410 



LIGHT AND LIFE 



preparations it survives prolonged sonication, lyophilization, cold 

 hexane extraction, probably complete removal of phycocyanin, and 

 a partial removal of chlorophyll a — in short, any treatment which 

 leaves a particulate chlorophyll protein complex intact. However, 

 it slowly tends to disappear with aging and disappears much faster 

 during dialysis. We observed that such inactive preparations could 

 be revived by the addition of 10-^ to 10-^ M phenazine methosul- 

 fate (PMS) (and with some reducing agents, such as ferrocyanide, 

 and palladium-hydrogen; PMS is probably reduced by endogenous 

 substrate) . Noteworthy also in table 2 is that ferricyanide, flavin 

 adenine mononucleotide (FMN) , and various dyes, often used as 

 Hill reagents, markedly inhibit the negative shift in the concentration 

 range 10-^ to 10-^ M. 



The positive light effect, on the other hand, is extremely suscepti- 

 ble to aging and heating and requires all the usual precautions to 

 retain activity in chloroplast preparations. 



The extreme sensitivity of the positive shift to 3 (3,4-dichloro- 

 phenyl)-l,l-dimethylurea (DCMU) , the most potent known inhibi- 

 tor of photosynthetic oxygen evolution (22) , is illustrated in Fig. 9. 

 The effect is reduced to zero at a concentration of 10-^ M. Note that 

 higher concentrations induce phycocyanin to sensitize the negative 

 shift, i.e., to act exactly like chlorophyll a. 



10-5 ji^ hydroxylamine causes complete inhibition, but no "flip- 

 over" effect is induced by higher concentrations of this poison. We 

 may conclude from these inhibitor data that the positive shift must 

 be intimately connected with the oxygen evolution reaction. 



TABLE II 



Effect of Various Agents on 700 my. Shift 



