S. SPIEGELMAN AND A. M. CAMPBELL 



yeasts are able to form enzymes when suspended in nitrogen-free 

 solutions of inducer, whereas the Gram-negative bacteria in 

 general require an exogenous supply of nitrogen as a necessary 

 concomitant of enzyme synthesis. The work of Taylor (96) 

 suggests a reasonable explanation for this apparent independence 

 of the yeast enzyme-synthesizing mechanism. This investigator 

 surveyed a variety of yeast and bacteria for the presence of free 

 amino acids in their internal environment. Of the three yeast 

 types examined, all possessed detectable quantities of the five 

 amino acids looked for. Among the bacteria, the Gram-posi- 

 tives possess primarily glutamic acid and lysine. None of the 

 Gram-negatives included in the survey contained detectable free 

 amino acid by the procedures employed. 



The possession of an internal pool was subsequently found to 

 be a universal attribute of a wide variety of yeasts (85). It 

 would appear that the ability of yeast to get along without an 

 external source of nitrogen for enzyme synthesis is due to the fact 

 that they have internal supply. To examine the question then 

 of the effect of free amino acid availability on enzyme formation 

 in the yeast it was necessary to devise and employ procedures 

 capable of modifying these pool levels both quantitatively and 

 qualitatively. Using such procedures, Halvorson and Spiegel- 

 man (33) demonstrated a strong correlation between enzyme- 

 forming capacity and pool level in both depletion and replenish- 

 ment cycles. The results obtained in these studies supported the 

 conclusion that free amino acids constitute the quantitatively 

 predominant source of nitrogen in the formation of new enzyme 

 molecules. Again, no evidence was uncovered which suggested 

 the existence of a detectable amino-acid-independent transfor- 

 mation of a preexisting complex prescursor into active enzyme 

 molecule, 



ENZYME FORMATION BY AMINO ACID AUXOTROPHIC MUTANTS 



The third approach mentioned which could provide rele- 

 vant information involves the use of the auxotrophic mutants 

 deficient in the ability to synthesize one or another of the amino 



122 



