COHEN, HALVORSON, AND SPIEGELMAN 



103 



to obtain the increase AX; this expression is independent of the rate of growth. 

 Figures 3 and 4 show that the differential rates of synthesis of hot-TCA-soluble 

 material and of protein are the same in the presence and absence of p-FPhe. 



/ig Hot TCA Soluble C/m 



H-t Protein C/m 



o control 

 • p-FPhe 



100 200 



eg Dry Weight/ml 



100 200 



CO Dry Weight /ml 



Fig. 3. Differential rate of synthesis of 

 total hot-TCA-soluble material in the pres- 



Fig. 4. Differential rate of synthesis of 

 proteins in the presence or absence of 



ence or absence of p-fluorophenylalanine. See p-fluorophenylalanine. See text for details. 

 text for details. 



Incorporation of C li -p-Eluorophenylalanine in Exponentially Growing Yeast. 

 Radioactive /7-FPhe was added to two exponentially growing cultures at final 

 concentrations of 3.87 X 10 4 M and 1.04 X 10" 3 M, and the cultures were allowed 

 to grow linearly (300 minutes) until their mass increased by 3.6 times. Samples 

 were taken at given intervals, and the radioactivity of the protein fraction was 

 determined. The differential rate of incorporation of /7-FPhe was calculated, 

 and from the slopes of the straight lines obtained (fig. 5) the content of /7-FPhe 

 in the proteins was found. As in E. coli, increasing the external concentrations 

 of p-FPhe increases the analog content of the proteins. The incorporation is 

 far from negligible, reaching 12 per cent of the normal phenylalanine content 

 for a concentration of the analog of 1.03 XlO" 3 M (31.2 umoles /7-FPhe/g dry 

 wt.). Munier (personal communication) and Kerridge (quoted in Gale and 

 McQuillen [5]) have also shown that /7-FPhe is incorporated into yeast proteins. 

 The relative amount of phenylalanine in yeast was determined from its differ- 

 ential rate of incorporation. In two experiments, where the C 14 phenylalanine 

 concentration was 3.12 X 10" 5 M and 6.25 X 10~ 5 M respectively, identical differen- 

 tial rates of incorporation were observed. 



In all experiments, radioautograms of the acid hydrolysates of the protein 

 fractions were made. Both for cells grown on C 14 -Phe and for those grown on 

 C 14 -/7-FPhe, the radioactivity of the protein hydrolysates was identified exclu- 

 sively with the added isotope. 



