Regulation of the int gene of bacteriophage lambda: activation by the cII and cIII gene products and the role of the Pi and Pl promoters.

The activation of the int gene by the cII and cIII gene products was studied by analysing int expression following infection of UV-irradiated cells by various phage mutants. Residual expression of int, probably from Pl, takes place in the absence of cII/cIII activation. Activation of the int gene, like that of the cI repressor gene, is poor at low multiplicities of infection. The mutation intC, which allows constitutive int expression in the lysogenic state, partially relieves the requirement for cII and cIII activation. The kinetics of Int synthesis after addition of the inhibitor rifampicin suggest that the activation occurs at the transcriptional level.