Reconstitution of electron transport in photosystem I with PsaC and PsaD proteins expressed in Escherichia coli

Abstract
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 terminal FA/FB acceptors. Reconstitution was found to be absolutely dependent on reinsertion of the iron‐sulfur clusters in the PsaCI 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.