Probing the structure and function of a super-rogue photosystem II complex involved in chlorophyll f synthesis

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There is an urgent need to develop new strategies to improve crop yield to feed the ever-growing global population. Crop plants grow because they use the energy of sunlight to drive the conversion of atmospheric carbon dioxide into biomass. This process of photosynthesis is relatively inefficient with much less than 1% of the incident solar energy converted into stored chemical energy. One straightforward way to improve photosynthetic efficiency is to capture more of the sunlight in the first place. Plants rely on chlorophyll pigments (as well as some accessory pigments) to absorb light to drive photosynthesis. The chemical nature of the chlorophyll pigments found in plants necessarily means that photosynthesis is restricted to the visible region of the solar spectrum. In recent years, however, several strains of cyanobacteria, which perform plant-like photosynthesis, have been discovered that make modified forms of chlorophyll that absorb light in the far-red region of the spectrum. If these far-red chlorophylls could be made in plants and assembled correctly in the photosynthetic apparatus, the number of photons of light that could be used to drive photosynthesis could be increased by up to 19%, a considerable increase in efficiency. One of the far-red absorbing chlorophylls is chlorophyll f (Chl f). In order to make Chl f in plants, an important first step is to identify and characterise the cyanobacterial enzyme that synthesises Chl f. In a recent breakthrough, Don Bryant and colleagues in the USA showed that Chl f synthesis was dependent on the ChlF protein subunit which, somewhat surprisingly, was found to be related to one of the proteins present in the well-studied photosystem II complex which catalyses the light-driven oxidation of water to oxygen characteristic of plant photosynthesis. In follow-up work, we have discovered that ChlF does not act alone, as was originally thought, but is part of a new type of PSII complex, which we term the super-rogue PSII complex. The super-rogue PSII complex shows clear similarities to regular PSII but has evolved to make Chl f rather than split water into oxygen. Chl f is made from the Chl a pigment through an oxidation reaction involving molecular oxygen; but the chemistry involved in this process is currently unknown. In this application, we propose to study the structure and mechanism of the newly identified super-rogue PSII complex in unprecedented detail. We aim to investigate whether the super-rogue complex is photochemically active and will test the hypothesis that the super-rogue PSII complex activates molecular oxygen into a reactive form that oxidises a Chl a molecule bound to a specific site in the super-rogue PSII complex. The project involves a team of scientists with skills in microbiology, molecular biology, biochemistry and spectroscopy. Our experimental approaches are diverse and involve working on biochemically pure protein complexes as well studying cyanobacterial mutants expressing Chl f. Ultimately our studies will provide important new knowledge on a new type of photosystem II complex that will underpin future work producing Chl f in crop plants.