Interaction between photosystem I and flavodoxin from the cyanobacterium Synechococcus sp. PCC 7002 as revealed by chemical cross-linking.

The interaction between photosystem I (PS I) and flavodoxin from the cyanobacterium Synechococcus sp. PCC 7002 was investigated by covalent cross-linking in the presence of a hydrophilic cross-linker, N- ethyl-3-(3-diaminopropyl)carbodiimide. Under the experimental conditions employed, five distinct cross-linking products of flavodoxin and PS I subunits are formed. Immunoblot analyses show that these species are the result of cross-linking of flavodoxin to PsaC, PsaD, an unidentified low-molecular-mass PS I polypeptide, and a 15-kDa subunit. The latter has been indirectly identified as the PsaF subunit. Analysis of the interaction of flavodoxin with PS I from a psaE mutant indicates that the PsaE subunit is required for correct complex formation between flavodoxin and PS I, although this subunit is not directly cross-linked to flavodoxin. In addition, the cross-linking products of PsaD with PsaC and PsaL, and PsaE with PsaF, are observed. The covalent complex of flavodoxin and PS I is shown to be fully inhibited with respect to electron transfer to soluble flavodoxin, ferredoxin or ferredoxin:NADP+ oxidoreductase.