Gene Regulatory Circuitry of Phage λ

Abstract

Abstract The gene regulatory circuitry of phage λ was the first complex regulatory circuit to be analyzed in molecular detail. λ was chosen as a model system for this purpose because it displays a wide range of interesting regulatory properties. It can adopt two alternative life-styles, the lytic and lysogenic states, and it can switch from the lysogenic state to the lytic state in the process of prophage induction, often called the ‘‘genetic switch’’ (13, 28). A choice is made between these two alternative pathways soon after infection; the lysogenic state, in particular, is highly stable, and the switch can be highly efficient. It was clear early on that these properties would be of general relevance to other organisms, including higher eukaryotes. λ proved a fortunate choice as an experimental system: its genetics is easy, and many of the regulatory proteins are also biochemically tractable, allowing a highly productive interplay between genetics and biochemistry. As a result of work by many investigators, the proteins and cis-acting sites involved in these regulatory events are well known and intensively studied, and most of their regulatory interactions are known in great mechanistic detail. Hence, λ remains the best-understood complex regulatory system. At the same time, new surprises continue to appear (12).