Electron transfer in photosystem I reaction centers follows a linear pathway in which iron-sulfur cluster FB is the immediate electron donor to soluble ferredoxin

Reaction centers of photosystem I contain three different [4Fe-4S] clusters named F-X, F-A, and F-B. The terminal photosystem I accepters (F-A, F-B) are distributed asymmetrically along the membrane normal, with one of them (F-A or F-B) being reduced from F-X and the other one (F-B or F-A) reducing soluble ferredoxin. In the present work, kinetics of electron transfer has been measured in PSI from the cyanobacterium Synechocystis sp. PCC 6803 after inactivation of F-B by treatment with HgCl2. Photovoltage measurements indicate that, in the absence of F-B, reduction of F-A by F-X is still faster than the rate of F-X reduction [(210 ns)(-1)]. Flash-absorption measurements show that the affinity of ferredoxin for HgCl2-treated PSI is only decreased by a factor of 3-4 compared to untreated photosystem I. The first-order rate of ferredoxin reduction by F-A(-), within the photosystem I/ferredoxin complex, has been calculated from measurements of P700(+) decay. Compared to control PSI, this rate is several orders of magnitude smaller (6 s(-1) versus 10(4)-10(6) s(-1)). Moreover, it is smaller than the rate of recombination from F-A(-), resulting in inefficient ferredoxin reduction (yield of 25%). After reconstitution of F-B, about half of the reconstituted photosystem I reaction centers recover fast reduction of ferredoxin with kinetics similar to that of untreated photosystem I. These results support F-B as the direct partner of ferredoxin and as the more distal cluster of photosystem I with respect to the thylakoid membrane, in accordance with a linear electron-transfer pathway F-X --> F-A --> F-B --> ferredoxin.