Reconstitution of iron-sulfur center B of Photosystem I damaged by mercuric chloride

Incubation of thylakoid membranes from spinach with low concentrations of mercuric chloride induces the loss of one of the iron-sulfur centers, F_B, in Photosystem I (PS I) and inhibits the electron transfer from PS I to the soluble electron carrier, ferredoxin. Reconstitution of this damaged iron-sulfur center has been carried out by incubating treated thylakoid membranes with exogenous FeCl₃ and Na₂S in the presence of-mercaptoethanol under anaerobic conditions. Low temperature EPR measurements indicate that center F_B is largely restored. Kinetic experiments show that the restored F_B can be photoreduced from P700. However, these reconstituted thylakoid membranes are still incompetent in the photoreduction of ferredoxin and NADP⁺, even though ferredoxin binding to the modified membranes was not impaired, indicating additional changes in the structure of the PS I complex must have occurred.