520 



LIGHT AND LIFE 



o 



10 



15 20 25 



minutes 



Fig. 9. Effect of vitamin K3 and phenazine methosiilfate (PMS) on anaerobic 

 cyclic photophosphorylation in spinach chloroplasts at a limiting light intensity 

 (3000 Lux). Reaction mixture included chloroplast fragments (Cj,) containing 1 

 mg chlorophyll and in micromoles: Tris buffer, pH 8.3, 80; KoH^TO^, 15; MgSOi, 

 5; ADP, 15; vitamin K3 or PMS, 0.3; gas phase nitrogen (Tsujimoto, Hall, and 

 .\rnon, 156). 



pyocyanin. This difference was persistent and gave a straiglit-line 

 relationsliip for a considerable period of time. 



These experiments were extended by comparing the rates of photo- 

 synthetic phosphorylation at different light intensities. The results 

 shown in Fig. 10 confirm and extend those illustrated in Fig. 9. 

 At low light intensity, cyclic photophosphorylation catalyzed by either 

 vitamin K3 or FMN proceeded at a much higher rate than that 

 catalyzed by phenazine methosulfate. However, at higher light in- 

 tensities the phenazine methosulfate system gave much greater rates 

 of phosphorylation (15G) . 



These results suggest that at high light intensity the vitamin K 

 and FMN systems became limited by enzymatic reactions which were 

 unable to keep pace with the rapid electron flux. The increasing 

 rates of phosphorylation obtained at high light intensity with the 

 phenazine methosulfate system are consistent with the explanation 

 that tiiis agent docs indeed serve as a bypass around some rate- 



