442 



LIGHT AND LIFE 



PHOTOCHEMICAL APPARATUS 



REDUCTANT 



DPN DPNH 



cytochrome chal n, includi no 

 cytochrome cj and probably 

 aA-type cytochrome 



ADP 



ADP- 



OXIDANT 



CYTOCHROME CHAIN 



INCLUDING A C-TYPE 

 andaA-TYPE CYTOCHROME 



oxidizable substrate 



Fig. 5. Schematic representation of the postulated electron transport chains in 



photosynthetic bacteria. 



chain which can oxidize substrates with oxygen in the dark or can 

 pass electrons to the oxidant formed upon ilhnnination of the bacteria. 

 In the latter reaction, an equivalent amount of photoreductant brings 

 about the reduction of DPN. The reduction of DPN upon illumina- 

 tion of R. rubrum has been demonstrated by Duysens and Sweep (.8) . 

 The photosynthetic bacteria do not evolve oxygen, but instead oxidize 

 a substrate (13). In the system pictured, ATP is formed from ADP 

 on oxidation of substrate either by chain B reacting with oxygen 

 or the photo-oxidant. In addition a second electron-transport chain 

 (chain A) exists, over which the photoreductant and the photo- 

 oxidant can interact with the phosphorylation of ADP. All of our 

 observations, particularly those on the effects of inhibitors (9) , in- 

 dicate that the latter electron-transport chain is similar in properties 

 to the extensively studied enzyme system which mediates oxidation of 

 substrates by molecular oxygen with accompanying phosphorylation. 

 This agrees with the idea that similar pigments, such as b- and c-type 

 cytochromes, are components of the chain, and that the light-induced 

 phosphorylation may have a similar mechanism to that of "oxidative 

 phosphorylation." This is all in agreement with the suggestions of 

 Dr. Hill on the role of the cytochromes in the oxidation-reduction 

 reactions of photosynthesis. 



REFERENCES 



1. Cliance, B.. Rev. Sci. Instr., 22, 619 (1951). 



2. Chance, B., and Smith, L., Nature, 175, 803 (1955). 



3. Duysens, L. N. M., and Sweep, G.. Diochim. el Iiiol)liys. Acta, 25, 13 (1957). 



4. Elsdcn, S. R., Kamcn, M. D., and Vernon, L. P., ,/. Am. Cliein. Soc, 75, 6347 



(1953). 



5. Frcnkel, A., J. Am. Client. Soc, 76, 5568 (1954). 



