Control of lambda repressor synthesis.

Direct measurements of the intracellular level of lambda repressor have been made by a DNA-filter assay and a radioimmune assay. Transcription of cI, the structural gene for repressor, appears to initiate at two different promoters, prm and pre. Promoter pre is activated during the establishment of lysogeny by the action of cII and cIII proteins at the DNA site cY. Phage mutated in cII, cIII, or cY do not make a normal burst of repressor after infection and do not efficiently lysogenize the cell. Cro product stops repressor synthesis midway in the infective cycle. Promoter prm maintains the repressor level in established lysogens. Delection mapping places it very near the right operator (Or). Prm is activated by repressor bound to the right operator. In the absence of cII or cIII protein, repressor synthesis requires active repressor and only proceeds on genomes able to bind repressor at Or.