Control of cI gene expression in bacteriophage lambda imm434, studied in an immunity/trp fusion made in vitro

The trp genes of a lambda imm434 trp-transducing phage have been fused to the immunity region by deletion, in vitro, of the DNA between two targets for the restriction enzyme R.EcoRI. The resulting phage has been used to study the control of expression of the cI gene in vivo. The constitutive rate of expression of the cI gene is between 2 and 5% of the maximally stimulated rate. The products of the cII and cIII genes enhance expression of cI on infection of a sensitive host. The requirement for the cII product is more stringent than that for the cIII protein. The phage 434 repressor present in a 434-immune cell stimulates the rate of cI expression from a superinfecting homoimmune phage about fifteen-fold. This result strongly suggests that repressor stimulates its own synthesis by a direct effect on transcription of the cI gene.