Structural and functional studies on the cytochrome b6f complex from higher plants and cyanobacteria

Oxygenic photosynthesis plays a pivotal role at the very heart of global ecosystems, providing the food, fuel and oxygen that sustains virtually all life on Earth (Hohmann-Marriott and Blankenship, 2011). Despite the essential and highly intricate role that photosynthesis plays on Earth, the actual process of photosynthesis appears to be remarkably inefficient with only ~ 9-12% of useable solar energy (wavelengths between 400 – 700 nm) being converted to biomass (Zhu et al., 2010). Given the rising demands on the global food chain associated with climate change and a rising population, it is widely recognised that improvements in the efficiency of this vital process will be required to ensure food security for an ever-increasing population over the coming decades (Long et al., 2015; Zhu et al., 2010). Among the multiple targets which have been identified for potential improvement is cytochrome b6f (cytb6f), an integral membrane complex found at the heart of oxygenic photosynthesis. As well as facilitating the rate-limiting step in light dependent electron and proton transfer, the cytb6f complex also plays a key role as a redox-sensing hub in higher plants involved in the regulation of light-harvesting, electron transfer, photosynthetic gene expression and adaptation to environmental stress. Together, these characteristics make cytb6f a judicious target for genetic manipulation to enhance photosynthetic yield and promote stress tolerance in crop plants. While a number of studies show great promise in this regard (Simkin et al., 2017), further progress is hindered by the lack of a detailed understanding of the structure and function of the cytb6f complex from higher plants. Here we present the first structures of the cytb6f complex from a higher plant (Spinacia oleracea), shedding light on the internal mechanics of the Q-cycle and providing new clues as to how this extraordinary complex fulfils its various roles in photosynthetic regulation. We offer further opportunities to explore these insights by presenting a second structure, from the model cyanobacterium Synechocystis sp. 6803. Together these structures provide a number of key mechanistic insights into the cytb6f complex, many of which may now be further explored through structure-based mutagenesis of the Synechocystis complex and molecular dynamics simulations.