The missing link between Photosynthesis and SULFur metabolism in cyanobacteria – PhotoSULF

One of the most intriguing question is how oxygenic photosynthesis evolved presumably from an anoxygenic, sulfur-based one. Primitive cyanobacteria were the “inventors” of oxygenic photosynthesis, and in an early and not completely oxygenated ocean they were able to perform both oxygenic and anoxygenic photosynthesis. Some of the biochemical mechanisms that allowed them to swap between metabolisms depending on the environmental conditions are still present in modern cyanobacteria. For instance, the reaction center D1 protein of Photosystem II can be replaced with a phylogenically old D1 isoform when cells experience low oxygen. In addition, a new hemoprotein called Tll0287 has been found expressed only with the low oxygen-induced D1 protein in the thermophilic cyanobacterium Thermosynechococcus elongatus. Widespread in cyanobacteria, tll0287 is found in the same operon than another ancient protein, the sulfide-quinone oxidoreductase (SQR). SQR catalyzes the oxidation of sulfide fueling electrons to PSI via the plastoquinone pool enabling anoxygenic photosynthesis. These findings suggest an intriguing but currently unknown linkage between modified PSII-expressing low oxygen D1 isoforms and sulfide exposure.
PhotoSULF aims at studying and understanding the regulation mechanisms that allow photosynthesis to occur in low oxygen and in presence of sulfide using a combination of biochemistry, biophysics and molecular biology techniques. The understanding of these mechanisms is of relevance to comprehend photosynthesis evolution, particularly the unknown biochemical switch that allowed the transition between the anoxygenic and the oxygenic world.