Repression of the lysogenic P R promoter in bacteriophage TP901-1 through binding of a CI-MOR complex to a composite O M -O R operator

A functional genetic switch from the lactococcal bacteriophage TP901-1, deciding which of two divergently transcribing promoters becomes most active and allows this bi-stable decision to be inherited in future generations requires a DNA region of less than 1 kb. The fragment encodes two repressors, CI and MOR, transcribed from the P R and P L promoters respectively. CI can repress the transcription of the mor gene at three operator sites (O R , O L , and O D ), leading to the immune state. Repression of the cI gene, leading to the lytic (anti-immune) state, requires interaction between CI and MOR by an unknown mechanism, but involving a CI:MOR complex. A consensus for putative MOR binding sites (O M sites), and a common topology of three O M sites adjacent to the O R motif was here identified in diverse phage switches that encode CI and MOR homologs, in a search for DNA sequences similar to the TP901-1 switch. The O R site and all putative O M sites are important for establishment of the anti-immune repression of P R , and a putative DNA binding motif in MOR is needed for establishment of the anti-immune state. Direct evidence for binding between CI and MOR is here shown by pull-down experiments, chemical crosslinking, and size exclusion chromatography. The results are consistent with two possible models for establishment of the anti-immune repression of cI expression at the P R promoter.