198 METABOLISM AND PHYSIOLOGY 



agents such as HOQNO (2) and antimycin A (12) and obtain a bypass 

 around the site of inhibition by adding phenazine methosulfate (12,13) 

 (PMS, methyl-phenazonium sulfate) has made it possible to investigate 

 the question of whether one or several phosphorylation sites are linked 

 to the electron transport. Our evidence for the presence of two sites 

 in the "physiological" chain and one in the "PMS" chain has been 

 given (14-17), The discussion of the phosphorylation reactions will be 

 limited to some possible implications of the results which we obtained 

 with the uncoupling agent valinomycin, which is known to uncouple the 

 phosphorylation at all three phosphorylation sites in the electron trans- 

 port system in animal mitochondria (18), but which appears to uncouple 

 only one of two existing phosphorylation sites in cyclic photophosphory- 

 lation. 



Fig. 1 shows our general interpretation of the effects obtained with 

 valinomycin on bacterial photophosphorylation (14-16), If this inter- 

 pretation is correct, there is a valinomycin-sensitive phosphorylation 

 site in a region of the cyclic electron transport chain containing only 

 dark electron transport reactions and a valinomycin-insensitive phos- 

 phorylation site in a region which contains the photochemical reactions 

 and possibly one or several dark electron transport steps. 



It may well be more than a coincidence that, when sensitivity to 

 valinomycin is used as an indicator, the bacterial photophosphorylation 

 reactions are similar to those in oxidative phosphorylation in the re- 

 gion where only dark reactions occur, but different in the region where 

 the photochemical reactions aie localized. Obviously, a possible site 



(The arrows indicate the direction of the electron 

 transport. A, B, C and D indicate physiological elec- 

 tron carriers.) 



Chlorophyll 



Valinomycin-sensitive phosphorylation 

 linked to the region: 



Valinomycin-insensitive phosphorylation 

 linked to the region: 



D — ^Chlorophyll — ^A — ^B 



Fig. 1. Interpretation of the effects obtained with 

 valinomycin on bacterial photophosphorylation. 



