Evidence for the identity of P430 of Photosystem I and chloroplast-bound iron-sulfur protein

The EPR spectrum of Photosystem-I subchloroplast particles, which had been pre-illuminated in the presence of methyl viologen, showed a large P700⁺ signal whereas bound iron-sulfur proteins were not detected. This observation is consistent with a “one-way” electron discharge by the primary electron acceptor, P430⁻, subsequent to the primary photochemical charge separation, and an accumulation of photooxidized P700⁺. Subsequent illumination of the same sample at 77 °K did not change the EPR spectrum. However, if the pre-illuminated subchloroplast particles were allowed to recover at room temperature by standing in the dark for 10 min or by addition of a chemical reductant, subsequent illumination of the sample at 77 °K yielded an EPR spectrum consisting of signals due to both P700⁺ and reduced iron-sulfur protein.