506 LIGHT AND LIFE 



an electron transport chain which catalyzed a stepwise recombination 

 of the products ol photodeconiposition of water (6) . It was suggested 

 that the energy liberated in this stepwise reconstitution of water be- 

 came available for ATP formation. The role of the first electron 

 or hydrogen acceptor was tentatively assigned to either FMN (6) 

 or, as proposed by Wessels (167) , to vitamin K (6, 16, 12) . 



Further studies of photosynthetic phosphorylation made it apparent 

 that the initial hypothesis required revision. TPN, not included in 

 earlier formulations, was identified as a catalyst of photosynthetic 

 phosphorylation (17, cf. 101) . Evidence Avas also obtained that, 

 under modified experimental conditions, FMN and vitamin K may 

 be active in catalyzing separate pathways of photosynthetic phos- 

 phorylation (171, 172). Maximal rates of phosphorylation occurred 

 with either FMN or vitamin K as catalysts. At a saturating con- 

 centration of one catalyst, the addition of the other gave no further 

 increase in photosynthetic phosphorylation, but little phosphorylation 

 occurred unless one of them was added. 



The distinction between the FMN and the vitamin K pathway of 

 photosynthetic phosphorylation was made on the basis of their dif- 

 ferential dependence on TPN and differential sensitivity to dinitro- 

 phenol and o-phenanthroline. A similar conclusion that FMN and 

 vitamin K catalyze alternative pathways for electron transport in 

 photosynthetic phosphorylation was independently reached by Wessels 

 (168), but on the basis of findings which in some respects did not 

 agree with our own (171, 172). 



Apart from the catalytic effect of FMN and vitamin K, photosyn- 

 thetic phosphorylation may also be increased by the addition of non- 

 physiological cofactors (76) . Among these, of particular interest is 

 phenazine methosullate, since this dye is knoAvn to be a strong re- 

 ducing agent for cytochromes (105) . Phenazine methosulfate was 

 foimd to stimulate jihotosynthetic phosphorylation in bacterial prep- 

 arations by Geller (59) and Kamen and Newton (81) and in spinach 

 (hlorop)'ists by Jagendorf and Avron (76). 



With respect to inhibitors, the phenazine methosullate system re- 

 sembled the vitamin K pathway in its resistance to dinitrophenol and 

 o-phenanthroline (172) . Hut in other respects phenazine methosulfate 

 differed from either the FMN or the vitamin K pathways. The use 

 of jjhenazine methosulfate in i)h()ioph()sj)horylation seemed to dis- 

 pense with the requirement for other cofactors, such as ascorbate, and 

 also gave higher rates of ]}hotophosphorylation than were obtained 

 with either FMN or vitamin K (76). 



