Further purification of "Triton subchloroplast fraction I" (TSF-I particles). Isolation of a cytochrome-free high-P-700 particle and a complex containing cytochromes f and b6, plastocyanin and iron-sulfur protein(s).

The "Triton Subchloroplast Fraction I" or "TSF-I particles" can be further fractionated into a cytochrome fraction and a P-700-containing fraction essentially free of cytochromes. The cytochrome complex contains cytochromes f and b6 in approx. equimolar amounts, and, in addition, also plastocyanin and one iron-sulfur protein, all in the bound state. Bound plastocyanin was characterized by EPR spectroscopy. The EPR spectrum of the bound iron-sulfur protein resembles that previously detected in Phostosystem I particles under highly reducing conditions at lower than -560 mV. The redox potential of P-700 in the cytochrome-free high-P-700 particles was measured to be +468 mV; those of cytochromes f and b6 are +345 and -140 mV, respectively. Among the four components present in the complex, only cytochrome f can be coupled to a Photosystem I particle and undergoes photooxidation. This coupled photooxidation is totoally inhibited by KCN and only partially inhibited by HgCl2. The similarity of the complex containing cytochromes f and b6, plastocyanin, and an iron-sulfur protein to complexes III and IV of the mitochondrial respiratory redox chain and a possible involvement of the complex in cyclic photophosphorylation are noted and discussed.