Electron-Transfer Events Leading to Reconstitution of Oxygen-Evolution Activity in Manganese-Depleted Photosystem II Membranes

O₂-evolution activity and the Mn complex can be reconstituted in photosystem II by a process called photoactivation. We have studied the elementary steps in photoactivation by using electron paramagnetic resonance spectroscopy to probe electron transport in Mn-depleted photosystem II membranes. The electron donation reactions in Mn-depleted photosystem II were found to be identical with those in untreated photosystem II, except that electron donation from the Mn complex was absent and could be replaced by slower electron donation from exogenous Mn²⁺. Mn²⁺ photooxidation by Mn-depleted photosystem II membranes correlates with reconstitution of O₂-evolution activity. However, photooxidation of Mn²⁺ occurs in competition with photooxidation of the tyrosine residue Y_D, and cytochrome b-559. Thus, these two species are excluded from direct participation in the initial steps in the assembly of the Mn complex. Because photooxidation of Mn²⁺ is slower than photooxidation of the competing electron donors, cytochrome b-559 and chlorophyll, as well as recombination of the charge-separated states chlorophyll⁺Q_A⁻ or Y_Z⁺Q_A⁻, these other reactions dominate in a single photochemical turnover reaction. This provides a molecular basis for both the low yield and low quantum yield of photoactivation. The first photochemical step in the assembly of the Mn complex results in photooxidation of one Mn²⁺ ion. Therefore, the first intermediate in assembly of the Mn complex contains Mn³⁺. On the basis of these results and previous kinetic studies [Miller, A.-F., & Brudvig, G. W. (1989) Biochemistry 28, 8181], we conclude that the second intermediate of Mn complex assembly contains Mn²⁺Mn³⁺, which is photooxidized to Mn³⁺₂.