314 Gene Action 



produce a darker color such as brown or black. Both enzymes 

 are oxidizing enzymes, but enzyme II has no effect unless en- 

 zyme I is also present. The exact color produced will depend 

 not only upon the presence of these two enzymes but also upon 

 their relative amounts and potencies. As these factors may 

 vary with different genes, a wide variety of colors may be pro- 

 duced. Thus a number of different possibilities exists even 

 with a system involving only two enzymes. The important 

 thing is that all these chemical- differences are the result of gene 

 action. 



Vitamins in Neurospora 



Beadle and Tatum have shown that certain gene mutations 

 can be induced which will prevent the synthesis of certain vita- 

 mins by the ascomycete Neurospora. They grew strains of this 

 fungus and then subjected them to X-rays just before meiosis 

 with the purpose possibly of inducing gene mutations. Single- 

 spore cultures from the X-rayed material were then grown on a 

 medium containing as many as practicable of the chemical sub- 

 stances which this fungus normally synthesizes. When estab- 

 lished, they were transferred to culture media containing none 

 of these substances. The theory was that if a strain grew on 

 the "complete" medium but not on the other, a gene mutation 

 had occurred which prevented the mutated strain from synthe- 

 sizing something, since the untreated fungus will grow well on 

 media which do not contain any of these substances. By then 

 growing the mutated strain on a series of media lacking dif- 

 ferent ones of these constituents, it could be determined which 

 particular substance the fungus could not synthesize. About 

 2000 such strains were grown, and three were found that could 

 not grow on the '^incomplete or minimal" medium. Subsequent 

 tests showed that one strain could not synthesize vitamin Be 

 (pyridoxine), the second could not synthesize vitamin Bi (thia- 

 mine), or more exactly the thiazole half of the Bi molecule, and 

 the third strain could not synthesize para-aminobenzoic acid. 

 All three mutations were proved to be single-gene mutations. 

 Since all these substances are essential for growth, and since 

 they are normally synthesized by this fungus, this is an example 

 of genes which normally act by the synthesis of certain chemical 

 growth substances. 



