N-terminal amino acid sequence analysis of small subunits of photosystem I reaction center complex from a thermophilic cyanobacterium, Synechococcus elongatus Nägeli

Abstract

Four small subunits (14, 13, 10, and 8 kDa) of the photosystem I reaction center complex were isolated from a thermophilic cyanobacterium Synechococcus elongatus and their N-terminal amino acid sequences determined. Sequence analysis of the 10-kDa subunit revealed that the distribution of cysteine residues, Cys-X-X-Cys-X-X-Cys-X-X-X-Cys-Pro, is characteristic of bacterial-type ferredoxins, and that its partial sequence is highly homologous to that deduced from the chloroplast gene frx A of liverwort. This indicates that the 10-kDa polypeptide is an apoprotein carrying two iron-sulfur centers, FA and FB, assigned as [4Fe-4S] clusters, which mediated the light-activated transfer of electrons from P700 in photosystem I reaction center complex to soluble ferredoxin. The amino acid sequence of the 14-kDa polypeptide also showed similarity to that of the 20-kDa polypeptide from spinach chloroplast that can be chemically crosslinked with soluble ferredoxin. Thus, the 14-kDa polypeptide appears to be the ferredoxin 'docking' protein.