45^ PLANT GROWTH SUBSTANCES 



tures at which adenine was required, it was expected that the adenine- 

 deaminase in the mutant might have a different temperature sensitivity 

 from that in the wild-type. McElroy and Mitchell (49) found, however, 

 no demonstrable differences in the enzyme from the wild-type and 

 mutant strains. It was pointed out by these authors that this instance 

 might not represent a critical test of the general concept since adenine 

 synthesis might normally proceed by a reaction other than a reversal of 

 the deaminating system. Another instance which has been examined in 

 Neurospora is that of the enzyme involved in pantothenic acid synthesis 

 from pantoic acid and |S-alanine. Wagner and Guirard (88) first reported 

 the enzyme which brings about this condensation to be missing in the 

 pantothenicless mutant, and to be present in the wild-type strain. 

 However, Wagner has been able to demonstrate that the enzyme is 

 actually present in the mutant strain (87). A third instance recently 

 investigated is that of the enzyme bringing about the synthesis of 

 tryptophan by condensation of serine with indole, first demonstrated 

 by Umbreit, Wood, and Gunsalus (86). In an investigation of a mutant 

 strain of Neurospora which is unable to grow on indole but requires 

 intact tryptophan, Mitchell and Lein (55) reported that this enzyme 

 was not present in the mutant. This example apparently provides ex- 

 cellent evidence in support of the general concept discussed above. 



In two examples in Neurospora, which may prove to be pertinent, 

 carboxylase has been demonstrated in a strain which requires acetate 

 or ethanol for growth, and asparaginase has been found in a strain which 

 has a specific requirement for asparagine (83). Although generahzations 

 from relatively few specific examples are always dangerous, the instances 

 just discussed might indicate that gene mutation may affect metabolism 

 in some microorganisms by modifying intracellular conditions so that 

 the enzyme is normally inoperative; or alternatively by leading to the 

 production of a specific enzyme inhibitor, and thus indirectly to the 

 production of an enzyme inactive in vivo. Gene mutation, therefore, 

 in at least some instances, may modify a biochemical reaction in the cell 

 not by direct modification or elimination of a particular enzyme, but 

 rather by controlling its //; vivo activity in some manner not as yet 

 understood. 



Some additional suggestions as to the nature of the gene-enzyme 

 relation may come from examination of the three general types of 

 behavior of biochemically deficient strains of fungi which have been 



