DEAN B. COWIE AND RICHARD B. ROBERTS 



11 



T. utilis on the other hand removed 76% of the original radioactivity. From 

 this loss of radioactivity from the immersion fluid it can be calculated that the 

 water space would exceed the total cell volume by more than a factor of 3. This 

 absurd result clearly indicates that the supernatant analysis method, though 



Fig. 4. Radiosulfate uptake by 7\ iililis. Curve \ represents radioactivity of TCA pre- 

 cipitable fraction observed during the growth of the cells shown in Curve B. The ratio A/B 

 shows the direct correlation between growth and sulfur uptake of the TCA precipitable frac- 

 tion (proteins). 



satisfactory for studies of E. coli permeability, is not necessarily adequate 

 when other organisms are investigated. For T. utilis it is also necessary to 

 measure the radioactivity retained by the cells through metabolic incorpora- 

 tion. Table 6 shows that even when the quantity of sulfur in the medium was 

 low only a slight amount of radioactivity was retained by E. coli. In contrast, 

 T. utilis retained 61 % of the total sulfur originally available. When this meta- 



