362 DANIEL I, ARNON 



These experiments were extended by comparing the rates of photo- 

 synthetic phosphorylation at different hght intensities. The results shown 

 in Fig. 10 confirm and extend those illustrated in Fig. 9. At low intensity 

 cyclic photophosphorylation catalyzed by either vitamin K3 or FMN 

 proceeded at a much higher rate than that catalyzed by phenazine metho- 

 sulphate. However, at higher light intensities the phenazine methosulphate 

 system gave much greater rates of phosphorylation [92]. 



These results suggest that at high light intensity the vitamin K and 

 FMN systems became limited by enzymic reactions which were unable to 



10000 20000 30000 40000 50000 

 light intensity (Lux) 



Fig. io. Effect of light intensity on anaerobic cyclic photophosphorylation. 

 Gas phase nitrogen. Illumination period 30 min. The reaction mixture included 

 chloroplast fragments (C,,) containing 01 mg. chlorophyll and chloroplast 

 extract equivalent to i mg. chlorophyll. 0-3 /xmole TPN and 0-3 /xmole FMN 

 were included in the FMN system. Other conditions as given for Fig. 9 (Tsujimoto, 

 Hall, and Arnon [92]). 



keep pace with the rapid electron flux. The increasing rates of phos- 

 phorylation obtained at high light intensity with the phenazine metho- 

 sulphate system are consistent with the explanation that this agent does 

 indeed serve as a bypass around some rate-limiting step, probably by 

 catalyzing the reduction of cytochromes [90]. These findings are inter- 

 preted as an indication of enzymic steps that may limit cyclic photophos- 

 phorylation at high light intensities, when physiological catalysts such as 

 vitamin K or FMN (or their analogues) are involved. 



The findings that, when light is limiting, vitamin K and FMN catalyze 

 higher rates of photophosphorylation than phenazine methosulphate 

 (Fig. 10), suggest the involvement of at least two phosphorylation sites in 

 the vitamin K and FMN pathways. A diagrammatic representation of this 

 mechanism is given in Fig. 5. 



