ELECTRON TRANSPORT IN R. RUBRUM 



299 



illumination ceased (Fig. 17). This dephosphorylation of "bound" ATP is 

 distinct from dephosphorylation of externally added ATP since washed 

 chromatophores dephosphorylated externally added ATP into ADP and 

 Pi at a negligible rate (less than one-tenth) either in light or in dark- 

 ness. Also, the dephosphorylation of the "bound" ATP in the dark 

 period was influenced by the addition of either oxidant (ferricyanide) or 

 reductant (ascorbate) in the same manner as photosynthetic ATP 

 formation was. It is probable that, when the light is turned off, the 

 "bound" ATP is converted into "bound" ADP and free Pi by the back- 

 ward reaction of photophosphorylation. The backward reaction of 



4 8 12 



TIME (MINUTES) 



Fig. 19. Effect of "light on" and "light off on ATP forma- 

 tion from bound ADP in "less intact" chromatophores of R. 

 nibruni. Experimental conditions were the same as for Fig. 

 18, except that R. nibtiim chromatophores (S) were used 

 instead of chromatophores (G). Pi32 was incorporated into 

 the organic phosphate fraction by the reactions standing in 

 darkness throughout the reaction time. The compound cap- 

 able of incorporating Pi32 in darkness liberated all the in- 

 corporated Pi32 by boiling in N-HCl for 7 minutes, but was 

 not ATP. Identification of this compound is under investiga- 

 tions. 



