COHEN, HALVORSON, AND SPIEGELMAN 107 



inducer, induced synthesis became more and more refractory to the inhibitor. 



One feature of the effect of /7-FPhe on resting yeast cells requires special 

 attention. Previous studies showed that, in the presence of /7-FPhe, the contents 

 of the free amino acid pool remained essentially the same as in unstarved cells 

 (Halvorson and Spiegelman [1]). On the basis of the present experiments, the 

 nondisappearance of the pool in the presence of p-FPhe can be attributed to: 

 (1) partial replenishment of the free amino acid pool from the nitrogen of 

 /7-FPhe, and (2) an increased rate of protein degradation in the presence of 

 /7-FPhe. 



The reversal of /7-FPhe inhibition by Phe can be related not only to a com- 

 petition between these two amino acids for an accumulating system (Halvor- 

 son and Cohen [6]) but also to its incorporation into proteins. Halvorson and 

 Spiegelman [1] had derived from their experiments with /7-FPhe the conclu- 

 sion that there were no intermediate precursors of induced maltozymase in 

 yeast. Although amino acid antagonists may prove valuable for studies on 

 protein synthesis, it would seem on the basis of these and other experiments 

 (Cohen and Munier [8]) that conclusions derived from their use as a tool in 

 the study of intermediates in protein synthesis are unwarranted. The de novo 

 nature of induced enzyme synthesis has been established on other grounds, 

 however (Hogness, Cohn, and Monod [10]; Rotman and Spiegelman [11]). 



ACKNOWLEDGMENTS 



We wish to express our appreciation to Dr. R. Munier for advice and assist- 

 ance in the early phases of these experiments and to Dr. Pichat of the Com- 

 missariat a l'Energie Atomique, France, for the synthesis of 3-C 14 -DL-/7-fluoro- 

 phenylalanine. 



SUMMARY 



The addition of 0.01 M /7-fluorophenylalanine to a growing culture of Sac- 

 charomyces italicus Y1225 results in (1) an incorporation of the analog into 

 cellular proteins, (2) a linear rather than an exponential rate of growth as a 

 function of time, and (3) an inhibition of cell division. The antagonist does 

 not influence the differential rates of synthesis of protein and hot-TCA-soluble 

 material. 



Under "resting conditions," p-fluorophenylalanine does not inhibit either pro- 

 tein synthesis or the utilization of the free amino acid pool. Elevated pools in 

 the presence of the antagonist were attributed to pool replenishment from the 

 nitrogen of antagonist and to an increased rate of protein degradation. 



REFERENCES 



1. H. O. Halvorson and S. Spiegelman, Incorporation d'analogues structuraux 

 The inhibition of enzyme formation by d'amino acides dans les proteines bacterien- 

 amino acid analogues, /. BacterioL, 64, 207 nes, Biochim. et Biophys. Acta, 21, 592- 

 (1952). 593 (1956). 



2. R. L. Munier and G. N. Cohen, 3. R. L. Munier and G. N. Cohen, 



