250 



F. R. Whatley 



PMS in a similar fashion emphasizes the idea that redox reagents 

 other than oxygen can regulate ("poise") the system by partly 

 oxidizing the intermediates. It appears likely from the results 

 in Table 1 that even PMS itself in larger amounts (IO-3O fagrams) 

 can bring about the redox regulation of the electron transport 

 chain. When photoreaction B was prevented by the addition of 

 the specific inhibitor, CMU, the flow of electrons from water 

 to the electron transport chain was stopped, and no overreduc- 

 tion could occur. There was thus no need for oxygen as a regu- 

 lator under argon in the presence of CMU (Table 4). On the 

 other hand, in the presence of air some contribution of elec- 

 trons from water by photoreaction B appears to be necessary in 

 order to maintain a suitable redox balance. The addition of 

 CMU caused a large inhibition of cyclic photophosphorylation in 

 air (Table 4), probably because oxygen overoxidized the inter- 

 mediates of the electron transport chain, thus preventing a 

 supply of electrons to photoreaction A. 



A possible explanation of the result shown in Table 1 might 

 have been that in air the PMS is converted to the oxidation 

 product pyocyanine (cf. Hill and Walker, 13) ^^<^ that pyocyanine 

 functions more effectively as a catalyst at lower concentrations 

 than PMS. However, as is shown in Table 5> pyocyanine-catalyzed 

 cyclic photophosphorylation behaves like the PMS catalyzed sys- 

 tem in its reaction toward aerobic and anaerobic conditions and 

 towards CMU. 



Experimental conditions as described in legend to Table 4, 

 except that pyocyanine was substituted for PMS. 



Thus with PMS or pyocyanine as the cofactor, just as with 

 ferredoxin, oxygen is able to play a regulatory role to combat 

 the apparent tendency of photoreaction B to overreduce the 

 intermediates of the electron transport chain. When photo- 

 reaction B is suppressed the system is already in a suitable 



