The λ Switch: cI Closes the Gap in Autoregulation

Bacteriophage λ provided one of the early paradigms for studying gene control and understanding how molecular switches work [1]. After infection by bacteriophage λ, two possible fates, reflecting alternative patterns of gene expression, await the bacterial host cell. Either the bacteriophage enters the lytic pathway, in which case the host cell ultimately lyses to release a burst of progeny phage, or the bacteriophage enters the lysogenic pathway, in which case the phage genome is stably integrated into the bacterial chromosome and normal bacterial growth continues. Although lysogeny is extremely stable — recent data indicate that significantly fewer than 1 in 107 lysogenic cells spontaneously lyse per generation ([2] and J.W. Little, personal communication) — a population of lysogenic cells can be induced to switch to lytic growth with essentially 100% efficiency. This process is known as prophage induction, and is triggered by exposure of the lysogenic bacterial cells to UV light or other agents that cause DNA damage [3]. At the heart of this regulation is the cI protein (λcI), which is both a repressor and an activator of transcription and is required for the maintenance of lysogeny [1]. In a lysogen, λcI binds to several operator sites located within two control regions, OR and OL, which are separated by 2.4 kilobases (kb), repressing transcription of the phage’s lytic genes while simultaneously activating transcription of its own gene [1]. Treatments that trigger prophage induction lead to the proteolytic cleavage of λcI and derepression of the early lytic genes [4]. In addition to functioning as a positive autoregulator, λcI can also repress transcription of its own gene [5]. This negative autoregulation is mediated through a low-affinity operator site (OR3), but the physiological relevance of negative autoregulation has been unclear because early experiments suggested that the fractional occupancy of OR3 in a lysogen was too low (<20%) to lead to significant repression of cI synthesis [5]. This inference was based on results obtained with promoter–lacZ reporter constructs. The rationale behind this reporter-based approach was to dissect the switch into its component pieces so that the function of each element could be analyzed separately. This approach was enormously productive and led to a comprehensive account of the molecular interactions underlying the function of the switch [5–7]. Nevertheless, one level of regulation was missed because it depends on long-range interactions between λcI molecules bound at OR and OL, and the reporter constructs contained only one control region. It is this level that Dodd et al. [8] have now uncovered: their new findings reveal that the occupancy of OR3, and hence negative autoregulation, depends on interactions between λcI molecules bound 2.4 kb away from one another at OR and OL. In fact, the ability of DNA-bound λcI molecules to interact over very large distances had previously been demonstrated [9]. To discuss the new results of Dodd et al. [8] in greater detail, it is necessary to review briefly what was already known about the switch and how it works. Figure 1A depicts the arrangement of cI molecules at OR and OL in a λ lysogen. OR and OL each contain three λ operator sites: OR1 through OR3 and OL1 through OL3. In the case of OR, these three sites are flanked by two promoters, PR and PRM, which Dispatch Current Biology, Vol. 12, R87–R89, February 5, 2002, ©2002 Elsevier Science Ltd. All rights reserved. PII S0960-9822(02)00667-X