244 NITROGEN NUTRITION AND METABOLISM 



also true of Neurospora crassa (511). Detailed study of the phenome- 

 non (345, 373, 375) brings out that nitrate utilization is completely 

 stopped by the addition of ammonium salts, provided the ammonia 

 is assimilated, i.e., that a carbon source is available. Since nitrite uti- 

 lization is not blocked by ammonium, it must be assumed that in these 

 fungi ammonium prevents the reduction of nitrate to nitrite; this is 

 borne out by enzymatic studies, which show that the nitratase system 

 declines in quantity soon after ammonia assimilation begins. 



Analytical data on utilization of ammonium nitrate by Helmintho- 

 sporium gramineum show a different relation: ammonium ion is used 

 much more rapidly than nitrate, but there is a slight assimilation of 

 nitrate even in the early stages of growth (110). This relation is 

 stated to prevail in some other fungi (375). 



The preferential utilization of ammonia from ammonium nitrate is 

 influenced by pH. Foster (163) reviews several studies in Aspergillus 

 niger, all of which agree that at very low pH ammonium utilization is 

 reduced and at least some nitrate is assimilated. Attempts have been 

 made to explain this phenomenon as one of permeability or as based 

 on the adsorption capacity of cell colloids (86, 264); neither of these 

 explanations is convincing, and it would appear desirable to reinvesti- 

 gate the phenomenon itself before speculating on its theoretical im- 

 plications. As shown later (Figure 3), the optimum pH for nitrate 

 uptake in Scopulariopsis brevicaulis is about 6.0, far above the "critical" 

 pH at which Aspergillus niger is claimed to assimilate nitrate (439). 



Nitrate reduction in the fungi is believed to be strictly assimilatory 

 in nature, that is, to occur only in the incorporation of nitrate nitrogen 

 into the cell. This process is in contrast to that of the bacteria, many 

 of which reduce nitrate without assimilation of the nitrogen; nitrate, 

 if it has an essential function, is acting as a hydrogen acceptor lor sub- 

 strate oxidations (556). This "nitrate respiration" appears to be 

 mediated by the cytochrome system in Micrococcus denitrificans and 

 other bacteria (460, 557); whereas, as will be seen, nitrate reduction in 

 the fungi is mediated, so far as we know, by flavin enzymes. It should 

 be obvious, of course, that utilization of nitrate in fungi does depend 

 upon substrate oxidations and affects, therefore, the respiratory quo- 

 tient (521). 



The nitrate reductase of Neurospora crassa has been studied inten- 

 sively; this work is reviewed by Nason (384). The enzyme is formed 

 only on nitrate or nitrite media, i.e., it is inducible. Flavin adenine 

 dinucleotide is the prosthetic group; sulfhydryl groups are essential to 

 activity, and molybdenum is an essential factor in electron transport. 



