Microbial Mutants 47 



Let us return to quinic acid, which is a growth factor for 

 certain Aerobacter mutants but not for any available E. 

 coll mutants. The enzyme that catalyzes the oxidation of 

 this compound to DHQ in Aerobacter has been extracted 

 from the wild type by Mitsuhashi and has been called 

 quinic dehydrogenase (23). It is specific for diphospho- 

 pyridine nucleotide (DPN), in contrast to the correspond- 

 ing enzyme that interconverts DHS and shikimic acid, al- 

 ready noted to be TPN-specific. Quinic dehydrogenase is 

 much more restricted in its distribution than are the two 

 enzymes previously described; it could not be detected in 

 extracts of wild-type E. coli or in various other microbial 

 and plant extracts that did contain these other enzymes. 



The enzymes concerned with phosphoshikimic acid and 

 with compound Zl have not yet been investigated. The 

 enzyme that converts prephenic acid to phenylpyruvic acid 

 has been shown to be present in wild-type extract, absent 

 from an extract of a mutant blocked in this reaction, and 

 present in the expected amount in a mixture of the two 

 extracts (10, 31). It has not been purified, but on structural 

 grounds it seems probable that only a single enzyme would 

 be required to catalyze the reaction. Finally, Rudman and 

 Meister (27) have described a transaminase from E. coli 

 that converts phenylpyruvic acid and several other a-keto 

 acids to their corresponding a-amino acids; no mutant 

 deficient in this enzyme has yet been recognized. 



And now, what criteria shall we use to decide on the 

 metabolic role of each of these compounds? Historically, 

 the first criterion to be used with mutants, and the only 

 one used for a number of years, was growth-factor activity. 

 Accordingly, one was hesitant to accept as an intermediate 

 a compound that could not support growth, or even one 

 that did so slowly or only at excessive concentrations. 

 There has been increasing evidence, however, that growth- 

 factor activity is not a necessary condition for function as 

 an essential intermediate. (This point is discussed later.) 



