End- Product Inhibition of the 



Initial Enzyme in a Biosynthetic 



Sequence as a Mechanism 



of Feedback Control 



H. Edwin Umbarger ^ 



The pathways by which small molecule building blocks are bio- 

 synthesized have been studied in yeast, Neurospora and such Gram 

 negative bacteria as Aerobacter aerogenes, Escherichia colt, and 

 Sahnonello tijphimuritnn. These organisms are able to synthesize 

 their entire spectrum of protoplasmic constituents and grow in a 

 medium containing essentially glucose, ammonia, and inorganic salts. 

 A striking feature of such organisms is not only that they contain all 

 the enzymes to catalyze these biosynthetic steps but also that the 

 activities of these enzymes are so integrated that there is an orderly 

 flow of metabolites to the sites of synthesis of macromolecules such as 

 proteins and nucleic acids. 



In the case of amino acids, though there is a small but measurable 

 pool of free amino acids within the cells, very little of any amino acid 

 appears in the medium. Thus, the cell forms virtually only as much 

 of each amino acid as it requires. Furthermore, if an amino acid 

 or a freely permeable precursor had been added to the medium, the 

 exogenous compound is used and, for some small molecule building 

 blocks, endogenous formation of the compound stops. 



1 Biological Laboratory, Long Island Biological Association, Cold Spring Harbor, 

 New York. The experiments performed in the author's laboratory were supported by 

 grants from the United States Public Health Service. 



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