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 cI 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.