A simplified model for lysogenic regulation through DNA looping

The lysogenic state of bacteriophage lambda has been a key model for understanding gene regulation. A single protein, CI repressor, maintains this state by blocking gene expression from two promoters separated by ∼2.3 kbp of DNA. CI controls its own expression by positive and negative feedback by binding to OR to regulate the PRM promoter. Not only does CI interact directly with operator DNA, but CI tetramers bound to OL and OR can form an octamer, looping the DNA that lies between them. Previous studies show that OL can assist with negative regulation of PRM, and we recently showed that DNA looping can also enhance looping activation. In this paper we present a new interpretation of our recent experimental data based a new crystal structure of CI. The simpler model suggested by the new structural information predicts that the most common forms of the DNA loop have similar activation behavior, and are enhanced ∼2.2-fold relative to unlooped activation.