E. L. TATUM 455 



homoserine, and isoleucine. Teas, Horowitz and Fling (85) and Teas (84) 

 have shown that isoleucine and threonine are interconvertible in Neuro- 

 spora and that threonine and homoserine are likewise interchangeable. 

 These relationships and the demonstrated activity of a-aminobutyric 

 acid as a precursor of threonine and homoserine (84) are intriguing 

 relationships which remain to be elucidated. One promising lead in the 

 investigation of these relationships may prove to be the examination 

 of mutant strains oiNeurospora which require both isoleucine and vaHne. 

 Continuing the work initiated by Bonner (10), a precursor of isoleucine 

 accumulated by such a mutant strain has been identified as a:,/3-dihy- 

 droxy/S-ethylbutyric acid (i). The results of studies using carboxyl- 

 labeled acetate are consistent with the fairly direct conversion of acetate 

 to the 18-ethyl side chain in the isoleucine precursor (2). These results 

 strongly suggest that isoleucine and therefore probably threonine and 

 homoserine are synthesized in Neurospora from a common four-carbon 

 precursor. 



Of fundamental importance in studies in biochemical genetics in 

 fungi as well as in other organisms is the concept that gene mutation is 

 specifically related to biochemical reaction. Implicit in this concept is 

 the hypothesis that each gene controls a specific biochemical reaction 

 through the action of a specific enzyme (13). The gene and the enzyme 

 are pictured as being specifically related in such a way that gene muta- 

 tion with a change in the spatial or configurational specificity of the 

 gene results in corresponding changes in enzyme specificity. Mutation 

 of a given gene could then result in failure of the biochemical reaction 

 either if no enzyme at all is produced, or if an enzyme is formed with 

 altered properties such that it can no longer carry out the specific 

 enzymatic function. The examination of particular mutant strains of 

 Neurospora for the presence of specific enzymes is of considerable im- 

 portance in experimentally testing the general concept. In this examina- 

 tion it is, of course, vital to test for differences in the presence or activity 

 of the specific enzyme between the wild-type and the mutant strain. 



One of the first examinations of an enzyme system from this point 

 of view was carried out by McElroy and Mitchell (49) in their studies 

 of the adenine-deaminase system of a temperature-sensitive adenme- 

 requiring strain of Neurospora. Since this strain could synthesize adenine 

 at temperatures under 28°C. but could not do so at higher temperatures, 

 and since hypoxanthine was inactive for the mutant at higher tempera- 



