141 



Ellen C. Weaver and Norman I. Bishop 



The difference spectrum of wild type Scenedesmus , agrees in its 

 major features with other published difference spectra for this 

 organism^ -' . In contrast is the spectrum of Mutant 8, a "CO2" niu- 

 tant, which displays virtually no spectral shifts; yet Mutant l8^ 

 in all other respects quite similar to Mutant 8, has a difference 

 spectrum which resembles wild type (Fig. 3). Among the "O2" mu- 

 tants the same situation holds true: some (e.g. No. 11) have no 

 spectral shifts, while others (e.g. No. ko) look very much like 

 wild type. There is no obvious correlation between either of the 

 EPR signals and any of the prominent positive or negative shifts 

 in the region we were able to scan. 



+0.5 



10 

 o 



" 



o 

 o 



-0.5- 



— 1 1 1 



NO. 8 "CO2" MUTANT 



1 r 



-I 1 1 r 



■H*^HH^f»^M^^ 



Fig. 3 Changes in absorption determined as in Fig. 2. The upper 



trace is that of No. 8, a "CO2" mutant, and displays al- 

 most no changes. No. 11, an "O2" mutant, is similar in its lack 

 of signal. The lower trace is of another "O2" mutant, No. I8; 

 the "O2" mutant No. kO resembles it. 



Fortunately, it was also possible to examine the time course of 

 an absorption change at any one wavelength by using the machine in 

 the following way: a neon flash bulb was substituted for the chop- 

 per, allowing the time sequence of light and dark to be completely 

 controlled. With the very short time constants employed, sensi- 

 tivity had to be sacrificed. This difficulty was circumvented by 

 use of a coirputer of average transients (CAT); this device 're- 



