306 INORGANIC NUTRITION AND METABOLISM 



organic molecule, but the scheme is in agreement with nutritional 

 data on genetically different mutants of Neurospora crassa (89), Asper- 

 gillus nidulans (83, 188), and Venturia inaequalis (104). Shepherd 

 (219) proposes a dual pathway of cysteine biosynthesis in A. nidulans. 



The naturally occurring fungi which require reduced sulfur are, we 

 may suppose, unable to carry out one or more of the reactions of Equa- 

 tion 1. Thus Blastocladiella emersonii grows with either cysteine or 

 methionine (13) and is presumably blocked prior to cysteine synthesis; 

 Allomyces arbuscula, on the other hand, appears unable to use any 

 source of sulfur other than methionine (99). The same considerations 

 apply to other fungi which require cysteine or methionine (131, 254). 



Bulk reduction of sulfate with evolution of hydrogen sulfide is not 

 effected by fungi, as it is by some bacteria. Candida spp. reduce sulfite 

 (173), many fungi enzymatically reduce elemental sulfur to H 2 S (125, 

 144, 216), and Neurospora crassa reduces selenite to selenium (259). 



Sulfur and sulfur compounds are oxidized by fungi: elemental sulfur 

 by Penicillium luteum (1) sulfur-containing amino acids by Aspergillus 

 niger (68, 148), cysteine by Microsporum gypseum (224). Cysteine-sul- 

 finic acid has been proposed as an intermediate in cysteine oxidation 

 (224). 



A few minor aspects of sulfur metabolism may be mentioned in 

 conclusion. Commercial enzyme preparations from Aspergillus spp. 

 contain a phenolsulfatase, hydrolyzing compounds of the type R — 

 OS0 3 K (2, 162, 163, 165); other sulfatases have not been reported 

 from fungi (66). Penicillium notatum is able to split a carbon-sulfur 

 bond (32). The toxicity of barium to Aspergillus niger results from 

 the removal of sulfate, and barium is not toxic if sulfur is supplied in 

 some other form (228, 234). Finally, the reduced sulfur requirement 

 of the yeast phase of Histoplasma capsulatum (186, 209) is a require- 

 ment for reducing conditions, not for reduced sulfur as such. Transfer 

 of methyl groups to sulfur is summarized in Chapter 6; reductive steps 

 occur in the over-all metabolism of sulfur, selenium, and tellurium. 



4. MAGNESIUM 



Magnesium, customarily provided as the sulfate at about 0.001 M, 

 probably has as its principal essential function the activation of en- 

 zymes necessary to normal metabolism and growth. The optimum 

 is dependent on the concentration of the carbon source (241), the 

 production by the organism of hydroxy acids or other chelate-forming 

 molecules which tend to render magnesium unavailable, and the con- 

 centrations of other ions to which magnesium is antagonistic. Asper- 



