Amino acid sequence of the 8-kDa protein in photosystem I reaction center complex from a thermophilic cyanobacterium, Synechococcus elongatus.

The 8-kDa protein in Photosystem I (PS I) reaction center complex was isolated from a thermophilic cyanobacterium, Synechococcus elongatus, by SDS-polyacrylamide gel electrophoresis using TRIS-Tricine buffer system. The complete amino acid sequence of the protein was determined. The 8-kDa protein consisted of 73 amino acid residues giving a calculated molecular weight of 7,472. No significant sequence homology were observed with the known other small subunits in PS I reaction center complex, except for the 6.5-kDa protein in PS I from another thermophilic cyanobacterium, S. vulcanus. The 8-kDa protein was characteristically rich in hydrophobic amino acid residues, especially the content of leucine. These suggest that the 8-kDa subunit is an intrinsic structure component in PS I core complex for stabilization of the reaction center.