BEN r LEY (.LASS 881 



tlic course ol bioclicinkal cvolulioii pholosyiilhcsis "liisl cincij^cd 

 as a process lor converting light energy into ATP and [that] this 

 'primitive' photosynthesis only later became a process linked to 

 COo reduction." The second part ol his present review constitutes 

 an examination of the evidence lor this theory. 



11 cyclic photophosphorylation is indeed a primitive lorm of photo- 

 synthesis in general, then one might ho])e to find in nature some 

 organism in which the only function of light in carbon assimilation, 

 at least inider some circmnstances, is that of forming ATP. Arnon's 

 group has been miable to provide an elegant demonstration of such a 

 situation in cell-free extracts of Clnoinatiion. This organism, when 

 living, utilizes either acetate or CO2 as the sole source of carbon. 

 \V^hen acetate is assimilated, no hyrdogen sotuxe is required; in the 

 case of CO2 hydrogen gas may serve as reductant. In the cell-free 

 extracts of Chromatium ATP is required for the assimilation of 

 either acetate or CO^, in the former case for activation of the carbon 

 source in forming acetyl-CoA, in the latter case for formation of 

 an activated intermediate (ribulose diphosphate, phosphoenolpyru- 

 vate, or 1,3-diphosphoglycerate) . Evidence that the action of light 

 in this system is solely that of supplying ATP was found by substitut- 

 ing exogenous ATP in the dark, and finding that assimilation went 

 on "in the same manner," although not, it must be confessed, quite 

 as actively. Since Chromatium is both an obligate anaerobe and 

 also an obligate photoautotroph, it cannot be expected to make ATP 

 by the usual oxidative phosphorylative mechanism. Thus it is 

 highly probable that photophosphorylation is the sole source of ATP 

 in this bacterium. Arnon cites R. rubrum, a facultative anaerobe, 

 as likewise utilizing light only for cyclic photophosphorylation, and 

 proposes that even in green plants photophosphorylation may con- 

 tinue under conditions when CO2 assimilation is stopped or greatly 

 reduced, as during the midday closure of stomata. 



Arnon's group was able in addition to demonstrate that Chroma- 

 tium hydrogenase can reduce DPN or TPN in the dark (although 

 R. ruhrum hydrogenase is unable to do so in a cell-free system) . 

 As in other bacterial systems, this reduction of DPN by hydrogenase 

 required the presence of benzyl viologen. Arnon regards this cell- 

 free system (in which ATP is formed by light, DPNH by a simple 

 dark hydrogenase) as representing the pathway in the living cell; 

 but the requirement for benzyl viologen shows that the cell-free 

 system is not quite identical with that in the intact cell. 



Various algal species contain hydrogenases that will assimilate 



