ELECTRON TRANSPORT IN R. RUBRUM 



295 



(RF) is present, the ascorbate- induced photophosphorylation is maxi- 

 mally accelerated (Fig. 14), but the rate is depressed in the presence 

 of higher concentrations of riboflavin (43). FMN and FAD accelerate 

 photophosphorylation in the same manner, but at much higher concen- 

 trations. The ascorbate-induced and the PMS-inducedphotophosphory- 

 lations are also inhibited by atabrine to the same extent, with a Km 

 for atabrine of 2 x 10-5 M. 



ATP and PP were found to be powerful inhibitors of the photo- 

 phosphorylation; the lower the concentration of ADP present, the 

 greater was the inhibition by either ATP or PP (Fig. 15) (56). Kinetic 

 studies showed that the inhibition by ATP was competitive with Pi, but 

 uncompetitive with ADP, while PP was competitive with both ADP and 

 Pi. 



Log (PYROPHOSPHATE ADDED^Nl) 



Fig. 15. Effect of varied concentration of pyrophosphate on 

 photophosphorylation by R. nibriim chromatophores. Experi- 

 mental conditions were the same as for Fig. 12, except that the 

 concentration of ascorbate was fixed at 0.07 M. 



