Microbial Mutants 55 



To summarize their biochemical implications, the enzy- 

 matic studies described here have convinced us that some 

 (but not all) of the compounds accumulated by aromatic 

 auxotrophs are true essential biosynthetic intermediates. 

 These results have therefore increased our confidence that 

 most of the compounds accumulated by other mutants will 

 have similar metabolic significance. Furthermore, a com- 

 bination of enzymatic, nutritional, and accumulation 

 studies, applied to mutants, has led us to previously in- 

 accessible intermediates. Also, such an approach has made 

 it possible to define more accurately the physiological role 

 of the tricarboxylic acid cycle. In general, it appears that 

 nutritional studies on mutants, like isotopic studies, have 

 provided most valuable exploratory tools in the search for 

 biosynthetic intermediates; that the identification of com- 

 pounds accumulated by mutants, even when these com- 

 pounds are devoid of nutritional activity, is a valid ex- 

 tension of this approach; and that enzymatic comparisons 

 between mutants and the wild type can offer the most 

 rigorous proof presently possible that a compound is an 

 essential intermediate on a metabolic path. 



To summarize their genetic implications, the enzymatic 

 studies described here, and similar studies on other enzyme 

 systems in this (19) and other laboratories (13, 25, 27), have 

 shown that Beadle and Tatum were correct in their original 

 assumption that an auxotrophic mutant lacks a specific 

 enzyme. I believe we can feel confident that only an ex- 

 ceptional auxotrophic mutant requires a substance for some 

 other reason: for example, the inhibition rather than the 

 absence of an enzyme (34), or the destruction of the sub- 

 stance by an exaggerated side reaction (17). 



There has not been time to describe other enzyme studies 

 that have thrown a little further light on the relation be- 

 tween a gene and either the structure or the amount of the 

 corresponding enzyme. With respect to enzyme structure, 



