44 Perspectives in Microbiology 



This is as far back as we have been able to penetrate 

 by the method of searching for growth factors that could 

 replace the aromatic supplement. At the other end of the 

 chain, beyond shikimic acid, we have encountered three 

 accumulated compounds that are completely devoid of 

 nutritional activity for any mutant. One of these is found 

 in filtrates of all mutants that accumulate shikimic acid, 

 and on acid hydrolysis it was found to yield shikimic acid 

 (11). It has been identified by Weiss as 5-phosphoshikimic 

 acid. The second compound is accumulated by mutants 

 that are blocked still later, and it too ^vas found to yield 

 shikimic acid on acid hydrolysis (11). This compound, 

 provisionally termed Zl, is as yet unidentified. 



The third inactive compound, prephenic acid (PPA), 

 is accumulated by mutants that require phenylalanine or 

 phenylpyruvic acid for growth (7, 18) and also by some that 

 require tyrosine or the corresponding a-keto acid. It has 

 been isolated and identified by Weiss and Gilvarg (31). 

 PPA is a nonaromatic compound, and it has particular 

 biochemical interest because it is the substrate of the actual 

 aromatization step in phenylalanine biosynthesis, losing 

 both CO2 and water to yield phenylpyruvic acid. 



I should now like to describe some studies on the en- 

 zymes linking certain of these compounds. These enzymes 

 can easily be obtained in cell-free solution after disrupting 

 wild-type E. coli cells, either by grinding with glass powder 

 or by sonic oscillation. 



The enzyme of this series that has been most extensively 

 studied, from a physiological point of view, is 5-dehydro- 

 quinase, investigated by Mitsuhashi (22). It removes a 

 molecule of water from DHQ to form DHS, and its ac- 

 tivity is readily measured by virtue of the fact that DHS 

 has a high absorption peak at 234 mfi, whereas DHQ does 

 not absorb light significantly at this wavelength. Activ- 

 ity is strictly proportional to enzyme concentration. The 

 reaction is reversible, with the equilibrium constant 



