390 DANIEL I. ARNON 



lower concentrations of chloroplast material suggesting that it required 

 less or fewer 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-chloro- 

 phenyldimethylurea) (a gift of Dr. C. E. Hoffman). The results are shown 

 in Table XV." 



TABLE XV 



Effect of o-Phenanthroline (o-P) and Dichlorophenyldimethylurea 

 (CMU) on Cyclic Photophosphorylation in Nitrogen or Air 



(Tsujimoto, Hall, and Arnon [92] 



Percentage inhibition 



Treatment , * , 



CMU o-P 



Nitrogen, FMN 25 20 



Nitrogen, vit. K3 19 27 



Air, FMN 97 77 



Air, vit. K3 85 64 



In the nitrogen series the illumination was 2000 Lux for 30 min. and the 

 reaction mixture included, in a final volume of 3 ml. chloroplast fragments (Ci,) 

 containing i • 5 mg. chlorophyll and o ■ 3 /umole of FMN or vit. Kg. In the air 

 series the illumination was 50 000 Lux for 5 min. and the reaction mixture in- 

 cluded chloroplast fragments (Cj,) containing i mg. chlorophyll and 0-003 Mi^iole 

 of FMN or vitamin K3. The final inhibitor concentrations were, 3 x 10^^ M 

 for o-phenanthroline and 2 x 10" m for CMU. Other common components of 

 the reaction mixture were, in micromoles: tris buffer, pH 8-3, 80; K2H^-P04, 

 15; and MgS04, 5. 



In agreement with findings of Wessels [54], Jagendorf and Avron [55] 

 and Nakamoto et a/. [56], 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, phen- 

 azine methosulphate 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 evolu- 

 tion by illuminated chloroplasts (cf. [54-56]). It seems likely, therefore, 

 that as was concluded earlier by Wessels [54] and Nakamoto et al. [56], 



