256 NITROGEN NUTRITION AND METABOLISM 



nomenon in the usual sense; about 25 per cent of the nitrogen supplied 

 appears in the medium, and it is not reutilized. 



The enzymatic basis of peptide synthesis in fungi is at present un- 

 known. Extracts of Neurospora crassa mycelium show y-glutamyl 

 transferase activity; conceivably this system could function in the syn- 

 thesis of peptides of glutamic acid (563). In Bacillus subtilis glutamyl 

 di- and tripeptides are synthesized by transfer reactions of this general 

 type (600). 



3. UTILIZATION OF AMINO ACIDS AND AMIDES 



Amino Acids and Amides in Nutrition. The specificity of response 

 to different amino acids indicates that in fungi as in other organisms 

 amino acids are assimilated primarily as such, and that conversion to 

 ammonia before assimilation of the nitrogen is at best a secondary 

 pathway (44, 542). It can be shown by analysis that amino acids are 

 assimilated at varying rates from a complex medium and that they 

 exist in part, as mentioned earlier (Chapter 2), as free acids in the 

 mycelium. Typically, the amino acid content of a medium drops as 

 the acids are taken up, then rises again as they are liberated during 

 autolysis (181, 413, 423). 



The value of individual amino acids for fungi is customarily as- 

 sayed by growth on media containing the amino acid of interest as the 

 sole nitrogen source. Studies of this type are numerous but generaliza- 

 tion from them is difficult; we find that a given amino acid allows very 

 good growth of one organism and only very little growth of another, 

 but we are not able to say whether this reflects permeability, enzymatic 

 capacities, or merely such secondary problems as acidity changes conse- 

 quent upon utilization. In addition, amino acids and amides from 

 natural sources may be contaminated with vitamins and there may 

 also be metabolic consequences of the chelation of inorganic ions by 

 amino acids. 



Most investigations more or less agree that glycine, asparagine, glu- 

 tamic acid, and aspartic acid are the most likely to support good 

 growth. However, it is relatively easy to find fungi which grow only 

 poorly with asparagine (164, 252, 440, 625), aspartate (110, 278), or 

 glutamate (275, 278). Conversely, leucine is generally not an adequate 

 source of nitrogen (409), but for Trichophyton persicolor it is one of 

 the most easily utilizable of the amino acids (506); tryptophan, usually 

 not well utilized, supports normal growth of Streptomyces coelicolor 

 (106). Examples of this type could be multiplied to establish the 

 point that, although generalizations may be made which fit most 



