The 110-kDa reaction center protein of photosystem I, P700-chlorophyll a-protein 1, is an iron-sulfur protein.

Germination and growth of barley (Hordeum vulgare L.) in the presence of 59Fe2+ or 35SO4(2-) allows heavy incorporation of both isotopes into the thylakoid membranes and into isolated photosystem I particles. Analysis of 59Fe-labeled preparations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under mild conditions demonstrates that a minimum of four iron atoms/P700 is carried on P700-chlorophyll a-protein 1. When isolated from 35S-labeled preparations, P700-chlorophyll a-protein 1 binds zero valence 35S, which is converted into acid-labile [35S]sulfide by dithiothreitol reduction. Isolated photosystem I particles contain 14 acid-labile sulfide atoms and 10 iron atoms for each molecule of P700 and are composed of polypeptides of 110, 18, 15, 10, and 8 kDa of which the 10-kDa component is loosely bound. Under the electrophoretic conditions used, none of the low molecular weight polypeptides could be shown to be specifically associated with iron or acid-labile sulfide. Carboxymethylation of cysteine residues shows a high cysteine content in the 8-kDa polypeptide and an intermediate content in the 110- and 18-kDa polypeptides, whereas the 15-kDa polypeptide is devoid of sulfur amino acids. The experiments with the 59Fe-labeled thylakoids reveal other labeled polypeptides not associated with photosystem I, namely cytochrome f and possibly cytochromes b6 and b559.