RECONSTITUTION OF ELECTRON-TRANSPORT IN PHOTOSYSTEM-I WITH PSAC AND PSAD PROTEINS EXPRESSED IN ESCHERICHIA-COLI

A fusion protein, denoted PsaC1, 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 PsaC1 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 terminal F(A)/F(B) acceptors. Reconstitution was found to be absolutely dependent on reinsertion of the iron-sulfur clusters in the PsaC1 apoprotein and on the presence of the PsaD protein. This implies that the PsaC1 holoprotein does not bind solely to the PsA/PsaB heterodimer but rather that its interaction with these proteins is mediated through the PsaD protein.