174 



Bernard D. Davis 



biosynthesis of a group of aromatic metabolites (Davis, 

 1954-55). 



HO COOH COOH 



COOH 



/ 1 X 



O^ljOH 



O^OH 



5-Dehydroquinic 5-Dehyroshikimic 

 acid (DHQ) acid (DHS) 



■A- 



hOqIjOH 



Shikimic 

 acid 



Tyrosine 

 -> Phenylalanine 

 Tryptophan 

 p-Aminobenzoic 



acid 

 p-Hydroxybenzoic 

 acid 

 -> 6th factor 



Yet mutants blocked before DHQ, though they contain the 

 normal amount of the enzyme converting DHQ to DHS, are 

 able to grow on DHS but not on DHQ. However, a secondary 

 mutation of these strains, selected for by exposing large 

 populations to DHQ, permits them to grow on DHQ (Davis 

 and Weiss, 1953). Since the enzyme is there all the time, and 

 is essential for biosynthesis, it is difficult to escape the con- 

 clusion that the secondary mutation in question has created a 

 mechanism for the permeation of DHQ. The specificity of such 

 permeation systems is shown by the fact that some mutations 

 promote the penetration of DHQ and others similarly affect 

 its close structural relative DHS ; but neither mutation affects 

 the other compounds (Davis and Weiss, 1953). 



A permeability barrier has been similarly demonstrated for 

 citrate, which has long been known to be inert for many 

 organisms that contain the enzymes for its utilization. Much 

 as in the case of DHQ and DHS it has been shown, with 

 mutants of Esch, coli and Aerobacter aerogenes blocked before 

 the compound, that citrate is an essential intermediate in 

 glutamate formation (Gilvarg and Davis, 1956). Hence the 

 enzymes between citrate and glutamate, which are readily 

 demonstrated in extracts of these mutants, must be present 

 in active form, and the inability of the organisms to utilize 

 citrate as a replacement for glutamate must be due to a 

 permeability barrier. 



An important development in the study of bacterial 



