DANIEL I. ARNON. 



521 



Ci 







10000 20000 30000 40000 50000 

 light intensify (Lux) 



Fig. 10. Effect of light intensity on anaerobic cyclic photophosphorylation. Gas 

 phase, nitrosjen. Illumination period. 30 minutes. The reaction mixture included 

 cliloropiast fragments (C,s) containing 0.1 mg chlorophyll and chloroplast extract 

 equivalent to 1 mg chlorophyll. 0.3 ^moles TPN and 0.3 yamoles FMN were included 

 in the FMN system. Other conditions as given for Fig. 9. 



limiting step. These findings provide the first indication of enzymatic 

 steps that may limit cyclic photophosphorylation at high light in- 

 tensities, when physiological catalysts such as vitamin K or FMN (or 

 their analogues) are involved. 



The higher rates of photophosphorylation catalyzed by vitamin K 

 and FMN than those catalyzed by phenazine methosulfate, when light 

 is limiting (Fig. 10) , suggest the involvement of at least two phos- 

 phorylation sites in the vitamin K and FMN pathways. A diagram- 

 matic representation of this mechanism is given in Fig. 5. 



A primary phosphorylation reaction, common to all pathways of 

 cyclic photophosphorylation, is envisaged as being coupled with the 

 oxidation of the terminal cytochrome in the photosynthetic particle, 

 i.e., the cytochrome that reacts with the excited chlorophyll molecule 

 (compare Figs. 4 and 5) . A second site of phosphorylation in the 

 vitamin K pathway is likely to occur upon oxidation of reduced 

 vitamin K (or its analogue) by cytochromes, as was suggested by the 

 model reactions proposed by Wessels (167) , Harrison (68) , and Clark, 

 Kirby, and Todd (38) . In the FMN pathway, an additional phos- 



