DNA looping can enhance lysogenic CI transcription in phage lambda

The lysogenic state of bacteriophage lambda is maintained by CI repressor, which negatively regulates two promoters to block lytic gene expression. Expression of CI is itself controlled by positive and negative feedback as CI binds to O_R to regulate the P_RM promoter. In addition to direct interactions with operator DNA, CI tetramers bound at O_L and O_R can come together to form an octamer, looping the DNA that lies between them and allowing O_L to assist with negative regulation of P_RM. We used a fluorescent reporter protein to measure the CI concentration for a set of constructs that differ in their ability to assume various forms of the looped structure. Based on the observed steady-state fluorescence for these constructs, the presence of O_L increases P_RM activation unless both operators can be fully occupied. By calculating the probabilities for the underlying operator configurations present in each construct, two different models for the mechanism of enhanced activation allow us to predict that when the DNA is looped, P_RM activation can be 2- to 4-fold higher than is possible for unlooped DNA. Based on our results, transcriptional regulation for lambda's lysogenic/lytic switch includes both activation and repression due to DNA looping.