REDOX REGULATION OF PHOTOSYNTHESIS BY THE CYTOCHROME bf COMPLEX: MECHANISMS AND CONSEQUENCES
Brantmier, Paul J.
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Photosynthesis is a fundamental and central metabolic process of our planet. Honed by billions of years of evolution, it forms the basis for the food chain that sustains our ecosphere. Photosynthesis begins with sunlight capture by light harvesting complexes associated with photosynthetic membranes. The captured energy is funneled to photosystems II and I (PSII & PSI) reaction centers. In the PSII reaction center, light energy pulls electrons from water, evolving oxygen. These electrons drive a series of redox (oxidation-reduction) reactions passing through the cytochrome (Cyt) bf complex and PSI to generate chemical energy as adenosine triphosphate (ATP) and nicotine adenine dinucleotide phosphate (NADPH). The danger in shuffling electrons through sequential redox transfers is the risk of unintended electron transfer to molecules such as oxygen, forming reactive radical species. The Cyt bf complex has been proposed as a source of superoxide, one of many forms of reactive oxygen species (ROS) that impair growth, function, and survival. My thesis addresses the regulation of electron transport and light harvesting processes by which photosynthetic organisms optimize light energy distribution between the photosystems (so called 'state transitions') and minimize ROS production. The Cyt bf complex has also long been implicated in sensing redox changes in electron transport and signaling the redistribution of light harvesting between PSII and PSI by mechanisms that are not well understood. I used specific inhibitors and a sensitive fluorescent probe (H2DCFDA) to characterize ROS production in the Cyt bf complex of the cyanobacteria (blue-green micro-algae), Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002. Inhibitors were also used in room-temperature and 77 degree K fluorescence studies to investigate the role of the Cyt bf complex in redox sensing and signaling that mediates the redistribution of light-harvesting phycobiliproteins (PBS) in these cyanobacteria. Findings from this work show that the Cyt bf complex plays a central role in these processes. These and previously published data allowed me to formulate a detailed model of redox signaling by the Cyt bf complex, and regulated redistribution of light energy by the formation of dynamic light-harvesting (PBS) supercomplexes involving the Cyt bf complex, PSII, and PSI. The model proposes two distinct means by which these 'state transitions' occur. I propose Cyt bf-independent and - dependent sensing-signaling mechanisms. The Cyt bf - dependent mechanism depends on the presence of light and appears to require binding events or conformational changes in the Cyt bf low-potential electron transfer chain or quinone-reductase (Qn) site. I propose that the Cyt bf - independent mechanism lies downstream of the bf complex. Together, these signal the formation of PSII-PBS-Cyt bf and PSI-trimer-PBS complexes during illumination, and the predominant formation of PSI-monomer-PBS complexes during darkness. These in turn determine the relative light-harvesting capacities of PSII and PSI. The model and supporting evidence are discussed in the context of strategies that have evolved to maximize photosynthetic efficiency and minimize ROS production.