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

迫切需要制定新的战略来提高作物产量，以养活不断增长的全球人口。农作物的生长是因为它们利用阳光的能量将大气中的二氧化碳转化为生物质。这种光合作用的过程相对低效，只有不到1%的入射太阳能转化为储存的化学能。提高光合效率的一个直接方法是首先捕获更多的阳光。植物依靠叶绿素色素（以及一些辅助色素）吸收光线来驱动光合作用。在植物中发现的叶绿素色素的化学性质必然意味着光合作用仅限于太阳光谱的可见光区域。然而，近年来，已经发现了几种进行植物样光合作用的蓝藻菌株，它们可以产生吸收光谱远红区光的叶绿素的修饰形式。如果这些远红叶绿素可以在植物中制造，并在光合机构中正确组装，那么可以用于驱动光合作用的光子数量可以增加19%，这是一个相当大的效率提高。叶绿素f（Chl f）是一种吸收远红外的叶绿素。为了在植物中制造叶绿素f，重要的第一步是鉴定和鉴定合成叶绿素f的蓝藻酶。在最近的一项突破中，美国的Don Bryant及其同事表明，Chlf合成依赖于ChlF蛋白亚基，令人惊讶的是，ChlF蛋白亚基被发现与充分研究的光系统II复合物中存在的一种蛋白质有关，该复合物催化植物光合作用中光驱动的水氧化为氧的特征。在后续工作中，我们发现ChlF并不像最初认为的那样单独起作用，而是一种新型PSII复合物的一部分，我们称之为超级流氓PSII复合物。超级流氓PSII复合物显示出与常规PSII的明显相似之处，但已经进化为制造叶绿素f，而不是将水分解为氧气。叶绿素f是由叶绿素a色素通过涉及分子氧的氧化反应制成的;但这个过程中涉及的化学过程目前尚不清楚。在本申请中，我们提议以前所未有的详细程度研究新发现的超级流氓PSII复合物的结构和机制。我们的目标是调查是否超级流氓复杂的光化学活性，并将测试的假设，超级流氓PSII复合物激活分子氧成反应形式，氧化叶绿素a分子结合到一个特定的网站在超级流氓PSII复合物。该项目涉及一组具有微生物学、分子生物学、生物化学和光谱学技能的科学家。我们的实验方法是多种多样的，涉及生物化学纯蛋白质复合物的工作，以及研究蓝藻突变体表达叶绿素f。最终，我们的研究将提供重要的新知识，一种新型的光系统II复合物，将支持未来的工作，在作物生产叶绿素f。

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.