M. LOSADA, M. NOZAKI, AND DANIEL 1. ARNON 575 



Reduction of Chromatium (PfS) 

 cytochromes by ttiiosulfate 



S 



^ -002 

 ^ -0.04 



0.04- 



0.02- 



0.00 



400 



500 

 X (my.) 



600 



Fig. 5. Reduction of Chromatiuvi cytochromes by thiosulfate in a cell-free sys- 

 tem. Reaction niixtiue included, in a final volume of 3.0 ml of 0.1 M Tris buffer, 

 pn 7.8, chromatophores (P) containing 0.1 mg bacteriochlorophyll and supernatant 

 fluid (S) corresponding to 0.3 mg bacteriochlorophyll. 20 micromoles of thio- 

 sulfate were added to one of a pair of Thunberg-type cuvettes, and the resulting 

 difference spectrum was measured in a Gary spectrophotometer after 20 minutes 

 at room temperature. Gas phase, argon. 



To summarize, illuminated Chromatium cells evolve molecular hy- 

 drogen from thiosultate, which becomes oxidized to sulfate. The re- 

 action is strictly light-dependent, is influenced by external pH, and 

 is inhibited by molecular nitrogen or ammonium ions. The reduc- 

 tion of oxidized Chromatium cytochromes by thiosulfate has been 

 demonstrated in a cell-free system. These findings are compatible with 

 the electron flow theory of photosynthesis (see Fig. 17, Arnon, this 

 symposium) . 



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