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Abstract

Purple non-sulfur bacteria follow a versatile life style. Depending on environmental conditions they can generate cellular energy from oxidative phosphorylation or cyclic photosynthesis. However, the regulatory mechanisms at the genetic level, which are associated with the transition from an oxidizing to a reducing environment, are only poorly understood. It has been hypothesized that the ubiquinone-10 pool, as part of the electron transport chain, might act as a redox sensor which directly interacts with the membrane-bound sensor kinase RegB. Under reducing conditions RegB is activated which, in turn,phosphorylates the transcriptional activator RegA leading to photo-synthetic gene expression. Recent experiments suggest that the gene induction process is highly cooperative in the sense that small changes in the reduction degree of the ubiquinone-10 pool result in considerable changes of the transcriptional activity of RegA. Here, we propose a simple mathematical model which exhibits the desired co-operative behavior if the RegB sensor kinase operates in the zero-order ultra sensitive regime. We also discuss the steady state behavior of the PpsR repressor which represents a second conserved regulator of photosynthetic gene expression in purple non-sulfur bacteria.