A fusion protein, denoted PsaCl, which contains an amino-terminal extension of five amino acids (MEHSM...) and is derived from an in vitro modified form of the psaC gene of Synechococcus sp. PCC 7002, has been over-expressed in Escherichia coli. The product of the psaD gene of Nostoc sp. PCC 8009 has similarly been over-expressed. The PsaCl and PsaD proteins can be combined with the photosystem I core protein of Synechococcus sp. PCC 6301 to reconstitute electron transport from P700 to the

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... nal F,/Fa acceptors. Reconstitution was found to be absolutely dependent on reinsertion of the iron-sulfur clusters in the PsaCl apoprotein and on the presence of the PsaD protein. This implies that the PsaCl holoprotein does not bind solely to the PsaA/PsaB heterodimer but rather that its interaction with these proteins is mediated through the PsaD protein. P-700; Iron-sulfur centers FA/FB; Photosystem I reconstitution; PsaC expression in E. coli; FA/FB iron sulfur reinsertion; PsaD expression in E. coli