Nat. Struct. Mol. Biol., published online 2 June 2013; doi:10.1038/nsmb.2597

Rhodobacter sphaeroides is able to generate energy via aerobic respiration (when O2 is abundant) or photosynthesis (when O2 is scarce). The O2- and light-sensing protein AppA and the transcriptional repressor PpsR regulate the expression of photosynthetic genes in this microorganism. Winkler et al. determined the X-ray crystal structures of AppA, PpsR and the AppA–PpsR2 complex, performed hydrogen-deuterium exchange experiments in the presence and absence of light, and carried out functional studies to characterize the structural changes that govern the transition from aerobic respiration to photosynthetic energy production. Their data suggest that PpsR8 and the AppA–PpsR2 complex are able to bind the promoter region of specific genes. When O2 is abundant, expression of AppA is low, and the predominant species in vivo are PpsR2, PpsR4 and a PpsR8–DNA complex; this PpsR8–DNA complex directly represses the transcription of photosynthetic genes. When the bacterium is in a dark, O2-scarce environment, expression of AppA is increased; AppA can bind PpsR and DNA, forming an AppA–PpsR2–DNA complex that prevents formation of the PpsR8–DNA complex and allows transcription of photosynthetic genes to occur. A shift of environmental conditions from darkness back to light reduces the affinity of AppA–PpsR2 for DNA, increasing the likelihood that the PpsR8–DNA complex will reform, once again repressing the transcription of photosynthetic genes.