614 



Mordhay Avron and Noun Shavit 



coupled to the reduction of ferricyonlde. It Is evident that whereas atebrin and methyl- 

 ammonium chloride Inhibited both types of phosphorylation to a similar degree, P-CF3O-CCP 

 was a much more potent Inhibitor of the phosphorylation associated with ferricyonlde reduc- 

 tion than of that dependent upon phenazine methosulfate. The simplest interpretation of 

 such data seems to us to invoke two different sites for ATP production. One associated with 

 the phenazine methosulfate dependent photophosphorylation, and another associated with 

 that coupled to ferricyonlde reduction. It may also be recalled that P-CF3O-CCP has been 

 previously noted to be the only known uncoupler of photophosphorylation which also uncoup- 

 les oxidative phosphorylation at similar concentrations. It Is therefore likely that it acts 

 at a site which differs from the site of action of the other uncoupling agents. 



To substantiate such a conclusion It was thought important to repeat these observations 

 under conditions where the large difference in rate between the phenazine methosulfate and 

 ferricyonlde dependent phosphorylations was not so apparent. This was achieved by lowering 

 the light intensity until It became the limiting factor in the rate of photophosphorylation. 



THOSULFATE 



100 160 



Fig. 5. The effect of light Intensity on the rote of ferricyonlde and phenazine 

 methosulfate dependent phosphorylations and their inhibition by carbonyl 

 cyanide p-trifluoromethoxyphenylhydrazone (p-CFsO-CCP). 

 Details as described under Fig. 1. 



The left side of Fig. 5 illustrates the response of both types of photophosphorylation to 

 different light Intensities. It is evident that whereas the rate of phenazine methosulfate 

 dependent phosphorylation was about four fold higher than that of the one coupled to ferri- 

 cyonlde reduction at 160,000 lux, the two rates were essentidl/ equal at 4,000 lux. The 

 extent of inhibition of both systems by P-CF3-O -CCP at different light intensities is plotted 

 on the right side of Fig. 5. It is evident that the relative inhibition of the two systems does 

 not vary significantly as one lowers the light intensity, and so equalizes the rates of photo- 

 phosphorylation In the two systems. This strengthens In our opinions, the suggetion that 

 two different sites of photophosphorylation are present In the two systems. 



The relation of the extent of uncoupling and inhibition to light intensity 



A curious, and as yet uninterpreted phenomenon, which became evident as these studies 

 progressed was the much stronger inhibition of photophosphorylation by the uncoupler as the 

 light intensity was lowered (Fig. 5, right). Fig. 6 Illustrates that this effect was found not 

 only with P-CF3O-CCP, but with all the uncouplers tried. It is also very strongly evident 

 in the case of the inhibition by 3-(3,4, dichlorophenyl)-!, 1, -dimethylurea (DCMU). The 



