216 PHYSIOLOGY OF THE FUNGI 



casei was used but were increased when L. arahinosus was the test 

 organism. 



Riboflavin. Lactohacillus casei. Fatty acids stimulate growth. 

 Growth may be measured by titrating the acid formed, or turbidimetri- 

 cally (Roberts and Snell, 1946). It is probable that mutants of Neuro- 

 spora deficient for this vitamin may also be used in assay. 



AMINO-ACID ASSAYS 



The importance of the amino-acid composition of proteins used in 

 animal nutrition makes any advance in analytical methods of great 

 interest and value. The general techniques for amino-acid determina- 

 tions by microbiological procedures are the same as for other assays. 

 The first requirement of this type of microbiological assay is a suitable 

 test organism. Few fungi isolated from nature are deficient for amino 

 acids. For this reason bacteria have been extensively used. The follow- 

 ing references will give an entry into the literature on the use of bacteria 

 for amino-acid assay: Hutchings and Peterson (1943); Shankman (1943); 

 Dunn et at. (1944); Snell (1945); and Horn et at. (1950). 



Some mutants of N'eurospora have been found to be deficient for amino 

 acids. Mutants having the following amino-acid deficiencies have been 

 studied: leucine, isoleucine, valine, lysine, methionine, serine, or glycine. 

 Only the mutant deficient for leucine appears to have been much used in 

 microbiological assay (Ryan and Brand, 1944; Brand et al., 1945). The 

 growth of a lysine-deficient mutant was completely inhibited by arginine 

 when the molecular ratio of arginine to lysine was 2 to 1 (Doermann, 

 1944). 



Ryan (1948) has considered the possibility of microbiological assay 

 of amino acids by observing the percentage of germination of conidia 

 from deficient mutants in the presence of different concentrations of the 

 specific amino acid. An assay can be completed within a few hours by 

 this method. Unfortunately the inhibiting action of certain amino acids 

 introduces complications into the proposed method. 



Mutations of Neurospora and certain other fungi have been induced 

 by chemicals, such as nitrite or nitrous acid, colchicine, nitrogen mustard 

 gas, and hydrogen peroxide, or by irradiation with ultraviolet and X rays. 

 These mutants are frequently characterized by inability to synthesize 

 various metabolites. They differ from the parent wild type in that one 

 or more genes have been inactivated. It is thought that each gene con- 

 trols a single biochemical reaction. Mutants having the same gross 

 deficiency may differ in the specific gene inactivated. 



Horowitz (1947) studied four mutants of Neurospora which were unable 

 to synthesize methionine from inorganic sources of nitrogen and sulfur. 

 One of these mutants was able to grow in the presence of cysteine, cysta- 



