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 (σA-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 σA-RNAP from P
R. Consequently, Cro abolishes the binding of σA-RNAP to the genetic switch of A2 and, presumably, its own synthesis, contributing indirectly to the entry of phage development into its late stages.
New Insights into the Phage Genetic Switch: Effects of Bacteriophage Lambda Operator Mutations on DNA Looping and Regulation of P R , P L , and P RM
