PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 36 1 



The marked effectiveness of phenazine methosulphate could be ex- 

 plained by its acting as an electron carrier that bypasses a rate-limiting step 

 in photosynthetic phosphorylation [66]. This is suggested by the observa- 

 tion of Geller [66] that the severe inhibition of photophosphorylation in 

 R. riibrian by antimycin A does not occur in the presence of phenazine 

 methosulphate. On this hypothesis phenazine methosulphate might give 

 higher rates of phosphorylation at high light intensity when there is a rapid 

 flux of electrons from excited chlorophyll. Assuming, however, several 

 sites of phosphorylation in the cyclic pathway, the advantage of phenazine 

 methosulphate might disappear at loic light intensity when the overall rate 



Low light Intensity 

 (3000 Lux) 



vit. K, 



PMS 



10 15 20 25 

 minutes 



Fig. 9. Effect of vitamin K^ and phenazine methosulphate (PMS) on anaerobic 

 cyclic photophosphorylation in spinach chloroplasts at a limiting light intensity 

 (3000 Lux). Reaction mixture included chloroplast fragments (C,,) containing i mg. 

 chlorophyll and in micromoles : tris buffer, pH 8-3, 80; K2H^'-P04, 15; MgS04, 

 5; ADP, 15; vitamin K:, or PMS, 03; gas phase nitrogen (Tsujimoto, Hall, 

 and Arnon [92]). 



of the process is limited by the electron flux. Under such conditions the 

 highest rate of photophosphorylation would be observed in a system in 

 which none of the phosphorylation sites was bypassed. Thus, a com- 

 parison of photosynthetic phosphorylation, catalyzed by vitamin K and 

 phenazine methosulphate under conditions of limiting light, seemed 

 desirable. 



The results of such a comparison are shown in Y'\<g. 9. At low light 

 intensity photophosphorylation catalyzed by vitamin Kg was markedly 

 greater than that catalyzed by phenazine methosulphate (or pyocyanin). 

 This diflFerence was persistent and gave a straight-line relationship for a 

 considerable period of time. 



