UTILIZATION OF AMINO ACIUS AND AMIDES 259 



or D-alanine, but not with D-leucine or D-phenylalanine as nitrogen 

 source (62). 



It seems likely that utilization of the D-amino acids depends upon 

 oxidative deamination by D-amino acid oxidase (p. 260); ammonia 

 liberated by the oxidase system is then presumed to be incorporated 

 into i.-acids (245). Specific racemases effecting direct interconversion 

 of the d- and L-forms, are known in bacteria (22, 28, 286, 383, 608), 

 not as yet in fungi. Presumably a second factor in utilization of the 

 D-amino acids is their occasional toxicity (149). 



Specific amino acid requirements appear with reasonable frequency 

 as induced mutations in fungi. They are, however, uncommon in 

 nature and, when found, are more apt to be conditioned or partial 

 deficiencies than absolute requirements. Thus, Mycena rubromargi- 

 nata responds in growth to tyrosine or phenylalanine, but grows with- 

 out either (169), and Cenococcum graniforme has a partial require- 

 ment for histidine (358). Eremothecium ashbyii requires several 

 amino acids only in acid media (619). An absolute requirement for 

 histidine has been reported in Trichophyton spp. (135, 136. 196), as 

 has a requirement lor arginine in T. tonsurans (515); these reports 

 are based on responses in agar media. Labryinthula minuta var. at- 

 lantica requires leucine (559), Lentinus omphalodes tryptophan (171). 



Requirements for methionine or cysteine are assumed to be related 

 to sulfur rather than nitrogen metabolism, and are considered in 

 Chapter 9; cysteine may also promote the growth of an organism re- 

 quiring a low oxidation-reduction potential (275). 



Transport of Amino Acids into the Cell. In the present context, 

 the main purpose of this section must be to call attention to a problem 

 which has been only tentatively explored in the fungi, and for the 

 solution of which promising experimental techniques are available. 

 We know that a free amino acid pool exists in fungal cells, that trans- 

 port across the membrane occurs in both directions, and that proteins 

 and peptides are synthesized. In bacteria, yeast, and animal cells 

 there is good evidence that some or all amino acids enter the cell by an 

 active transport mechanism requiring an expenditure of metabolic 

 energy, but different organisms cannot at present be fitted into any 

 single pattern (100, 183, 188); lysine accumulation, for example, ap- 

 pears to be independent of an energy source in Gram-positive bacteria, 

 but glutamic acid enters only if glucose is being respired. Histidine 

 enters the cells of Ncurospora crassa against a concentration gradient; 

 the uptake is inhibited by arginine and methionine, which are, conse- 

 quently, toxic to a histidine-requiring mutant (350). An analogous 



