DANIEL I. ARNON 



543 



LIMITING LIGHT and HIGH CHLOROPHYLL 



12 



-10 



0.003 0.01 0.05 0.1 03 1.0 0.003 0.01 0.05 0.1 03 1.0 



^moles vitKs odded /umoles FMN added 



Fig. 20. Effect of FMN and vitamin Kg concentration on cyclic photophosphoryla- 

 tion by spinach chloroplasts in nitrogen and air, at a low light intensity. The 

 reaction mixture included, in a final volume of 3.0 ml, chloroplast fragments 

 (Ci,) containing 1.5 mg chlorophyll; and in micromoles: Tris buffer, pYi 8.3, 80; 

 MgSO,. ,".: K,HP-0,. 15; ADI', 15 TPN, 0.3 (only in the FMN series). FMN or 

 vitamin K^, was added as indicated. The reaction was run for 30 minutes at an 

 illumination of 2,000 Lux (Tsujimoto, Hall, and .\rnon, 15()). 



of the chloroplast factor (s) than were required for the anaerobic 

 FMN system. These chloroplast factors for the FMN system appeared 

 to be bound in the grana fraction and were not supplied by an 

 aqueous extract of chloroplasts. 



On comparing the aerobic and anaerobic systems under conditions 

 when they responded optimally to the addition of cofactors, a marked 

 difference was observed, depending on the presence or absence of 

 oxygen, in the effect of two inhibitors, o-phenanthroline and CMU 

 [p-chlorophenyl dimethylurea] (a gift of Dr. C. E. Hoffman) . The 

 results are shown in Table 11. 



In agreement with findings of Wessels (169) , Jagendorf and Avron 

 (77), and Nakamoto et al. (Ill) , o-phenanthroline and CMU, in the 

 presence of air, inhibited photophosphorylation in the FMN and 

 vitamin K systems. Relatively little inhibition by these two inhibitors 

 was observed in an atmosphere of nitrogen. In other experiments, 

 not reported here, phenazine methosulfate was found to differ from 

 FMN and vitamin K in that its pathway was resistant to inhibition 

 by o-phenanthroline and CMU, both in air and in nitrogen. 



o-Phenanthroline and CMU are powerful inhibitors of oxygen 

 evolution by illuminated chloroplasts (cf. 169, 77, HI). It seems 



