SYNTHESIS OF AMINO-ACIDS 65 



culture medium [40]. Since the amount of anthranilic acid 

 produced by an exacting strain was greater than the amount 

 of indole utilized, it was unlikely that the former was derived 

 from the latter. The inhibitory effect of 4- or 5-methylan- 

 thranilic acid on the growth of the non-exacting strain was 

 reversed by anthranilic acid, indole or tryptophan [16]. 

 These results are summarized in the following scheme, 

 vertical arrows denoting the site of action of inhibitors: 



4-methyl- indoleacrylate, methyl- 



anthranilate 4-methylindole tryptophan 



anthranilic ] >■ indole"! | 



acid Y j *■ tryptophan ' 



serineJ y 



'*" protein 



Another example of antagonism in amino-acid metabolism 

 comes from studies of the nutrition of Bacillus anthracis and 

 serves to emphasize that growth may be dependent not only 

 on the presence of certain amino-acids but also on their 

 relative concentrations [19]. Although B. anthracis would 

 not grow on a synthetic and complete amino-acid medium 

 from which valine, leucine or isoleucine had been removed, 

 growth did occur in the absence of all three of these amino- 

 acids. When valine was present, no growth was possible 

 unless a suitable amount of leucine had also been added, 

 whilst growth in the presence of isoleucine was conditional 

 on the addition of both valine and leucine. Since these 

 amino-acids are of comparable chemical structure, Glad- 

 stone concluded that they are synthesized by similar, if not 

 identical, routes and that the addition of only one of the 

 acids resulted in the competitive inhibition of one or more 

 of these routes. Several examples of this type of effect have 

 been found during experiments with Neurospora mutants 

 [cf. 45]- 



Mutants 



Much valuable information concerning the routes of bio- 

 logical synthesis of natural compounds has been derived by 

 the use of mutants, i.e. from organisms genetically different 

 from the parent strains. If this difference results in inability 



