COHEN, HALVORSON, AND SPIEGELMAN 



105 



ponential phase, washed twice, and resuspended in phosphate-succinate buffer 

 with glucose but without an exogenous source of nitrogen. Radioactive valine 

 was added at time, and the suspension was shaken for 8 minutes at 30° C. 

 The suspension was then centrifuged, washed twice in the cold, and divided 

 into two flasks in the same medium without valine, and with and without 

 10" 2 M p-FPhe. Valine incorporation proceeded at the same rate in the two 

 flasks (fig. 7) ; as shown previously (Halvorson and Cohen [6]), the valine pool 

 size is sufficient under these conditions for an unchanged rate of valine incor- 

 poration. These experiments show that, under growing or "resting" condi- 

 tions, p-FPhe does not inhibit protein synthesis. It was then interesting to 

 find out why, under the conditions of Halvorson and Spiegelman [1], the pools 

 were not depleted in the presence of p-FPhe. 



m/imoles Protein Valine/ml 

 I4 r 



"iMfl Protein S/ ml 

 3O0OT 



200C- 



» NH, 



1000 



10 12 



Time (mln) 



Fig. 7. Valine incorporation into the pro- 

 teins of resting yeast cells. See text for 

 details. 



a p-FPhe 



100 200 



Time(min) 



Fig. 8. Effect of exogenous nitrogen on 

 the incorporation of S 35 into proteins in ni- 

 trogen-starved cells. See text for details. 



The Effect of Nitrogenous Compounds on the Incorporation of Sulfur into 

 Proteins in Nitrogen-Starved Cells. A culture growing exponentially in broth 

 was centrifuged, resuspended with glucose in a phosphate-succinate buffer, and 

 then shaken for 18 hours (original dry weight: 1.55 mg/ml). These starved 

 cells were then washed and resuspended in nitrogen-free, phosphate-succinate 

 buffer with glucose and S 35 sulfate with or without 10~ 2 M NH 4 C1, 10 -2 M Phe, 

 or 10" 2 M p-FPhe. The curves of figure 8 show that /7-FPhe and Phe can act as 

 nitrogen sources for S incorporation, probably providing — NH2 groups through 

 transamination to carbon acceptors. 



Effect of p-Fluorophenylalanine on Protein Degradation. In the absence of 

 an exogenous source of nitrogen and energy, the degradation of cellular proteins 

 leads to elevated pool levels (Halvorson [7]). Since high amino acid pool levels 



