104 



MICROSOMAL PARTICLES 



m/i moles Protein p-FPhe /ml 

 1 



3 87-10" M 



100 150 



eg Dry Weight/ ml 



4 



0. 3 



0.2 



O.I 



Ht) Protein S/ml 



o control 

 • p-FPhe 



20 30 



Time ( mln) 



40 



Fig. 5. Differential rate of incorporation Fig. 6. Effect of p-fluorophenylalanine 



of p-fluorophenylalanine. See text for details. on S 35 sulfate incorporation into proteins of 



resting yeast cells. See text for details. 



Incorporation of p-Fluorophenylalanine under "Resting" Conditions. Since 

 the experiments of Halvorson and Spiegelman [1] were conducted with "rest- 

 ing" cells, it was interesting to know whether incorporation of p-FPhe occurred 

 also under these conditions. Exponentially growing yeast was washed twice 

 with buffer and resuspended in phosphate-succinate buffer, pH 4.7, with glu- 

 cose as energy source, but without an exogenous nitrogen source. The sus- 

 pension was divided in two flasks to which C 14 Phe and C 14 p-FPhe were 

 respectively added (final concentration: 6.25x10 3 M). In both flasks the in- 

 corporation of radioisotope was linear with time and ceased after 50 minutes' 

 incubation. At this time, 31.8 mumoles of Phe had been incorporated, whereas 

 19.4 mumoles of p-FPhe had been incorporated in the proteins per milliliter 

 of culture. Thus, the analog was incorporated at 61 per cent the extent of Phe 

 incorporation. 



Incorporation of Sulfur from Radiosulfate into Yeast Proteins under "Rest- 

 ing" Conditions. An exponential culture from synthetic medium was cen- 

 trifuged, washed, resuspended in nitrogen-free medium, and divided in two 

 flasks containing S 35 with and without 0.01 M p-FPhe. The incorporation of 

 S 35 into proteins was linear with time over 40 minutes with identical slopes of 

 incorporation in the two flasks (fig. 6). 



Incorporation of Radiovaline under "Resting" Conditions in Presence or 

 Absence of p-FPhe. Yeast was grown in broth, centrifuged while in the ex- 



