46 Perspectives in Microbiology 



of the mutant extract might be due to presence of an in- 

 hibitor or to absence of an essential activator; for, in order 

 to account for the results of the mixture experiments, 

 either an inhibiting or an activating factor would have 

 to be stoichiometrically attached to the enzyme, and such 

 a difference between two extracts would not be operation- 

 ally distinguishable from the presence of active enzyme 

 molecules in one and their absence from the other. We can 

 therefore conclude that the genetic block between DHQ 

 and DHS is indeed associated with inability of the mutant 

 cell to make the enzyme for that reaction, at least in active 

 form. Furthermore, the loss is specific; mutants blocked 

 in various earlier or later reactions of the aromatic se- 

 quence have yielded normal or sometimes even increased 

 amounts of dehydroquinase. 



The enzyme that catalyzes the next reaction in this 

 chain, the reduction of DHS to shikimic acid, has been 

 similarly studied in extracts by Yaniv and Gilvarg (32). 

 The enzyme is specific for triphosphopyridine nucleotide 

 (TPN) as hydrogen carrier, and the reaction is reversible. 

 There is strong evidence that this reaction also is catalyzed 

 by a single enzyme, and this enzyme has no demonstrable 

 cof actor or metal ion requirement other than TPN. As 

 in the previous case, the wild type yields the enzyme in 

 about the concentration expected from the growth rate; 

 mutants blocked in this reaction yield no enzyme activity; 

 and the addition of mutant extract has no effect on the 

 activity of wild-type extract. 



It might be noted at this point that both these enzymes 

 have been found present in extracts of a variety of micro- 

 organisms and plants that can synthesize their own aro- 

 matic amino acids, and absent from an animal tissue which 

 cannot synthesize these compounds. This path is therefore 

 widely distributed in nature, and may well be the only 

 path existing for the biosynthesis of this group of benze- 

 noid compounds. 



