Multilevel autoregulation of λ repressor protein CI by DNA looping in vitro.

The prophage state of bacteriophage λ is extremely stable and is maintained by a highly regulated level of λ repressor protein, CI, which represses lytic functions. CI regulates its own synthesis in a lysogen by activating and repressing its promoter, P(RM). CI participates in long-range interactions involving two regions of widely separated operator sites by generating a loop in the intervening DNA. We investigated the roles of each individual site under conditions that permitted DNA loop formation by using in vitro transcription assays for the first time on supercoiled DNA that mimics in vivo situation. We confirmed that DNA loops generated by oligomerization of CI bound to its operators influence the autoactivation and autorepression of P(RM) regulation. We additionally report that different configurations of DNA loops are central to this regulation--one configuration further enhances autoactivation and another is essential for autorepression of P(RM).