INDUCED ENZYME FORMATION 



acids. It was obvious that organisms such as yeast which ac- 

 cumulate an internal free pool would be difficult to employ in 

 such studies, and not unexpected difficulties arose when they 

 were attempted with the yeasts. Fortunately, however, this 

 approach was successfully applied almost simultaneously in two 

 laboratories, and it is interesting to note that in both instances 

 the organisms employed, Escherichia coli and Aerobacter aerogenes, 

 possess a vanishingly small internal supply of free amino acids. 



One of these investigations stems from the illuminating 

 studies of Monod and Cohn (55) and their collaborators into the 

 formation of beta-galactosidase by the ML strain of E. coli. It is 

 interesting to note that in the course of these studies Cohn and 

 Torriani (19,20) had discovered the existence of an enzymati- 

 cally inactive protein (Pz) which was serologically related to the 

 beta-galactosidase. In addition to this obvious structural 

 relationship, they established a suggestive correlation between 

 the distribution of the Pz protein and the capacity of the cells to 

 synthesize the beta-galactosidase. Finally, they also showed 

 that a significant decrease in Pz occurred in cells during the in- 

 duced synthesis of beta-galactosidase. Although not the only 

 possible hypothesis entertained by the authors, it is clear that all 

 of these observations would receive ready explanation if Pz 

 were indeed the precursor of the beta-galactosidase. In any 

 event, taken together, the observations noted offered the most 

 impressive evidence existent in the literature to support the sug- 

 gestion that a preexistent complex specific precursor is involved 

 in the synthesis of a known enzyme. 



There was, however, one fact that militated against the 

 acceptance of this view. Nitrogen-starved cells, though pos- 

 sessing normal amounts of Pz, showed no ability to snythesize 

 the beta-galactosidase unless an external source of nitrogen was 

 provided. It was nevertheless still possible to imagine that the 

 starvation procedure interfered in some way with the metabolic 

 step required for transformation of Pz into active enzyme. 



Monod, Pappenheimer, and Cohen-Bazire (56) undertook 

 to investigate this question further by employing a series of 



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