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F. R. What ley 



addition of CMU to the PMS system in air caused a profound in- 

 hibition of ATP formation (Column k) , which was only partly over- 

 come by ^0 iagrams of PMS. Jagendorf and Avron (9) have made 

 similar observations with both CMU and o-phenanthroline. These 

 results do not at first appear to support the conclusion that 

 PMS catalyzes a truly cyclic photophosphorylation, but are com- 

 patible with this conclusion when the data are interpreted in 

 terms of a "poising" action of oxygen (redox regulation). 



In a recent paper Tagawa et al. (8) have advanced such an 

 hypothesis to explain their results on cyclic photophosphoryla- 

 tion catalyzed by ferredoxin. They point out that noncyclic 

 electron flow is a unidirectional electron transfer from water 

 to TPN, driven by two photoreactions, B and A (systems 2 and 1, 

 respectively, in Duysens ' terminology (10)). The intermediates 

 in the electron transport chain will be kept in a partly re- 

 duced, partly oxidized state (i.e., "poised") as long as TPN 

 (or ultimately COg) is available, and no "overreduction" or 

 "overoxidation" can occur. However, to maintain a cyclic elec- 

 tron flow from a reduced cofactor back to the electron transport 

 chain, as is required for cyclic photophosphorylation, the inter 

 mediates of the electron chain must be kept in a suitable redox 

 balance. If they are kept fully reduced, as by photoreaction B, 

 they cannot accept electrons from the reduced cofactors. Tagawa 

 et al. proposed the hypothesis "that molecular oxygen normally 

 regulates the redox balance for the electron transport chain 

 involved in cyclic photophosphorylation by chlorop lasts . In 

 the presence of oxygen, the electron transport system does not 

 become overreduced by the flow of electrons from water. But 

 under anaerobic conditions the flow of electrons from photo- 

 reaction B overreduces the components of the electron transport 

 chain in chloroplasts and this overreduction cannot be counter- 

 balanced by the regulatory oxidizing action of oxygen." 



As shown in Table 1, the rate of ATP formation at lower con- 

 centrations of PMS was much less under argon than in air. This 

 is interpreted to mean that, under argon, electrons flowing from 

 photoreaction B overreduce the intermediates of the electron 

 transport chain; and this slows down or prevents the return of 

 electrons from photoreaction A via PMS to the chain. Oxygen 

 can counterbalance the overreduction by oxidizing some portion 

 of the intermediates and so bring about a regulation of the 

 electron flow. Smaller amounts of oxygen produce a proportion- 

 ately smaller response, although the action of oxygen is clearly 

 catalytic (Table 2). The fact that very small amounts of ferri- 

 cyanide (Table 3) stimulate the cyclic photophosphorylation with 



