Evidence that the intermediate electron acceptor, A2, in photosystem I is a bound iron-sulfur protein.

Absorption changes accompanying the formation of light-induced P-700+ were investigated in a highly enriched Photosystem I preparation where an intermediate electron acceptor preceding P-430 could be detected. In an enriched Photosystem I particle, light-induced reversible absorption changes observed at 700 nm in the presence of dithionite resembled those previously seen at 703 nm and 820 nm [9], thus indicating the presence of a backreaction between P700+ and A-2. After this same Photosystem I particle was treated to denature the bound iron-sulfur centers, the photochemical changes that could be attributed to P-700 in equilibrium A2 were completely lost. These results provide evidence that the intermediate electron acceptor, A2, is a bound iron-sulfur protein. Additional studies in the 400--500 nm region with Photosystem I particles prepared by sonication indicate that the spectrum of A2 is different from that of P-430.