Activation of the cycA P2 promoter for the Rhodobacter sphaeroides cytochrome c2 gene by the photosynthesis response regulator.

The Rhodobacter sphaeroides photosynthesis response regulator, PrrA, positively regulates cycA P2 expression. Deletion analysis has identified sequences within 73 bp upstream of the transcription initiation site that are required for the activation of cycA P2 by PrrA. A mutant form of the Rhodobacter capsulatus PrrA homologue, whose activity is independent of phosphorylation (RegA*), protects an approximately 26 bp region of cycA P2 that is centred at approximately -50 from DNase digestion, and activates transcription of a mutant -14T promoter with increased activity when using either R. sphaeroides RNA polymerase or Escherichia coli Esigma70. A 4 bp target site mutation that eliminated DNA binding and transcription activation by RegA* in vitro also abolished PrrA activation of cycA P2 transcription in vivo, indicating that this region contains a PrrA binding site. By analysing the behaviour of the -14T mutant cycA P2 promoter in vivo, we also found that PrrA uses the same target site to activate expression in both the presence and the absence of O2. However, the extent of transcription activation by PrrA at cycA P2 in vivo is greater under anaerobic conditions.