Characterization of an isolated chloroplast membrane Fe◀S protein and its identification as the photosystem I Fe◀SA/Fe◀SB binding protein

An iron‐sulfur protein of approx. 9 kDa has been isolated from spinach chloroplast membranes. Based on iron and sulfide content (∼900 nmol Fe and S2− per mg protein), optical absorbance and low‐temperature EPR spectra, it appears that this protein contains 8Fe and 8S2− in two [4Fe◀4S] clusters. The protein cross‐reacts with an antibody raised against a 9 kDa PS I subunit previously identified as the psaC (frxA) gene product. This identity was confirmed by the N‐terminal amino acid sequence of the isolated Fe◀S protein. It is concluded that the isolated Fe◀S protein binds PS I centers Fe◀SA and Fe◀SB and that each of these centers contains 4Fe and 4S2−.