CYCLIC AND NONCYCLIC PHOTO PHOSPHORYLATION 177 



2TPNH, 



2ATP + 4A 



(3) 



(4) 



2TPNH, 



2ADP 



O2 + 2H2O (2) 



This subdivision of noncyclic photophosphorylation of chloroplasts 

 into two component reactions revealed that Reaction 4, the noncyclic 

 photophosphorylation proper, was basically independent of oxygen 

 evolution and might, therefore, conceivably occur in photosynthetic 

 bacteria. According to this interpretation, noncyclic photophosphoryla- 

 tion in bacteria would lack Reaction 3, the photooxidation of water, 

 which in chloroplasts reduces an intermediate electron carrier, 

 probably plastoquinone (14,15), corresponding to A in Equation 3, In 

 photosynthetic bacteria, the intermediate donor (A inEq, 4) would not 

 come from a photochemical reaction but would be supplied by the 

 external medium, in accord with the well-known dependence of bac- 

 terial photosynthesis on such external hydrogen (or electron) donors 

 as thiosulfate or organic acids (12,16), 



The existence in photosynthetic bacteria of a noncyclic electron 

 transport pathway was suggested by the evidence in Chromatium cells 

 for a light- driven electron flow from thiosulfate as the electron donor 

 to H and N2 as the electron acceptors, resulting in the evolution of 

 H2 and reduction (fixation) of N2^ respectively (17,18). Likewise, the 

 photoreduction of pyridine nucleotide by succinate or the ascorbate- 

 DPIP couple in cell-free preparations of/?, rubrum (19-21) could also 

 be interpreted as evidence for a light-driven noncyclic electron flow 

 in photosynthetic bacteria. There was no evidence, however, that such 

 noncyclic, light- dependent electron transport in photosynthetic bacteria 

 could be coupled with a simultaneous ATP formation. 



Evidence for noncyclic photophosphorylation in chromatophores of 

 R. rubrum has recently been presented by Nozaki, Tagawa and Arnon 

 (22), They observed bacterial noncyclic photophosphorylation under 

 conditions when cyclic photophosphorylation was experimentally sup- 

 pressed, thereby making it possible to distinguish the ATP formed by 

 a noncyclic electron flow mechanism from that formed by a cyclic 

 mechanism. In chromatophores of R. rubrum, in which cyclic photo- 

 phosphorylation was made inoperative, ATP formation was coupled 

 with a light-dependent noncyclic electron flow from an ascorbate-DPIP 

 couple as the external electron donor to DPN as the terminal electron 

 acceptor. 



This article reports further work on the nature of bacterial cyclic 

 and noncyclic photophosphorylation in R. rubrum chromatophores, and 

 considers particularly the experimental conditions needed to demon- 



