THE 8-AMINO ACID INTERNAL LOOP OF PSI-C MEDIATES ASSOCIATION OF LOW-MOLECULAR-MASS IRON-SULFUR PROTEINS WITH THE P700-F-X CORE IN PHOTOSYSTEM-I

The PSI-C subunit of photosystem I (PS I) shows similarity to soluble 2[4Fe-4S] ferredoxins. PSI-C contains an eight residue internal loop a nd a 15 residue C-terminal extension which are absent in the ferredoxi ns. The eight-residue loop has been shown to interact with PSI-A/PSI-B (Naver, H., Scott, M. P., Golbeck, J. H., Moller, B. L., and Scheller , H. V. (1996) J. Biol. Chem. 271, 8996-9001). Four mutant proteins we re constructed. Two were modified barley PSI-C proteins, one lacking t he loop and the C terminus (PSI-C-core) and one where the loop replace the C-terminal extension (PSI-CcoreLc-term). Two were modified Clostr idium pasteurianum ferredoxins, one with the loop of barley PSI-C and one with both the loop and the C terminus of PSI-C. Wild-type proteins and the mutants were used to reconstitute barley P700-F-X cores lacki ng PSI-C, -D, and-E. Western blotting showed that PSI-CcoreLc-term bin ds to PS I, whereas PSI-C-core does not. Without PSI-D the PSI-CcoreLc -term mutant accepts electrons from F-X in contrast to PSI-C mutants w ithout the loop. Flash photolysis of P700-F, cores reconstituted with C. pasteurianum ferredoxin showed that only the ferredoxin mutants wit h the loop accepted electrons from F-X. From this, it is concluded tha t the loop of PSI-C is necessary and sufficient for the association be tween PS I and PSI-C, and that the loop is functional as an interactio n domain even when positioned at the C terminus of PSI-C or on a low m olecular mass, soluble ferredoxin.