167 



R. P. Levine 



with G. Gingras, measurements of the action spectrum of the ferricyanide Hill 

 reaction by chloroplast fragments of both wild type and ££-^- These measure- 

 ments, obtained with the aid of an oxygen electrode, have revealed so far that 

 the action spectrum for wild type is very similar to the action spectrum for 

 photosynthetic oxygen evolution by whole cells. 



Photophosphorylation 



Additional important information for the design of the model was obtained 

 from an investigation of photophosphorylation by wild type and the four mutant 

 strains. 



Photophosphorylation has been studied in wild type in several different 

 ways(ll). Cyclic photophosphorylation was obtained with either PMS or the 

 combination of vitamin K3 and FMN as electron carriers. Cyclic photophos- 

 phorylation with PPNR^-^^) has not been tested. Non-cyclic photophosphoryla- 

 tion coupled to the photoreduction of TPN was found to occur with either water 

 or DPIP and ascorbate as the reductant. However, non-cyclic photophos- 

 phorylation coupled to ferricyanide reduction could not be demonstrated. 



The ratio of one ATP produced per two electrons transferred during non- 

 cyclic photophosphorylation in C. reinhardi suggests that there is only one site 

 for non-cyclic photophosphorylation. Furthermore, in agreement with Losada, 

 Whatley, and Arnon^"^'^), non-cyclic phosphorylation lies at a point after the 

 entry of electrons from DPIP and ascorbate into the electron transport system. 



In the model for electron transport in C. reinhardi under consideration 

 here, the site for photophosphorylation has been placed tentatively between X 

 (a component of the electron transport system and the assumed point of entry 

 of electrons from DPIP and ascorbate) and cytochrome f. A photophosphoryla- 

 tion at this site would, accordingly, be coupled to the oxidation of a reduced X 

 and the reduction of an oxidized cytochrome f. Recently, Forti, Bertole, and 

 Parisi^'^'*) have shown that photophosphorylation in spinach chloroplasts can be 

 coupled to the reduction of cytochrome f and that the stoichiometry is one ATP 

 produced per two electrons transferred. It has also been shown that cyto- 

 chrome f can be oxidized by system l(35-37)_ Further, in Anacystis nidulans (35) 



cytochrome f can be reduced by system II. 



Cyclic photophosphorylation with either PMS, or the combination of 

 vitamin K3 and FMN, was obtained only with the mutant strains ac- 1 15 and 

 ac- 14I . This indicates that the cyclic pathway enters the electron transport 

 system after the block in these two strains and, accordingly, does not depend 

 upon a photoreductant produced in system II. The fact that neither ac-2_l^ nor 

 ac-208 gave cyclic photophosphorylation supports the contention that these 

 strains are blocked at sites after the point of entry of the cyclic pathway into 

 the electron transport system. 



