Biochemical evidence for histidine oxidation in photosystem II depleted of the Mn‐cluster for O2‐evolution

Flash excitation of Tris-treated photosystem II (PSII) membranes generated a thermoluminescence band peaking at -20 degrees C, indicating that a positively charged oxidizing equivalent is stably accumulated on donor side of PSII even in the absence of the Mn-cluster. This oxidizing equivalent was sensitive to low concentrations of exogenous Mn2+, and its stable accumulation was reversibly inhibited by diethylpyrocarbonate treatment that modifies histidine residues with high specificity. It was inferred that there is a photooxidizable histidine residue(s) on donor side of PSII and it provides a redox-active ligand for Mn.