165 



R. P. Levine 



PM5- 



TPN.^ PPNR.^ -4 Chli-^ Cyf-F-^ y^ — x-^ Q-4 ^ Ch,a,-< HoO 





(' 



ac-115 

 ac-141 \ 



Ascorbate 



Figure 1. A Model of the Electron Transport System of Photosynthesis in 

 C. reinhardi as Deduced from an Investigation of Four Mutant Strains. 



(See text for explanation) 



accommodates the facts we have obtained so far. and we believe it contains the 

 least number of assumptions. Quite clearly, the model is not unique, for it 

 draws heavily upon findings of other investigators whose approach has been 

 somewhat different from ours. 



Two light-dependent reactions and at least 

 one light -independent reaction 



Evidence for the two different light-dependent reactions of C. reinhardi 

 shown in the model comes from results of experiments with all four mutant 

 strains. All of the data obtained for ac-115 and ac- Ul ^^' ^^' are consistent 

 with the hypothesis that these two strains are blocked in a reaction associated 

 with system IL in which a photoreductant is produced coupled with the oxidation 

 of water. Chloroplast fragments obtained from these strains showed no Hill 

 reaction activity and were unable to photoreduce TPN. It was predicted that 

 if the block were only in system IL chloroplast fragments could carry out the 

 photoreduction of TPN if a reductant were supplied implying that system 1 was 

 able to function in these two strains. This prediction was borne out when it 

 was shown that chloroplast fragments of ac- 115 and ac- 141 could photoreduce 

 TPN from DPIP and. ascorbate. In this respect both strains resemble wild 

 type which has been inhibited with DCMU or o-phenanthroline, for under these 

 conditions wild type chloroplast fragments can photoreduce TPN in the presence 

 of DPIP and ascorbate. 



Further confirmation of the ability of these two mutant strains to carry 

 out part of the photosynthetic electron transport was obtained when it was es- 

 tablished that both were able to do cyclic photophosphorylation with PMS and 

 to fix carbon dioxide by photoreduction. 



