Mutant lambda phage repressor with a specific defect in its positive control function.

The lambda phage repressor is both a positive and a negative regulator of gene transcription. We describe a mutant lambda phage repressor that has specifically lost its activator function. The mutant binds to the lambda phage operator sites and represses the lambda phage promoters PR and PL. However, it fails to stimulate transcription from the promoter PRM. The mutation lies in that portion of repressor--namely, the amino-terminal domain--that has been shown [Sauer, R. T., Pabo, C. O., Meyer, B. J., Ptashne, M. & Backman, K. C. (1979) Nature (London) 279, 396-400] to mediate stimulation of PRM. We suggest that the mutation has altered that region of repressor which, in the wild-type, contacts RNA polymerase to activate transcription from PRM.