SYNTHESIS OF AMINO-ACIDS 



73 



metabolism of the other aromatic amino-acids. As a result 

 of a suggestion made by Stanier, Davis found that this 

 quadruple requirement could be replaced by shikimic acid, 

 an alicyclic compound known to occur in plants. This acid 

 has now been isolated from the culture filtrate of a mutant 

 of Esch. coli and unequivocally characterized. A precursor of 

 shikimic acid (SKA) has recently been isolated and iden- 

 tified as 5-dehydroshikimic acid (DSKA) which is in turn 

 probably derived from 5-dehydroquinic acid (DQA) [8]. 

 Since growth in the presence of the four aromatic acids was 

 slow and became maximal on the addition of shikimic acid 

 or filtrates from wild-type cultures, Davis deduced that the 

 mutants required at least one other aromatic substance, and 

 one of these has been identified as ^-hydroxybenzoic acid 



(DQA) 



(POB). Shikimic acid is only utilized by mutants exacting 

 to at least four aromatic compounds and this multiple 

 requirement is probably due to the mutation of a single 

 gene. It is not yet possible to state whether shikimic acid 

 is in fact a simple precursor of all these aromatic nitrogen 

 compounds. Davis has suggested [7, 10] that the apparent 

 complexity in growth requirements is the result of inter- 

 ference with the synthesis of a key substance which is 

 responsible for the integration of various parallel and related 

 pathways of biosynthesis (cf. valine-isoleucine, pp. 65, 76). 

 Unlike mammals, Neiirospora and Esch. coli cannot convert 

 phenylalanine to tyrosine. 



Cysteine and methionine 



Sulphur is found in organic combination in the amino- 

 acids cysteine and methionine, and most - organisms can 

 utilize inorganic forms of sulphur at any oxidation level as 



