PHOTO PHOSPHORYLATION AND ELECTRON TRANSPORT 197 



coenzyme Q can be photochemically reduced and thus may be a pri- 

 mary electron acceptor in isolated chromatophores of photosynthetic 

 bacteria. The presence of the electron carrier ferredoxin (the plant 

 enzyme has long been known as photosynthetic pyridine nucleotide 

 reductase or methemoglobin reducing factor) in photosynthetic bacteria 

 has been reported by Tagawa and Arnon (10). To further compare 

 bacterial photophosphorylation with mitochondrial oxidative phosphory- 

 lation, one may endeavourtostate that the site of cytochrome c oxidase 

 in oxidative phosphorylation is occupied by chlorophyll in bacterial 

 photophosphorylation, where the pigment in the light may serve as 

 a "cytochrome c^ photooxidase," While the general picture on the 

 oxidizing side of the chain today appears to be quite similar to its 

 mitochondrial counterpart, the present knowledge about the electron 

 transport sequence at the reducing side is still rather limited. For 

 example: is ferredoxin or coenzyme Q or another compound the 

 primary acceptor of electrons from chlorophyll, or may more than 

 one agent act in this capacity? 



The present uncertainty in this region is reflected in the multiple 

 choices of our very tentative scheme for the electron transport re- 

 actions in cyclic photophosphorylation of chromatophores from R. 

 rubrum: 



Z? :--- 



Light 



ferredoxin- flavoprotein- ^ ^ , ■ -c-tvpe cytochrome-chlorophyll 



6-type cytochrome -^ ^ 



pyridine nucleotide 



Recently one of us (H. B.) has tried to outline what similarities 

 and differences one may find between the above and various other 

 electron transport pathways (11), That outline and the above discus- 

 sion may be taken as an expression of our conviction that the variabil- 

 ity observed between different systems for biological electron trans- 

 port is, in the final analysis, due to nothing but variations on a general 

 and basically common theme. 



ABOUT THE PHOSPHORYLATION REACTIONS 



The fact that one may inhibit the "physiological" cyclic electron 

 transport chain in chromatophores from R. rubrum with site-specific 



