168 



R. P. Levine 



Since ac-21 can photoreduce TPK fronn DPIP and ascorbate it was 

 assumed thaTT like wild type, it would also carry out photophosphorylation with 

 DPIP and ascorbate; thus, it would be possible to localize the block in this 

 mutant strain to a position before the site of entry of electrons from DPIP and 

 ascorbate. As mentioned earlier it has thus far been impossible to obtain 

 photophosphorylation in this manner with ac-2J. even though TPN is reduced. 



There are several ways one may interpret the results obtained with ac - 2 1 . 

 First, it might be assumed that there is a block between X and cytochrome f, 

 and in order to obtain TPN photoreduction from DPIP and ascorbate, electrons 

 from these donors must enter the system at a point after X and the site of 

 phosphorylation. This might be at cytochrome £or at Chlj. Second, the block 

 might be in the formation of X itself; it may be either lacking, deficient, or 

 inactive. Once again, in order to explain TPN photoreduction from DPIP and 

 ascorbate, the electrons would have to enter at some alternate site. Third, X 

 might be essential for both electron transport and for the coupling of phos- 

 phorylation to the electron transport system. Consequently, if X were lacking, 

 deficient, or inactive in ac-22^ both electron transport and phosphorylation would 

 be blocked. 



P hotoreduction and cyclic photophosphorylation 



A mechanism exists in C. reinhardi for the production of ATP independ- 

 ently from the oxygen evolving" pathway of photosynthesis. Wild type cells can 

 fix carbon dioxide by photoreduction in the presence of a concentration of DCMU 

 that almost completely inhibits both photosynthetic oxygen evolution and carbon 

 dioxide fixation. Photoreduction also occurs in cells of ac- 115 and ac- 141 where 

 the oxygen evolving pathway is blocked as a consequence of mutation. Both of 

 these mutant strains were also found to carry out cyclic photophosphorylation 

 with PMS, thus confirming the finding of several investigators that photophos- 

 phorylation with PMS is independent of the oxygen evolving pathway. 



These results suggest that ATP is generated during photoreduction by a 

 process of cyclic photophosphorylation, and photoreduction in C. reinhardi , 

 therefore, may be similar to bacterial photosynthesis. That is, a hydrogenase 

 acts to reduce TPN by a light-independent reaction and ATP is produced via 

 cyclic phosphorylation. However, the possibility that the hydrogenase may 

 provide electrons for the production of ATP by a non-cyclic photophosphoryla- 

 tion cannot be excluded. 



Cyt ochrome f and quinones 



Among the possible components of the electron transport system in 

 C. reinhardi, the quinones and cytochrome £are the only ones for which marked 

 quantitative differences have been foundn2). xhe low level of plastoquinone in 

 both ac-115 and ac- 141 is of particular interest. The plasto- and tocopheryl- 

 quinolies have be'eii implicated in photosynthesis because they are localized in 



