Interaction of the Cro repressor with the lysis/lysogeny switch of the Lactobacillus casei temperate bacteriophage A2

The transcriptional switch region of Lactobacillus casei temperate bacteriophage A2 contains three similar 20 bp operator subsites, O-1, O-2 and O-3, which are interspersed between the divergent promoters P-R and P-L. The Cro protein binds initially to O-3, which overlaps the -35 region of P-L, excluding the RNA polymerase (sigma(A)-RNAP) from it. This results in the switching off of cl transcription and directs the incoming phage into the lytic cycle. At higher concentrations, Cro also binds to O-1, and/or O-2, which overlap P-R, probably introducing a bend in the intervening DNA. This interaction induces DNA looping, which provokes the subsequent displacement of sigma(A)-RNAP from P-R. Consequently, Cro, abolishes the binding of sigma(A)-RNAP to the genetic switch of A2 and, presumably, its own synthesis, contributing indirectly to the entry of phage development into its late stages.