30 



FIGURE 1.11. Terminal steps in arginine biosynthesis. The position of genetic blocks 

 in this pathway in various mutant strains can be found by comparing growth on various 

 intermediate compounds. In general, mutant strains grow on intermediate compounds 

 located beyond the blocked step. 



in turn, accumulated ornithine which could be utilized by mutants 

 of Group 3. 



In this experiment it was possible to localize the phenotvpic differ- 

 ence between wild-type and each of the fifteen mutants at *.» particular 

 biochemical reaction; this was achieved both by the accumulation test 

 and by arranging the mutants in a logical sequence with respect to 

 growth requirement. The fact that the inferred sequence coincided with 

 that established by biochemical methods provided a neat confirmation 

 of the biochemical findings, and assurance that the mutations had 

 blocked steps in a normal pathway. In the twenty years since this 

 method was developed, it has provided a wealth of information of value 

 both to genetics and to biochemistry. Auxotrophic mutants have pro- 

 vided biochemists with a fast and easy method to investigate unknown 

 biosynthetic pathways by testing the effects of suspected intermediates 

 upon the growth of mutant strains; a flood of new data has been forth- 

 coming. 



The impact of auxotrophic mutants upon the field of genetics has been 

 profound. Mutations could be selected to block most of the known 

 enzyme-catalyzed steps in the biosynthesis of cell constituents. Beadle 

 and Tatum generalized these findings into a hypothesis of gene action 

 which states that (1) gene function consists of "imposing specific con- 

 figurations on protein and other molecules in the final step in their 



