is also able to infect spheroplasts, although it has little infectivity in the helper assay.

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