270 



Aspartic acid 



NITROGEN NUTRITION AND METABOLISM 



Homoserine 



Methionine ■*- 



— Cysteine 

 Cystathionine 



Homocysteine 



Threonine Figure 9. The probable in- 



terrelations of aspartic acid, 

 threonine, and the sulfur 

 amino acids in Neurospora 

 crassa. Several of the steps 

 involve more than one re- 

 action. All amino acids are 

 of the L-configuration. 



Nutritional and metabolic studies on methionine-requiring mutants 

 of Neurospora crassa established an over-all sequence leading from 

 cysteine to methionine (159, 247, 533). A relation of threonine and 

 methionine biosynthesis is suggested by both inhibition studies (132) 

 and nutritional requirements of a single-gene mutant (533). These 

 relations are schematized in Figure 9, to which has been added the 

 postulate that aspartic acid is the precursor of both threonine and 

 methionine, as it is in yeast and bacteria (59). The scheme is by no 

 means complete; there is probably an intermediate between homocys- 

 teine and methionine (92), and the source of the methyl group of 

 methionine is not yet clear. An enzyme converting homoserine to 

 threonine occurs in fungi (578a). 



Sulfur-methylcysteine is formed by Neurospora crassa and the iso- 

 lated compound is utilizable as a source of sulfur (427). 



Canavanine has been mentioned as an inhibitor of arginine synthesis. 

 It has also been suggested that canavanine-resistant strains convert 

 canavanine to homoserine (329); Streptococcus faecalis carries out this 

 reaction (292). Such a conversion would explain why canavanine re- 

 places threonine for some N. crassa mutants (532). 



Isoleucine and valine are synthesized by Neurospora crassa from, 

 respectively, a,/3-dihydroxy-/?-ethylbutyric acid and a./J-dihydroxy-/?- 

 methylbutyric acid (13). An observed double requirement for both 

 isoleucine and valine (65) has been traced to a deficiency in the de- 

 hydrase — possibly dehydrases — which convert these dihydroxy acids to 

 keto acids (381). There is some evidence that the precursors of valine 

 and isoleucine ultimately arise from, respectively, pyruvic acid and 

 threonine (12, 13, 565). 



The keto analogue of leucine, a-ketoisocaproic acid, replaces leucine 

 for a mutant of Neurospora crassa, and is considered therefore to be a 

 precursor of it (432). Earlier steps in leucine biosynthesis by fungi are 

 not known. 



Studies on Neurospora crassa have shown that both a -aminoadipic 



