Repressor structure and the mechanism of positive control.

It has been suggested that the lambda repressor stimulates transcription of its own gene by binding to the lambda operator and contacting RNA polymerase bound to the adjacent promoter. We describe three different mutants (called pc) of the lambda phage repressor that are specifically deficient in the positive control function. We show that the amino acid residues altered in the pc mutants lie on the surface of the DNA-bound repressor that we predict, based on structural and other evidence, would most closely approach DNA-bound polymerase. Furthermore, we describe a pc mutant of the P22 repressor. We argue that in both the lambda and P22 repressors a structure comprised of two alpha helices has two functions: to bind DNA and to contact RNA polymerase. In the two cases, however, different regions of this structure contact polymerase to mediate positive control.