268 NITROGEN NUTRITION AND METABOLISM 



formation. It is not always realized that there are serious pitfalls in all 

 methods which seek to state unequivocally that this or that compound 

 is an obligatory precursor of a given metabolite. Adelberg (11) has 

 critically reviewed these problems with the conclusion that "present- 

 day techniques are unable to establish metabolic intermediates with 

 finality." In practice, one approaches certainty or conviction as more 

 and more evidence from different types of experiment is accumulated 

 (123). 



It seems appropriate to include here only a sketch of the more im- 

 portant biosynthetic pathways which are known, or are likely, to be 

 functional in one or more fungi. As will be seen, there is little evidence 

 from fungi other than Neurospora crassa. Following Davis (123), we 

 shall consider groups of amino acids which are known to be biosyn- 

 thetically related. 



A family of amino acids is derived more or less directly from glutamic 

 acid. Omitting details, the following over-all formulation may be sug- 

 gested (123, 500): 



glutamine 



Glutamic acid 



* proline — > hydroxyproline (11) 



Evidence from fungi is, however, limited at present to the glutamate- 

 proline interconversion (9, 64, 561, 624), and the appearance of A'- 

 pyrroline-5-carboxylic acid, the immediate precursor of proline in 

 Equation 11, in glutamate cultures of Aspergillus oryzae (486). 



Ornithine biosynthesis in Neurospora crassa starts from glutamic acid 

 also; the pathways, however, are not at all clear. Vogel (560) sug- 

 gests the following sequence in N. crassa and Torulopsis utilis: 



Glutamic acid -^ glutamic y-semialdehyde — > ornithine (12) 



This is rather different from the Escherichia coli system (123), which 

 depends upon acetylated intermediates. There may be in N. crassa 

 some other pathway of ornithine biosynthesis than that of Equation 12 

 (153, 560). 



Ornithine participates in the ornithine cycle, first worked out in 

 animal systems (307) and later found to function in Neurospora crassa 

 (156, 490). The basic elements of the cycle, leaving aside biochemical 

 details, are schematized in Figure 8. N. crassa, although forming 

 urease, does not liberate urea; we may suppose that the cycle here 

 serves primarily to supply arginine for protein synthesis. Nutritional 

 data suggest the same general pattern in Penicillium chrysogenum (64). 

 Urea is a common product of fungi, but direct evidence implicating 



