CYCLIC AND NONCYCLIC PHOTO PHOSPHORYLATION 191 



capacity for DPN reduction but, as mentioned previously, they retained 

 their capacity for cyclic photophosphorylation even after storage for 

 30 days. 



DISCUSSION 



The results of this investigation confirm and extend the previous 

 finding of noncyclic photophosphorylation in chromatophores of R. 

 rubrum (22). The capacity of R. rubrum chromatophores to catalyze 

 a noncyclic electron flow, i.e., a light-driven, "uphill," unidirectional 

 electron transfer against the thermodynamic gradient, has already 

 been seen in the photoreduction of DPN by the ascorbate-DPIP couple 

 (19-21). Our experimental findings provide evidence that this non- 

 cyclic electron flow in chromatophores is coupled with ATP formation 

 under experimental conditions which exclude ATP formation by cyclic 

 photophosphorylation . 



Noncyclic photophosphorylation is distinguished from cyclic photo- 

 phosphorylation in chromatophores by its joint dependence on an ex- 

 ternal electron donor system (ascorbate-DPIP couple) and an external 

 electron acceptor (DPN). Losada et al. showed (13) that a similar 

 "bacterial type" of noncyclic photophosphorylation, in which the 

 ascorbate-DPIP couple is the electron donor and TPN is the electron 

 acceptor, can be carried out by spinach chloroplasts once the use of 

 the natural electron donor for chloroplasts, water (0H~), is experi- 

 mentally suppressed. 



Bose and Gest have recently argued (23) that the noncyclic photo- 

 phosphorylation which we have previously found in R. rubrum chroma- 

 tophores is, in fact, a cyclic photophosphorylation catalyzed by the 

 dye DPIP, which acts as a bypass for the antimycin A- sensitive site. 

 They explain the joint requirement for an added reductant (ascorbate 

 + DPIP) and oxidant (DPN) as resulting from "their action in estab- 

 lishing a redox environment which permits efficient operation of 

 cyclic LIP [photophosphorylation]" (23). 



The experiment reported by Bose and Gest (Exp. II, Table 6 in 

 ref. 23) which comes closest to ours was carried out under hydrogen 

 gas, in the presence of 1 /imole DPN, 0.2 //moles DPIP and 0.2 fimoles 

 of ascorbate (in 3 ml), i.e., under conditions where the system was not 

 overreduced. The relevance of this experiment and the accompanying 

 arguments to our previous experiments and to those reported now is 

 not apparent. Our experiments (for example. Table 2 in ref. 22) were 

 carried out under argon gas, in the presence of 2 //moles DPN, 

 0,2 //moles DPIP, and 20/<moles ascorbate (in 3 ml), i.e., in the pres- 

 ence of 100 times more ascorbate than used by Bose and Gest, thereby 

 bringing about a degree of "overreduction" which suppresses the cyclic 

 photophosphorylation that might otherwise have been promoted by 



