42 PerspecHves in Microbiology 



a fusion of enzymatic and nutritional studies on mutants 

 has made it possible to solve problems that could not be 

 solved by either approach alone. 



Since several of the enzymatic studies to be described 

 have been concerned with a path of aromatic biosynthesis, 

 I shall first present this path in some detail. 



Our interest in aromatic biosynthesis arose through the 

 chance isolation of several bacterial mutants that required 

 a mixture of four aromatic compounds: tyrosine, phenyl- 

 alanine, tryptophan, and para-aminobenzoic acid (9). Sub- 

 sequent work showed that most of these strains had a rela- 

 tive requirement for traces of a fifth aromatic compound 

 not previously known to be a bacterial metabolite, para- 

 hydroxybenzoic acid (8). Since then, we have found that 

 under certain circumstances these strains require in addi- 

 tion traces of a sixth factor, as yet unidentified (Figure 1). 



The common aromatic structure of these several com- 

 pounds led to a search for a possible common precursor. 

 It was found that a rare plant acid, shikimic acid, could 

 satisfy the growth requirement of several of these aromatic 

 polyauxotrophs. Furthermore, the response was propor- 

 tional to shikimic acid concentration and was approxi- 

 mately the same as that produced by a molar equivalent 

 of the required mixed aromatic supplement. Finally, the 

 other strains with the same requirement, presumably 

 blocked in a later reaction in the same sequence, could 

 not respond to shikimic acid, but could be shown to ac- 

 cumulate large quantities of this substance in their culture 

 filtrates. We therefore felt justified in concluding that 

 shikimic acid is a precursor of these aromatic cell constitu- 

 ents (4). 



transamination of a-ketoisovaleric acid with alanine rather than with 

 glutamic acid (27)), normal intermediates would include both essential 

 and non-essential ones; where there are major alternatives (as may be 

 true in carbohydrate metabolism preceding specific biosynthetic paths (2)), 

 none of the normal metabolites would be essential. 



