S. SPIEGELMAN AND A. M. CAMPBELL 



b. What is the nature of the enzyme-forming mechanism 

 which converts the precursor material into active enzyme? 



c. What is the role of the inducer, the presence of which 

 specifically stimulates the appearance of the corresponding 

 enzyme? 



We can now turn our attention to an estimation of the ex- 

 tent to which answers can be provided to the questions just 

 proposed. 



The Nature of the Precursor 



The problem of the precursor is perhaps most dramatically 

 exhibited by considering inductions carried out under the 

 simplest circumstances. It has been shown by a number of 

 workers with a variety of enzyme systems (55,75,90) that enzyme 

 synthesis can be induced in the absence of a nitrogen source in 

 cells suspended in a buffer solution of the inducer. That the 

 appearance of enzyme activity actually involves the formation of 

 enzyme has been established in these cases by exhibiting the 

 homologous enzyme in extracts prepared from the induced cells. 

 In such inductions, the nitrogen employed by the cell in fabricat- 

 ing the new enzyme molecule must come from some preexisting 

 nitrogenous components, and one is immediately faced with the 

 obvious necessity of identifying the components so employed. 



Before considering the most recent experiments which have 

 led to a satisfactory resolution of this problem, it is of interest to 

 note briefly some of the earlier work which, although inconclu- 

 sive, nevertheless exerted a strong influence on the subsequent 

 development of this aspect of the problem. Monod (53), in his 

 classic investigation into the growth of bacteria, discovered the 

 existence of a severe interaction between enzyme-forming systems, 

 which was expressed by the fact that simultaneous synthesis of 

 two metabolically unrelated enzymes did not occur on exposing 

 cells to a mixture of the two relevant inducers. In general, only 

 one of the enzymes was formed at a time. A similar situation 

 was uncovered in the yeasts by Spiegelman and Dunn (80). 



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