169 



R. P. Levine 



chloroplasts(38) and undergo reduction in chloroplasts in the light' •^^"^^'. Fur- 

 thermore, it has been shown that they are essential for the Hill reaction^ 2'' 41-45)^ 

 the photoreduction of TPlsK^^), and for cyclic photophosphorylation(47, 48), Thus, 

 it appears that different plasto- and tocopherylquinones may function at different 

 sites in the electron transport system. 



The low plastoquinone content of both ac- 1 15 and ac- 141 is not sufficient 

 to account for the complete absence of a Hill reaction in these strains, at least 

 on the basis of the results obtained by Bishop^^^' for sugar beet chloroplasts. 

 However, it is possible that these two mutant strains lack or are deficient in a 

 specific plasto- or tocopherylquinone that functions at a site associated with 

 system II and the oxygen evolving portion of the system, whereas the quinones 

 that remain act at some different site or sites. 



The cytochrome f content of both ac- 1 15 and ac- 14n ^2) ^g about three to 

 four times greater than that of wild type and ac-21. Coupled with this increase 

 there is at least a doubling of the rate of TPN reduction from DPIP and ascorbate 

 (Table I), and also in cytochrome photo-oxidase activity( 10). These observa- 

 tions are consistent with the idea of Hill and Bendall'^) that cytochrome £is in- 

 volved with system I, and with the observations of the light -dependent oxidation 

 of cytochrome f in algae by system l(35-37). 



CONCLUSIONS 



Though our model for the electron transport system of photosynthesis in 

 C. reinhardi lacks complete documentation, it most easily accommodates the 

 observations that have been made with the wild type and four mutant strains. 

 All of the data presented are consistent with a model for electron transport in 

 which there are two light-dependent reactions separated by at least one light- 

 independent reaction. The data suggest that there is a single site for non- 

 cyclic photophosphorylation, aAd that cyclic photophosphorylation can occur 

 independently of the oxygen evolving portion of the system. In addition, one 

 or more plasto- or tocopherylquinones may play an integral role in the electron 

 transport system. 



The model presented here has the advantage of providing several predic- 

 tions that can be tested experimentally. For example, if ac- 2J_ lacks compo- 

 nent X (Fig. 1), then it should be possible to detect the oxidation of cytochrome £ 

 by system I but not its reduction by system II. Further, both ac- 115 and ac- 141 

 may show the oxidation of cytochrome f in the light followed by its reduction in 

 the dark(35). in contrast, if the block in ac-208 lies in system I the mutant 

 strain would be expected to show the reduction of cytochrome £by system II 

 but not its oxidation by system I. 



In addition to testing predictions such as these, the use of the mutant 

 strains provides an opportunity to search for the function of different possible 

 components of the electron transport system, for as mentioned at the outset 

 of this discussion, mutations may have occurred at gene loci that affect the 



