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ELECTRON TRANSPORT 



phores a.re ilhimimdedanaerobically . It was found that with the chroma- 

 tophore suspension under aerobic conditions a difference spectrum 

 of "illuminated" minus "nonilluminated" exibited a new trough at 415- 

 435 mju. It is not known whether the peak and the trough correspond 

 to the peaks in the action spectrum for light stimulation on respiration 

 of the aged cells. 



Respiration of the dark-grown cells was inhibited by carbon 

 monoxide, and the inhibition was removed by illumination. An action 

 spectrum for this light relief was found to be of the type designated 

 "cytochrome o" by Castor and Chance (21, also 20) (Fig, 7). It is 

 similar to that with a nonphotosynthetic bacterium Micrococcus 

 pyrogenes var, albus (27,28), The action spectrum is the same as the 

 absorption spectrum of a variant hemoprotein from R. rubrum (ab- 

 breviated as RHP) in its reduced form bound to carbon monoxide (29), 



450 



500 

 >^ ( m/j) 



550 



Fig, 7, Action spectrum of light relief on CO-inhibited respiration of dark- 

 growncellsof R. nibnim. R. nibrum cells ^\ilich had been grown for three days 

 aerobically in darkness (approximately fivefold multiplication in dry weight) 

 were used. Care was taken so tliat pH of the growth medium did not exceed 8 

 during incubation. Before this experiment, it was determined with the use of the 

 oxygen-electrode method that the respiration of the cell suspension was not 

 influenced by alternate darkness and light , although the cells possessed bacterio- 

 chlorophyll and carotenoids, as seen with a microspectroscope (20). 



