Determining the role of FNR location in the photoprotection of photosystem I

Life on earth depends on photosynthesis, the source of our food, oxygen and most of our energy. Approximately 50% of this life-giving photosynthetic process happens in single celled organisms that live in the world’s oceans and form the critical basis of global food chains. Solar cells within these tiny single-celled photosynthetic organisms, known as photosystems I and II (PSI and PSII), convert solar energy into proton and electron currents that can be converted into fuel molecules. PSI and PSII are combinations of protein and hundreds of chlorophyll molecules, exceptional at harvesting solar energy even when light is scarce. However, the flipside of this success is that on sunny days PSI and PSII are extremely susceptible to light-induced damage. The key element of PSI photoprotection is ‘photosynthetic control’ a regulatory mechanism which controls the flow of electrical current between PSII and PSI at the cytochrome b6f (cytb6f) complex. How this crucial photoprotective mechanism works remains a mystery. Our hypothesis is that the protein ferredoxin-NADP+-reductase (FNR) plays a key role in regulating photosynthetic control, activating the process when it is associated with cytb6f and inactivating it when it is associated with PSI. In this project we will use a CRISPR-CAS9 gene editing approach to create mutant algae with FNR tethered to either PSI or cytb6f to test these ideas.

We will characterise the photosynthetic physiology of the mutant cells expressing these chimeric proteins using a variety of in vivo spectroscopic approaches, before going on to biochemically purify the chimeric complexes for characterisation by Cryo-electron microscopy. Finally, we will investigate the regulatory consequences of each chimera by studying the interactions they make using TurboID, a genetically encoded protein tag which biotinylates surrounding proteins. These tagged partners can then be identified using mass spectrometry to reveal the unique interactions of each chimera.