The PSI core complex prepared from cucumber cotyledons, which contains 80 chlorophylls per reaction center (P700) and eight polypeptides with apparent molecular masses of 65/63, 20, 19.5, 18.5, 17.5, 7.6, and 5.8 kDa, has been shown to catalyze the light-dependent transfer of electrons from plastocyanin to ferredoxin. The “native” PSI complex, which contains more than fifteen polypeptides and 120 chlorophylls per P700, did not show higher activity. Any attempt to deplete subunit(s) of the core complex decreased its activity. These results suggest that in addition to light-harvesting chlorophyll a/b protein complexes, several genes of psaA-psaK, which have been proposed as components of PSI complex, are not involved in the activity of PSI complex. It was also found that the amount of 18.5-kDa polypeptide in the PSI complex affects the activity: when this polypeptide was largely depleted, the complex was almost inactive. The inactivation was due to inhibition of electron transfer from plastocyanin to photooxidized P700. Chemical cross-linking and N-terminal amino acid sequencing experiments indicated that the 18.5-kDa polypeptide is the plastocyanin-docking protein and the psaF gene product. The function of the psaF gene product was discussed.