ESSENTIAL NONMETALLIC ELEMENTS 93 



Volkoiisky (1933, 1934) observed that certain of the aquatic Phycocomy- 

 cetes failed to utiHze sulfate sulfur. These species were Saprolegnia 

 parasitica, Isoachlya monilifera, Achlya prolifera, A. polya7idra, A. oblon- 

 gata, A. conspicua, Dichtyuchus monosporus, and Aphanomyces sp. A 

 total of 26 isolates failed to utilize sulfate sulfur. This investigator 

 (1933a) designates ability to utilize 6-valent sulfur as euthiotrophy and 

 inability to utilize sulfate sulfur and ability to utilize reduced sulfur as 

 parathiotrophy . 



Fries (1946) was able to induce mutation in Ophiostoma (Ceratostomella) 

 rmiltianmilatum by irradiating the ascospores with X rays. Among 

 these mutants 13 strains were unable to utilize sulfate sulfur. Only five 

 of these strains regained this ability when cultivated on media containing 

 sulfate. These parathiotrophic strains of 0. multiannulaium utilized 

 ammonium sulfide as well as cystine and cysteine as sources of sulfur. 

 From the fact that these mutants could utilize sulfide sulfur, it is evident 

 that these strains were not deficient for specific sulfur-containing amino 

 acids. Bonner (1946) has, however, found induced mutants of Peni- 

 cillium to be deficient for specific sulfur-containing amino acids. Blasto- 

 cladia pringsheimii has been reported to require methionine (Cantino, 

 1949). 



Fries (1948) has reported the occurrence of natural mutants of Ophi- 

 ostoma multiannulatiim which require reduced sulfur, and also mutants 

 which are unable to synthesize methionine. Of a total of 51,037 single- 

 conidium cultures, 2 required reduced sulfur and 30 required methionine. 



The role of sulfur. The use fungi make of sulfur may be deduced from 

 the sulfur-containing compounds which are known to occur in mycelium 

 and spores. Among these are the proteins. In Chap. 4 it was noted that 

 the activity of many enzymes depends upon the sulfhydryl or thiol group, 

 ■ — SH. On hydrolysis, fungus protein yields the following sulfur-contain- 

 ing amino acids: cystine, cysteine, and methionine. Sulfur is thus a 

 structural element. Another sulfur-containing compound is the tripep- 

 tide, glutathione, which is abundant in yeast. The formula for gluta- 

 thione is given below: 



COOH CH2SH 



H2N— CH— CH2— CH2— CONH— CH— CONH— CH2— COOH 



This compound is sometimes represented by the symbol GSH. In spite 

 of intensive investigation the role of this compound is not fully under- 

 stood. Perhaps one of its functions is to protect sulfhydryl enzymes 

 from inactivation. 



The probable mechanism of the biosynthesis of cystine has been studied 

 using mutants of Aspergillus nidulans (Hockenhull, 1949). All these 

 cystine-deficient mutants were able to utilize thiosulfate sulfur, methio- 



