Activation of the lambda int gene by the cII and cIII gene products

The sequential synthesis of λ directed proteins, following infection of uv-irradiated Escherichia coli, has been studied using different λ mutants. The synthesis of phage-specified proteins involved in integration (Int) and repression (the cI repressor) was found to be activated by two other phage-specified proteins, the products of genes cII and cIII. The latter proteins had been thought to be involved only in regulation of cI synthesis, an effect abolished by a cis acting mutation, cy. Although the cII and cIII products regulate int gene expression, the cy mutation has no effect on this process. Moreover, intC, a mutation which maps immediately adjacent to the int gene and results in constitutive Int expression, has no effect on cI synthesis. It appears, therefore, that cII and cIII act at two different promoters to achieve the coordinated expression of the int and cI genes, and thereby ensure the successful establishment of lysogeny.