Repressor cleavage as a prophage induction mechanism: hypersensitivity of a mutant lambda cI protein to recA-mediated proteolysis

λcIind^s prophage is hypersensitive to derepression by ultraviolet-irradiation. We have utilized this mutant to test current models for prophage derepression. We find that cIind^s repressor is cleaved by RecA protein in vivo at lower ultraviolet doses and with more rapid kinetics than cI⁺ repressor, and that induction of the recA8 gene or other LexA-repressed genes is not required for cleavage. Our results support the concept that RecA-directed proteolysis is the primary mechanism for prophage derepression.