FUNGI AS TEST ORGANISMS 217 



thionine, homocysteine, and methionine. The second was unable to 

 utihze cysteine but was able to utilize the other three compounds. The 

 third isolate utilized either homocysteine or methionine, while the fourth 

 isolate utilized only methionine. From these results the steps in the 

 synthesis of methionine and the genes inactivated may be summarized as 



gene 4 gene 3 gene 2 gene 1 



follows : > cysteine > cystathionine > homocysteine — -^ methi- 

 onine. From similar studies Srb and Horowitz (1944) concluded that 



Neurospora synthesizes arginine as follows: ornithine — -^ citrulline > 



arginine. 



Fungus mutants have proved to be powerful tools for investigating 

 pathways of synthesis and utilization of vitamins, amino acids, and other 

 compounds, and in studies of biochemical mutations. From these studies 

 also comes the realization that each step in the synthesis or utilization of 

 a compound may be controlled or limited by a specific gene. The review 

 papers of Bonner (194G) and Beadle (1945, 1945a) should be consulted 

 for further information and literature citations. 



ASSAYS FOR ESSENTIAL ELEMENTS 



Microorganisms may be used to determine the presence of essential 

 elements. In view of the speed and accuracy of chemical and spectro- 

 scopic methods, it might be assumed that microorganisms would be of 

 little value in such applications. The value of microbiological tests 

 would appear to be in applications where availability as well as total 

 amounts are of importance. Problems of this sort frequently arise in 

 connection with mineral deficiencies in soil. It is recognized that the 

 absolute content of an essential element in a soil may not measure the 

 availability of that element for green plants. Microbiological and chem- 

 ical methods of analysis must be correlated with plant tests before they 

 are of much value. 



The possible number of test organisms is unlimited except for the 

 important considerations of sensitivity, ease of handling, and time 

 required to make an assay. In practice, only a few organisms have been 

 used. There exists a wide field for investigations dealing with the cor- 

 relation between availability to microorganisms and availability to green 

 plants of certain essential elements in soil. 



Copper. Mulder (1939-1940) used Aspergillus niger to determine 

 copper in soil. The range of concentrations in the standard series was 

 0.0 to 2.5 Mg Cu++ per culture; 40 ml. of medium was used in liter flasks. 

 One gram of sterile soil was used as the sample. The method of measur- 

 ing the response of A. niger to copper was very simple, inasmuch as the 

 number and color of the spores produced were functions of the copper 

 content of the medium. No spores developed on the control medium, but 



