DANIEL I. ARNON 519 



cytochroines which h;itl I)cen oxidized in hght support the electron 

 flow theory (9) lor cychc photophosphoryhition. This theory assigns 

 to vitamin K. or some analogous quinone, the role of an intermediate 

 electron carrier in the electron transport chain associated with 

 photophosphorylation''. 



10. Multiple Sites in Cyclic Photophosphorylation 

 As already discussed, cyclic photophosphorylation is catalyzed by 

 vitamin K and FMN and also by non-physiological factors such as 

 phenazine methosulfatc. The latter fact has given rise to questions 

 whether vitamin K and FMN, or equivalent quinone and flavin con- 

 stituents of photosynthetic tissues, are to be considered the physio- 

 logical catalysts of cyclic photophosphorylation or whether they are 

 to be regarded as non-specific agents in no way distinguishable from 

 non-physiological catalysts. The question w^as of special interest in 

 connection with phenazine methosulfate because, as already men- 

 tioned, this dye has given rates of photosynthetic phosphorylation 

 higher than either vitamin K or FMN (cf. 76) . 



The marked effectiveness of phenazine methosulfate could be ex- 

 plained by its acting as an electron carrier that bypasses a rate- 

 limiting step in photosynthetic phosphorylation. This is suggested 

 by the observation of Geller (59) that the severe inhibition of photo 

 phosphorylation in R. rubrum by antimycin A does not occur in the 

 presence of phenazine methosulfate. On this hypothesis, j^henazine 

 methosulfate might give higher rates of phosphorylation at high 

 light intensity when there is a rapid flux of electrons from excited 

 chlorophyll. Assuming, however, several sites of phosphorylation in 

 the cyclic pathway, the advantage of phenazine methosulfate might 

 disappear at low light intensity when the overall rate of the process 

 is limited by the electron flux. Under such conditions the highest 

 rate of photophosphorylation would be observed in a system in which 

 none of the phosphorylation sites was bypassed. Thus, a comparison 

 of photosynthetic phosphorylation, catalyzed by vitamin K or phena- 

 zine methosulfate under conditions of limiting light, seemed desirable. 

 The results of such a comparison are shown in Fig. 9. At low 

 light intensity photophosphorylation catalyzed by vitamin Kg was 

 markedly greater than that catalyzed by phenazine methosulfate or 



'^ Added note: After this Symposium, new experimental evidence for the electron 

 flow theory has come from the work of Arnold and Clayton (4), who, on illuminat- 

 ing bacterial chromatophores, observed temperature-independent (1°K to 300°K) 

 reversible spectral changes near the absorption bands of bacteriochloroph)Il. 



