Modeling the Light and Redox Dependent Interaction of PpsR/AppA in Rhodobacter sphaeroides

Switching energy generation mechanism from photosynthesis to respiration in facultative photosynthetic bacteria is a response to changes in availability of oxygen and light. An indispensable part of this transition consists of a conserved transcriptional repressor PpsR, which is responsible for specific repression of the components of photosynthetic apparatus under aerobic conditions. In Rhodobacter sphaeroides a light and redox sensitive flavoprotein AppA antagonizes the repressive activity of PpsR and interaction of PpsR with AppA is believed to be responsible for semi-aerobic high light repression of photosynthetic apparatus genes in order to reduce the risk of photo-oxidative stress. We developed a mathematical model to investigate the underlying mechanism of this phenotype based on experimental findings. The model takes into account light excitation of AppA as well as redox dependent complex formation between AppA and PpsR. Steady state investigations show that the high light repression of photosynthetic genes under semi-aerobic conditions can indeed occur: (i) If the PpsR is reduced on a faster time scale than AppA but not too fast. (ii) Electron transfer from AppA to PpsR is effectively irreversible. Our study also supports the earlier finding [1] that the transition from anaerobic to aerobic growth can occur via a bistable switch if AppA copy number exceed those of PpsR by at least a factor of two. References [1] R. Pandey, D. Flockerzi, M. J. B. Hauser and R. Straube, Biophysical Journal in press (2011)