A2 Cro, the lysogenic cycle repressor, specifically binds to the genetic switch region of Lactobacillus casei bacteriophage A2

Abstract

Lysogenic induction of temperate bacteriophage A2 of Lactobacillus casei is controlled by the action of its cl and cro products at the phage operator region. Three 20-bp inverted repeated DNA segments (subsites O1, O2, and O3) and the two divergent (P(L) and P(R)) promoters were mapped within the 153-bp operator region. The A2-encoded Cro product is shown to be the functional homolog of λ Cro. The binding of Cro to the three operator subsites is noncooperative and yields two discrete protein-DNA complexes of retarded migration in mobility shift assays. The K(app) value for the Cro- P(L)-P(R) DNA complex was estimated to be 6 nM. Cro shows a slightly higher affinity for O3 than for O1 and O2 subsites. The O3 subsite overlaps the -35 hexamer of the P(L) promoter, which directs cl expression. A Cro mutant protein, devoid of the last 12 residues (Cro*), allowed the assignment of the DNA-binding domain to the NH2 end of Cro. The C end enhances its affinity for the DNA and probably stabilizes bending induced by Cro.