PHOTOSYXTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 373 



established that Chromatium particles have enzymes catalyzing the transfer 

 of electrons from thiosulphate and succinate to cytochromes [136, i] : the 

 reduction of oxidized Chromatium cytochromes by thiosulphate is shown in 

 Fig. 14 and by succinate in Fig. 15. Second, illuminated photosynthetic 

 bacteria reduce pyridine nucleotides in the presence of succinate or some 

 other electron donor that is less reduced than pyridine nucleotides. The 

 photoreduction of DPN was observed by Frenkel [146] and Vernon and 



Reduction of Chromatium (P+S) 

 cytochromes by succinate 



0-2 



0-1 



0-0 



-0-1 



-0-2 



400 



450 500 



A (m//) 



550 



600 



Fig. 15. Reduction of Cliromathim cytochromes by succinate in a cell-free 

 system. Reaction mixture included, in a final volume of o • 3 ml. of o • 2 m tris buflfer, 

 pH 7-8, chromatophores (P) containing o-o6 mg. bacteriochlorophyll and super- 

 natant fluid (S) corresponding to 0-3 mg. bacteriochlorophyll. 10 /j.moles of 

 succinate was added to one of a pair of cuvettes and the resulting difference 

 spectrum was measured in a Gary spectrophotometer at the indicated time 

 intervals (Nozaki, Ogata, and Arnon [114]). 



Ash [147] in R. riihnim and by Ogata et al. in Chromatium [91]. In more 

 recent experiments we have found that in the presence of succinate and 

 light, unwashed chromatophores from R. rubriim, unaided by enzymes 

 from chloroplasts (cf. [147]) reduce both di- and triphosphopyridine 

 nucleotide. 



Additional support for the non-cyclic electron flow mechanism in 

 bacterial photosynthesis has come from recent experiments on the photo- 

 production of hydrogen gas and photofixation of nitrogen gas. We found 



