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.