ELECTRON TRANSPORT IN R. RUBRUM 281 



TABLE 1. 



Effect of temperature treatment on respiration 

 of light -grown cells ofR- rubrum 



O2 uptake {mfi-mole/2 min.) 



Ratio 



Dark Light Dark/Light 



78 2.1 



83 1.9 



99 1.3 



87 1.0 



78 1.0 



50 0.4 



17 0.2 



107 1.0 



Experimental conditions were the same as for Fig. 1, except that the suspen- 

 sions of light-grown cells of R. nibnim were heated or frozen and thawed at 

 pH 7.2 (18). 



After centrifugation of the resulting chromatopho re suspension, neither 

 the sediment nor the supernatant consumed oxygen in light, while the 

 mixture of the two showed the light- dependent oxygen uptake. It was 

 found that fresh growth medium or reduced cytochromes (see below) 

 could not substitute for the supernatant, providing a strong argument 

 against the postulate that this light- stimulated oxygen uptake resulted 

 from an enzymic reaction. It was noted that this light- dependent oxi- 

 dation still was cyanide sensitive, although only 30 per cent of oxygen 

 uptake could be inhibited by a high concentration (5 x 10"3 M) of 

 cyanide. The cyanide inhibition on oxygen uptake became less efficient 

 with increasing temperature. 



The light-grown cells, if aged, also showed the same light-stimulated 

 respiration as for the nontreated chromatophores. This change was 

 accelerated by allowing the cell suspension to stand under aerobic 

 conditions, especially in light. The light- stimulated respiration was, 

 in all likelihood, a photooxidation of some reducing substances catalyzed 

 by bacteriochlorophyll, probably in its modified form, analogous to the 

 photooxidation reactions described by Vernon and Kamen (19). 



An action spectrum of light inhibition on respiration of light-grown 

 cells was measured (20) (Fig. 5). It shows that bacteriochlorophyll 

 and carotenoids mediate this action, and it closely resembles the action 

 spectrum for photosynthesis obtained by Thomas (22) and Thomas and 



