454 PLANT GROWTH SUBSTANCES 



is related to shikimic acid. Shikimic acid will replace all four of these 

 substances for the multiple aromatic mutant of Neurospora, Y-7655. 

 That the suggested relationship between these substances is of significance 

 in other organisms is shown by the demonstration by Davis of a corres- 

 ponding mutant in E. coli, which also responds to shikimic acid (18). 

 Fischer has suggested that shikimic acid is derived in the plant directly 

 from hexose sugars (22). In microorganisms the aromatic nuclei of these 

 amino acids and of /7-aminobenzoic acid may thus be derived from hexoses 

 through a common precursor similar to shikimic acid. Previous results 

 (82) with the /'-aminobenzoicless mutant strain suggests that in the 

 synthesis of this compound nitrogen is normally introduced into a non- 

 aromatic structure. Since shikimic acid is active only for the multiple 

 mutant, and not for the /7-aminobenzoicless strain, this structure would 

 seem to be a substance closely related to shikimic acid. 



The other important contribution of investigations with micro- 

 organisms is in regard to the roles of many amino acids as biosynthetic 

 precursors of still other amino acids. The interconversion of glycine and 

 serine first indicated in the rat (74) and recently further investigated by 

 Sakami (69), seems also to be true for Neurospora and for certain bacteria 

 (31,40). Evidence obtained with mutant strains of Acetohacter suggests 

 that glycine is normally formed from serine, which may arise from a 

 nonnitrogenous precursor, since one strain will grow on either glycine 

 or serine and a second strain only on glycine (31). The interconversion 

 of cysteine and methionine by way of homocysteine has been amply 

 demonstrated in Neurospora as well as in bacteria (38,76). The role of 

 aminoadipic acid as a precursor of lysine in Neurospora (54) represents 

 another extremely interesting example of amino acid interconversions. 

 The role of serine in tryptophan biosynthesis in Neurospora is now well 

 known. There seems little question as to the reality of this conversion 

 in view of enzymatic studies which have been carried out (55,86) and 

 in view of the demonstrated incorporation of labeled nitrogen in the 

 form of serine into the tryptophan synthesized by this system (75). 

 The relationships of glutamic acid, proline, and ornithine suggested for 

 the rat (74) have been shown to hold for Pejiicillium (11) and more 

 recently for Neurospora (21). Fincham has shown that a-hydroxy- 

 5-aminovaleric acid is an intermediate in the interconversions of glutamic 

 acid, proline, and ornithine (21). Another extremely interesting series 

 of interrelations is that between the aliphatic amino acids, threonine, 



