232 



C, C. Black and A. San Pietro 



zoo 



too, c^. 



E I 

 a. o 



50 



L— + PMS 



./ 



-0- +NADP 



■*^ _^'*' Phosphodoxin 



only 



+ Ferncyanide 



.05 



.1 15 ,2 



il of Phosphodoxin 



Fig. 5. (Left) Effect of inhibitors on photophosphorylation with 

 spinach chloroplast fragments plus spinach phosphodoxin. 

 Fig. 6. (Right) Effect of spinach phosphodoxin on photophos- 

 phorylation with spinach chloroplast fragments in the presence 

 of other electron acceptors. 



ELECTRON PARAMAGNETIC RESONANCE SIGNAL 



The EPR signals observed with spinach phosphodoxin have been studied 

 in cooperation with Dr. John Heise ^^4) Aqueous solutions of spinach phos- 

 phodoxin exhibit a light- induced, pH- dependent EPR signal. The intensity of 

 the EPR signal is increased with alkaline conditions in a fashion similar to 

 the relative fluorescence intensity (Fig. 2). No dark EPR signal is observed 

 with spinach phosphodoxin alone and the light- induced signal decays in the 

 dark. 



Figure 8 demonstrates the effects of spinach phosphodoxin on the EPR 

 signal of spinach chloroplast fragments in 4. 2 x 10" 2 M Tris-HCl buffer, 

 pH 7. 8. The small characteristic light- induced EPR signal of spinach 

 chloroplast fragments alone (25) can be observed by comparing curves 1 and 

 4 of Fig. 8. A sharp decrease in the dark EPR signal of spinach chloroplast 

 fragments upon the addition of spinach phosphodoxin can be observed by com- 

 paring curves 1 and 2 of Fig. 8. Upon illumination of spinach chloroplast 

 fragments plus spinach phosphodoxin, an increased EPR signal was observed 

 (compare curves 3 and 4 of Fig. 8). This increased EPR signal was observed 

 immediately upon illumination and decayed with continuous illumination. 

 Examination of the data indicates that spinach phosphodoxin contributes 

 primarily to signal 2 ^25) of spinach chloroplast fragments '24)^ 



