The transcriptional switch region of Lactobacillus casei temperate bacteriophage A2 contains three similar 20 bp operator subsites, O₁, O₂ and O₃, which are interspersed between the divergent promoters P _R and P _L. The Cro protein binds initially to O₃, which overlaps the −35 region of P _L, excluding the RNA polymerase (σ^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 P _R, probably introducing a bend in the intervening DNA. This interaction induces DNA looping, which provokes the subsequent displacement of σ^A-RNAP from P _R. Consequently, Cro abolishes the binding of σ^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.