Photosynthesis and Synthetic Biology
A basic property of plants is their ability to carry out photosynthesis and thus to provide the organic carbon which forms the basis for the food chain on Earth. The photochemical reactions of photosynthesis takes place in the thylakoid membranes of the chloroplasts and are mediated by two photosystems: PS I and PS II, catalyzing vectorial electron transport across the thylakoid membrane. We can isolate PSI as a photochemically active pigment-protein complex containing at least 18 different subunits and new subunits belonging to this complex are still being identified.
Using gene suppression or gene knock-out in combination with biochemical techniques and physiological analysis we have studied the structure, function and biogenesis of the PS I complex. Functionally, PS I operate mostly in series with PS II. However, a fraction of the electrons cycle around PS I. This permits the plant to balance and optimize the ratio between ATP, NADPH and electrons for other processes. More importantly, the cyclic electron flow serves to control pH in the thylakoid lumen and thereby to regulate PSII activity. The tight regulation of electron transport ensures that damaging over-reduction of the photosynthetic apparatus does not take place under conditions of restricted carbon dioxide assimilation, e.g. during drought or low temperature.