ELECTRON TRANSPORT SYSTEM IN FACULTATIVE 

 PHOTOHETEROTROPH: RHODOSPIRILLUM RUBRUM 



T. HORIO and J. YAMASHITA 

 Division of Enzymology, Institute for Protein Research, 

 Osaka University , Osaka, Japan 



INTRODUCTION 



Regardless of their nature— strict aerobe, facultative anaerobe, or 

 strict anaerobe— photosynthetic tissues contain subcellular portions, 

 chloroplasts in green plants or chromatophores in photosynthetic 

 bacteria, where the photosynthetic pigments and all the hematin com- 

 pounds are present (1), The photo sjoithetic pigments are the main 

 constituents of the photochemical apparatus (2,3), and the hematin 

 compounds are parts of a special electron transport system coupled 

 to the absorption of light energy in the photochemical apparatus. In 

 green plants this system is wholly separate from the mitochondrial 

 respiratory system, but in the photosynthetic bacteria this system is 

 available for respiration as part of a common system for both anaerobic 

 photosynthetic metabolism and aerobic nonphotosynthetic metabolism. 

 The primary photochemical act gives electrons a driving force to re- 

 duce one terminus of the electron transport system, and the conse- 

 quent oxidation- reduction reactions in the system take them back to 

 their starting point, coupling to phosphorylation in a manner analogous 

 to oxidative phosphorylation in mitochondria (4,5). This series of re- 

 action has been designated as "cyclic" photophosphorylation by Arnon 

 (6), In green plants, but not in the photosynthetic bacteria, there exists 

 an additional system capable of drawing an electron photochemically 

 out of a water molecule to reduce the electron transport system on the 

 one hand and to evolve molecular oxygen on the other. In experiments 

 with chloroplasts, the photosynthetic oxygen evolution (Hill reaction) 

 can be demonstrated in the presence but not absence of an appropriate 

 oxidation- reduction substance (Hill reagent). This overall reaction 

 couples to a phosphorylation designated as "noncyclic" photophosphory- 

 lation. It is not known yet whether in chloroplasts electrons migrate 

 through the electron transport system in such a cyclic fashion as in 

 chromatophores. 



The outline of photosynthetic electron transport mentioned above 

 has been derived from the pioneering works published so far (7,8,9,1, 

 6), Most current research is aimed at elucidating the nature of the 



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