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

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 O-1, O-2, and O-3) 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 lambda 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-opp value for the Cro-P-L-P-R DNA complex was estimated to be 6 nM. Cro shows a slightly higher affinity for O-3 than for O-1 and O-2 subsites. The O-3 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, (C) 1999 Academic Press.