Sunlight is the ultimate source of energy on Earth. By means of photosynthesis, plants, algae, and certain bacteria convert the radiant energy of sunlight into chemical potential energy, producing food and feed for virtually all organisms as well as precursors for liquid fuels. In photosynthesis, photosystem II (PS II) carries out the light-driven cleavage of water to generate oxygen and extract the electrons and protons necessary for the synthesis of biomolecules. PS II owes its remarkable water-splitting chemistry to the catalytic prowess of its manganese cluster, which enables it to harness the potential of two virtually inexhaustible raw materials: sunlight and water. The valuable water-oxidation property of PS II, however, is also its weakness. Under intense sunlight or stress, incomplete water oxidation and charge recombination events produce reactive oxygen species that modify a number of key amino acid residues in the reaction center proteins of PS II. This proposal seeks to understand how plant PS II repairs its photodamaged reaction center core and the role of protein phosphorylation in this process. We propose that core phosphorylation releases the peripheral antenna complement from the reaction center core, decreasing reaction center damage in high light. We further suggest that phosphorylation at additional key sites trigger disassembly of the dimeric core, facilitating the repair of the damaged reaction center. The proposed research will test these hypotheses in the model plant Arabidopsis thaliana. Our approach involves the study of PS II antenna size and light harvesting as a function of core phosphorylation. Mutant plants that have higher or lower levels of core phosphorylation will be examined for impaired antenna size adjustment and PS II repair. Using chloroplast genetic transformation techniques, we will replace core phosphosites with alanine or the phosphomimetic glutamate residues. These mutants will be critical in testing the predicted existence of antenna dissociation and core monomerization phosphosites. We further aim to calculate the interaction energy between antenna and core at their precise in vivo phosphorylation stoichiometries. Nearly 70% of the productivity loss in major crop plants results from abiotic factors that impair PS II function and photosynthesis. PS II damage additionally manifests itself as the source of the characteristic midday depression of photosynthesis. The studies proposed here will inform our understanding of the light harvesting strategy, self-repair, and molecular assembly of PS II. These studies will thereby illuminate the design of an optimal antenna system and a light tolerant PS II. The overall objective of this proposal is to foster greater fundamental knowledge of the structure and function of PS II and to determine the physical and chemical rules that underlie the biological mechanisms of its photo-protective and self-repair pathways.