A pleiotropic regulatory mutation in λ bacteriophage

Lambda bacteriophage mutants, λsar, were isolated. These mutants can form plaques on a non lysogenic lawn and are unable to grow on nonimmune (imm^-), cro constitutive hosts. Analysis of the restriction of λsar by a set of defective lysogens suggested that both the cro and cII gene products participate in the inhibition. The sar mutations were mapped in the ori region between the genes cII and O. Complementation experiments showed that under the restrictive conditions λsar is defective in the expression of both the N and the O genes. Transcription analyses support these findings, as λsar is unable to serve as a template for transcription after infecting cro constitutive hosts. In addition λsar does not replicate under the restrictive conditions, although its DNA can bind to the host membrane to some extent. The Sar phenotype can be relieved by removing sites of action of cro either by a V2 mutation or by substituting the λ immunity region by imm434 or imm21. Similarly introducing a cy mutation, which interferes with the action of the cII gene product, also eliminates the Sar effect. The sar mutation can suppress cy mutations as manifested in plaque morphology, lysogenization frequency, cI repressor synthesis and the expression of rex function. Suppression takes place only when the sar mutation is present in cis to cy and it requires the action of the cII and cIII gene products. It is suggested that the sar mutation suppresses cy by activating a new promoter for repressor synthesis, pro. The results also suggest that the cII and cIII gene products may act at a site other than y.