DEAN B. COWIE AND RICHARD B. ROBERTS 7 



the trichloroacetic acid soluble and insoluble fractions of the cell is also evident 

 in figure 2. Glucose is required for this metabolic uptake of sulfate (10). Cells 

 immersed in synthetic medium, complete with the exception of glucose, do not 

 incorporate sulfur metabolically. Upon the addition of glucose, growth begins, 

 and there is a direct correlation among growth, nitrogen content, and sulfate 

 uptake. Except at extremely low concentrations of sulfate, the quantity of 

 sulfur metabolically bound is independent of the concentration in the medium. 



0.0 



0.4 o.a 1.2 1.6 2.0 2.4 



QUANTITY OF NEW CELLS GROWN 



2.8 



Fig. 2. Sulfate uptake during growth (£. coli). The upper curve shows the total radiosulfate 

 uptake as a function of the quantity of new cells grown. The lower curve shows the radio- 

 sulfur of the TCA soluble fraction as a function of growth. 



It is obvious from the preceding experiments that if the wash procedures were 

 not carried out, large quantities of nonmetabolized sulfur passively held by 

 these cells could constitute a potential source of error in the study of cell 

 metabolism. 



Exchange and Wash Losses During Growth. Further comparison of the 

 uptake of radiosulfate by resting and by growing cells may be made using 

 wash-loss measurements. Cells grown in radiosulfate from a light inoculum 

 were washed to remove water space sulfur and immersed in complete media 

 containing various nonradioactive sulfur sources. It can be seen in table 5 that 

 little of the labeled sulfur exchanged with the sulfur of the medium despite 

 continuous cellular growth. The fact that no large losses of metabolically bound 



