Chapt' 



er 



Nutritional and Enzymatic 

 Studies on Microbial Mutants 



By BERNARD D. DAVIS 



Some fifteen years ago, Beadle and Tatum 

 initiated the systematic investigation of auxotrophic mu- 

 tants of a microorganism. Nutritional studies on such 

 mutants indicated that each was blocked in a single bio- 

 synthetic reaction, presumably because it had lost the 

 ability to synthesize the enzyme that catalyzed the reaction. 

 On the genetic side, these results gave rise to the famous 

 one-gene-one-enzyme hypothesis, which proposed that each 

 gene provides information that determines the structural 

 specificity of a corresponding enzyme. On the biochemical 

 side, auxotrophic mutants of molds and also, more recently, 

 of bacteria have been studied in an increasing number of 

 laboratories, and this work has led to the detection of a 

 large number of intermediates in the biosynthesis of vari- 

 ous amino acids, nucleic acid bases, and vitamins. 



One criticism of the one-gene-one-enzyme hypothesis has 

 been that it was built on enzymatic interpretations that, 

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