Structural Aspects of Photosystem I from Dunaliella salina

A native PSI complex and a PSI core complex have been isolated from the halophilic green alga, Dunaliella salina. The composition and properties of these complexes are similar to previously described PSI complexes from spinach membranes. By growth on ¹⁴C-NaHCO₃, it has been possible to isolate uniformly labeled ¹⁴C-PSI complexes in order to determine PSI subunit stoichiometry. This analysis has shown a ratio of one copy of three low molecular weight subunits (22,000; 15,000; 8,000) per two copies of high molecular weight subunits (84,000). Using a ¹⁴C-labeled cytochrome b₆-f complex as an internal protein standard, it has been possible to estimate the molecular weight of a PSI core complex as about 330,000. This complex contains one P700, two 84,000 subunits, and one subunit of 22,000, 15,000, and 8,000.