Changing the mechanism of transcriptional activation by phage lambda repressor.

The first steps of transcription initiation include binding of RNA polymerase to a promoter to form an inactive, unstable, closed complex (described by an equilibrium constant, K(B)) and isomerization of the closed complex to an active, stable, open complex (described by a forward rate constant, k(f)). lambda cI protein activates the PRM promoter by specifically increasing k(f). A positive control mutant, cI-pc2, is defective for activation because it fails to raise k(f). An Arg to His change in the sigma70 subunit of RNA polymerase was previously obtained as an allele-specific suppressor of cI-pc2. To elucidate how the mutant polymerase restores the activation function of the mutant activator, abortive initiation assays were performed, using purified cI proteins and RNA polymerase holoenzymes. The change in sigma does not significantly alter K(B) or k(f) in the absence of cI protein. As expected, cI-pc2 activates the mutant polymerase in the same way that wild-type cI activates the wild-type polymerase, by increasing k(f). An unexpected and novel finding is that the wild-type activator stimulates the mutant polymerase, but not wild-type polymerase, by increasing K(B).