For more than 50 years, biology textbooks have taught that oxygenic photosynthesis requires two molecular machines working in series: photosystem II, which splits water, and photosystem I, which energises the electrons that ultimately fix carbon dioxide. A team led by Dario Leister, Chair of Plant Molecular Biology at LMU, has now shown that the second machine is not strictly necessary.
The researchers set out to replace the native photosystem I of the cyanobacterium Synechocystis with a plant version, combining genetic engineering with adaptive laboratory evolution, in which organisms are exposed to selection pressure over many generations. To their surprise, they obtained strains that had lost photosystem I entirely yet continued to grow, fix carbon dioxide and produce oxygen.
Analysis revealed that the photosynthetic electron transport chain had been extensively reorganised: a steep proton gradient allows the NDH-1 complex to run in reverse and generate NADPH, the job normally assigned to photosystem I. Published in Nature Communications, the finding revises ideas about how oxygenic photosynthesis may have evolved and suggests photosynthetic systems are far more adaptable than assumed, with implications for engineering more efficient photosynthetic organisms.