368 DANIEL I, ARNON 



formation of ATP, was obtained by replacing light with a supply of 

 exogenous ATP and finding that carbon assimilation would then proceed 

 in the same manner in the dark as in the light [121]. Other evidence for 

 the equivalence of light and ATP is given in Table V. Here assimilation 

 occurred either in the dark with added ATP or in the light when ATP was 

 allowed to form photosynthetically. If, however, the ATP formed in light 

 was trapped by an added hexokinase-glucose system then acetate assimila- 

 tion ceased. The addition of hexokinase alone, without glucose as the ATP 

 acceptor, was not inhibitory (Table V). 



TABLE V 



Equivalence of ATP and Light in the Assimilation of "C-Acetate by 

 Cell-Free Preparations of Chromatinm 



(Losada, Trebst, Ogata, and Arnon [121]) 



^''Carbon fixed in 

 Treatment soluble compounds 



(Thousands of counts /min) 



1. Dark, control 27 



2. Dark, ATP 180 



3. Dark, ATP, hexokinase 186 



4. Dark, ATP, hexokinase, glucose 6 



5. Light, control 414 



6. Light, hexokinase 348 



7. Light, hexokinase, glucose 20 



Each vessel included, in a final volume of i -5 ml., cell-free extract, containing 

 o • 3 mg. bacteriochlorophyll and the following in micromoles : tris buffer, pH 7 • 8, 

 80 ; cysteine, 20 ; magnesium chloride, 5 ; manganese chloride, 2 ; potassium 

 chloride, 20; coenzyme A, 0-3; oxalacetate, 10; [i-^*C]-acetate, 3. 1-5 mg. hexo- 

 kinase, type III (Sigma Chemical Co.), 10 /xmoles glucose, and 4 /^imoles ATP 

 were added as indicated. In treatment 5, 6 and 7 no addition of ADP was necessary 

 to supplement the catalytic amounts present in the cell-free extracts. 



The experimental substitution of ATP for light was considered 

 particularly significant because it was found in photosynthetic bacteria 

 such as Chromatiiwi, that are unique in the living world in being strict 

 phototrophs. Chromotiiim, unlike, for example, Chlorella or photosynthetic 

 bacteria of the genus Rhodospir ilium, cannot replace its light-dependent 

 mode of life by a heterotrophic, aerobic metabolism in the dark [122, 123, 

 124]. Chromatinm grows only in the light [122, 123], and being an obligate 

 anaerobe, does not possess an alternative way for forming ATP by the 

 mechanism of oxidative phosphorylation. 



As regards the photoassimilation of acetate in another photosynthetic 

 bacterium, the facultative anaerobe R. nibrum, a similar view that the 



