Bacteriophages Virus were originally thought to be lethal agents, killing their host cells afer infection. In the early 1920s, a previously unknown interaction was discovered, in which the host cell not only survived the infection but also stably inherited the genetic information of the virus. It was also observed that certain bacterial strains could lyse spontaneously and produce bacteriophages afer a period of growth in culture. Such strains were called lysogenic, and the phenomenon, lysogeny. Studies of lysogeny uncovered many previously unrecognized features of virus-host cell interactions
Recognition of this phenomenon came from the work of many scientists, but it began with the elegant experiments of André Lwoff and colleagues at the Institut Pasteur in Paris. Lwoff showed that a viral genome exists in lysogenic cells in the form of a silent genetic element called the prophage. This element determined the ability of lysogenic bacteria to produce infectious bacteriophages.
ubsequent studies of the E. coli phage lambda established a paradigm for one mechanism of lysogeny, the integration of a phage genome into a specific site on the bacterial chromosome. Bacteriophages became inextricably associated with the new field of molecular biology (Table 1.1). Teir study established many fundamental principles: for example, control of the decision to enter a lysogenic or a lytic pathway is encoded in the genome of the virus. Te first mechanisms discovered for the control of gene expression, exemplified by the elegant operon theory of Nobel laureates François Jacob and Jacques Monod, were deduced in part from studies of lysogeny by phage lambda. The biology of phage lambda provided a fertile ground for work on gene regulation, but study of virulent T phages (T1 to T7, where T stands for “type”) of E. coli paved the way for many other important advances (Table 1.1). As we shall see, these systems also provided an extensive preview of mechanisms of animal virus reproduction.