The polypeptide composition of the Photosystem I complex from Synechococcus sp. PCC 6301 was determined by sodium-dodecyl sulfate polyacrylamide gel electrophoresis and N-terminal amino acid sequencing. The PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaK and PsaL proteins, as well as three polypeptides with apparent masses less than 8 kDa and small amounts of the 12.6 kDa GlnB (PII) protein, wee present in the Photosystem I complex. No proteins homologous to the PsaG and PsaH subunits of eukaryotic Photosystem I complexes were detected. When the Photosystem I complex was treated with 6.8 M urea and ultrafiltered using a 100 kDa cutoff membrane, the resulting Photosystem I core protein was found to be depleted of the PsaC, PsaD and PsaE proteins. The filtrate contained the missing proteins, along with five proteolytically-cleaved polypeptides with apparent masses of less than 16 kDa and with N-termini identical to that of the PsaD protein. The PsaF and PsaL proteins, along with the three less than 8 kDa polypeptides, were not released from the Photosystem I complex to any significant extent, but low-abundance polypeptides with N-termini identical to those of PsaF and PsaL were found in the filtrate with apparent masses slightly smaller than those found in the native Photosystem I complex. When the filtrate was incubated with FeCl3, Na2S and beta-mercaptoethanol in the presence of the isolated Photosystem I core protein, the PsaC, PsaD and PsaE proteins were rebound to reconstitute a Photosystem I complex functional in light-induced electron flow from P700 to FA/FB. In the absence of the iron-sulfur reconstitution agents, there was little rebinding of the PsaC, psaD or PsaE proteins to the Photosystem I core protein. No binding of the truncated PsaD polypeptides occurred, either in the presence or absence of the iron-sulfur reagents. The reconstitution of the FA/FB iron-sulfur clusters thus appears to be a necessary precondition for rebinding of the PsaC, psaD and psaE proteins to the Photosystem I core protein.