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Daniel I. Arnon 



recently obtained by Tagawa et al. (11) who found that ferredoxin 

 catalyzes an anaerobic cyclic photophosphorylation in chloroplasts 

 which proceeds in the presence of CMU and without the addition of 

 other cofactors. A notable feature of this endogenous, ferre- 

 doxin-catalyzed cyclic photophosphorylation, which distinguishes 

 it from other types of cyclic photophosphorylation in chloro- 

 plasts, is its sensitivity to antimycin A and to low concentra- 

 tions of dinitrophenol (11,12). Since, in mitochondria antimycin 

 A inhibition is considered to be indicative of the participation 

 of cytochrome b in electron transport (50>51)> the sensitivity 

 to antimycin A suggests a possible participation of the cyto- 

 chrome bg component of chloroplasts (52,53) in the ferredoxin- 

 catalyzed cyclic photophosphorylation. 



A previously puzzling feature of cyclic photophosphorylation 

 in isolated chloroplasts, a feature which distinguished it from 

 cyclic photophosphorylation in bacterial chroma tophores, was a 

 dependence on an added electron carrier such as vitamin K or 

 phenazine methosulfate. A possible, though heretofore experi- 

 mentally unsupported, explanation of this difference was that 

 chloroplasts, but not chroma tophores, lost a soluble constituent 

 in the process of isolation. The recent findings point to chloro- 

 plast ferredoxin as being the water-soluble constituent of cyclic 

 photophosphorylation which is, at least in part, lost from chlcto- 

 p lasts when they are removed from the cell. However, it is still 

 premature to say what role bacterial ferredoxins play in the 

 mechanism of bacterial photophosphorylation. 



As previously mentioned, ferredoxin appears to be a junction 

 in chloroplasts for the electron transport systems that lead to 

 either cyclic or noncyclic photophosphorylation. Since we now 

 know that TPN is required for noncyclic photophosphorylation but 

 not for cyclic photophosphorylation, it follows that when the 

 photoreduced ferredoxin is reoxidized by TPN ( via the flavopro- 

 tein reductase), noncyclic photophosphorylation would result. 

 When oxidized TPN is unavailable as an electron acceptor, the 

 photoreduced ferredoxin would be reoxidized (directly or indirect- 

 ly) by a cytochrome component of the grana, and cyclic photophos- 

 phorylation would result. It is thus possible to envisage that 

 the availability of TPN as an electron acceptor might serve as a 

 physiological regulator between cyclic and noncyclic photophos- 

 phorylation. Evidence for this view has recently been reported 

 (11). 



Analysis of chloroplast reactions with monochromatic light . 

 The photoreduction of ferredoxin and the resultant cyclic or 



