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 c yanobacterium 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 empl oyed, five distinct cross linking products of flavodoxin and PS I subu nits are formed. Immunoblot analyses show that these species are the r esult of cross-linking of flavodoxin to PsaC, PsaD, an unidentified lo w-molecular-mass PS I polypeptide, and a 15-kDa subunit. The latter ha s been indirectly identified as the PsaF subunit. Analysis of the inte raction of flavodoxin with PS I from a psaE mutant indicates that the PsaE subunit is required for correct complex formation between flavodo xin and PS I, although this subunit is not directly crosslinked to fla vodoxin. In addition, the cross-linking products of PsaD with PsaC and PsaL, and PsaE with PsaE are observed. The covalent complex of flavod oxin and PS I is shown to be fully inhibited with respect to electron transfer to soluble flavodoxin, ferredoxin or ferredoxin:NADP(+) oxido reductase.