Role of the cro gene in bacteriophage λ development

Previous experiments have shown that the product of the cro gene of baeteriophage λ can exert an anti-repression activity, defined by the capacity of certain "cro-constitutive" defective lysogens to channel a superinfecting λ phage toward lytic development. We have used a combination of biological and biochemical assays to draw two main conclusions concerning this anti-repression activity: (1) after infection of a cro-constitutive cell, the superinfecting phage is unable to establish repression because it is unable to commence synthesis of cI protein (λ repressor) at a substantial rate; (2) the cause of this diminished synthesis of cI protein is the capacity of cro product to repress synthesis of the cII and cIII proteins, which normally activate the cI gene to establish repression in an infected cell. From our experiments and those of others, we suggest that cro product possesses a repression activity which is similar to that of the cI protein itself, but normally exerts a very different physiological role: the turnoff of synthesis of replication, recombination and regulation proteins as the virus enters the late stage of lytic development.