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, O1 ,O 2 and O3, which are interspersed between the divergent promoters PR and PL. The Cro protein binds initially to O3, which overlaps the N35 region of PL, excluding the RNA polymerase (r A -RNAP) from it. This results in the switching off of cI transcription and directs the incoming phage into the lytic cycle. At higher concentrations, Cro also binds to O1 and/or O2, which overlap PR, probably introducing a bend in the intervening DNA. This interaction induces DNA looping, which provokes the subsequent displacement of r A -RNAP from PR. Consequently, Cro abolishes the binding of r 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.