LIGHT REACTIONS IN R. VANNIELII 



437 



For purposes of analysis it is convenient to number the inflection 

 points in the cytochrome curve (Fig. 2) I, II, III and IV in order of in- 

 creasing light intensity and to assume that each inflection is associated 

 with a cytochrome reaction which is more or less independent of the 

 others. By this approach it is possible in principle to elucidate kinetic 

 and spectral differences between the various reactions. 



From Fig. 2 it is clear that the steady- state level of cytochrome 

 oxidation is proportional to intensity up to the first inflection point 

 approximately. The initial rate of cytochrome oxidation upon illumina- 

 tion, however, is linear with intensity beyond the third inflection point. 

 From the dependence of the initial rate on intensity, one can estimate 

 the quantum efficiency of cytochrome oxidation for the most sensitive 

 electron transfer system (10). From the slope in Fig, 3 the maximum 

 quantum efficiency estimate is O.lmole/einstein based on the following 

 assumptions: 



^M22.5 " ^^445 



60 mM~ cm" , 



800 



0.5. 



Effect of oxygen . 



Oxygenation by itself causes absorption changes in whole cells, and 

 in addition modifies the light effects to some extent, as shown in Fig. 

 4. The absorption spectrum changes in the region 540 to 570 m/i indi- 

 cate the oxidation of c-type cytochromes upon the addition of oxygen. 



4 6 



TIME IN MINUTES 



10 



Fig. 4. Kinetic trace of absorbancy changes caused by 0.80 ii light and oxygen. 

 Cytochrome oxidation denoted by a decrease in A422.5 - A445. 



The absorbancy changes induced by high intensity light reach comple- 

 tion more rapidly and also decay more rapidly in the presence of 

 oxygen, while the extent of change is slightly diminished. Nevertheless, 



