A simple procedure is described for the preparation of photosystem I (PSI) particles from Triton X-100-solubilized thylakoid membranes of the unicellular cyanobacterium Synechococcus 6301. The purified PSI complex contained the full complement of antenna chlorophylls, 130 +/- 5/P700, displayed the electron paramagnetic resonance signals characteristic of iron-sulfur centers X, A, and B, and had a protein/chlorophyll ratio of 2.9. Determination of the polypeptide composition, utilizing a uniformly 14C-labeled complex, showed that it contained polypeptides of 70, 18, 17.7, 16, and 10 kDa, in a molar ratio of 4.0:0.7:1.0:0.5:1.6. The relative amount of the lower molecular weight polypeptides showed progressive decrease with increase in Triton X-100 concentration and time of exposure to detergent. Consequently, it is proposed that in vivo the composition of the complex is [70 kDa]4 [18 kDa]1 [17.7 kDa]1 [16 kDa]1 [10 kDa]2. Relative to 130 mol of chlorophyll a, the PSI complex contained 16 mol of carotenoids, 13.7 +/- 1.0 g atoms of Fe, and 12.2 +/- 1.1 g atoms of labile sulfide. The properties of complexes fully depleted of the low-molecular weight polypeptides by treatment with sodium dodecyl sulfate or with proteinase K are also described.
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