IDS 



CHAPTER 32 



pend upon what is considered to be primarj 



and what is identified as a phenotypic effect. 

 An additional implication of the one gene- 

 one primary effect hypothesis is that it we 



know the nature oi a primar\ etleet. it should 

 always be the result of one gene. To test 

 this prediction, it is accessary to decide 

 which particular aspects of the phenotype arc 

 primary effects of gene action. 



The cases just discussed indicate that a 

 mutant gene has a primary effect upon the 

 catalvtic ability of an enzyme. If we assume 

 that the catalytic abilities of all enzymes re- 

 sult from the primary action of genes, it 

 should be possible to study any particular 

 enzyme and find that its catalytic ability can 

 be altered or abolished as the result of mu- 

 tation. Experimental support for this one 

 enzyme-one gene hypothesis would also 

 provide specific — though limited — support 

 for the more general concept of one gene-one 

 primary phenotypic effect. 



One Enzyme-One Gene Hypothesis 



Not only is Neurospora a good organism 

 for studying genetic recombination (p. 124), 

 but it has certain very favorable characteris- 

 tics for biochemical studies. Neurospora 

 can manufacture all the components it needs 

 to exist and to reproduce from a basic, very 

 simple, food medium consisting of water; an 

 array of inorganic salts; sources of nitrogen, 

 phosphorus, sulfur; various trace elements; 

 a carbon and energy source such as a sugar; 

 and a single vitamin, biotin. From these raw 

 materials it can synthesize all twenty different 

 amino acids, all essential vitamins (except 

 biotin), purines and pyrimidines, and every- 

 thing else needed for its total activity. Ac- 

 cording to the hypothesis under considera- 

 tion, it should be possible to induce mutants 

 that change the catalytic ability of enzymes, 

 thereby blocking various chemical syntheses. 

 Previous work has established that the 

 last step in the synthesis of vitamin B x , or 

 thiamin, is normally accomplished by the 



enzymatic combination of a particular thi- 

 azole with a particular pyrimidinc. If the 

 catalytic action of every enzyme depends 

 upon the primary action of genes, it should 

 be possible to induce a mutation in the gene 

 that normally specifies this B,-forming en- 

 zyme. If the mutant no longer produces the 

 active B,-forming enzyme, no B, will be 

 made. Since B. is required for growth, the 

 mutant mold will be auxotrophic and require 

 B, in its diet to grow. 



An experiment can be performed -' in 

 which asexually-produccd, haploid spores 

 (p. 26) are treated with a mutagenic agent 

 such as X rays or ultraviolet light. The 

 treated spores are then grown on the basic 

 medium supplemented with vitamin Bi. The 

 spores that grow include prototrophs for B, 

 as well as auxotrophs which obtain their B, 

 from the culture medium. Once the spores 

 have grown sufficiently, a portion of each 

 of the growths is placed on a basic, minimal 

 medium supplemented with the particular 

 thiazole and pyrimidine that are the immedi- 

 ate precursors of vitamin B,. (All other 

 imaginable nutritional factors except B, it- 

 self can also be added, but they will have no 

 effect on the outcome. ) Cultures which fail 

 to grow on a medium which contains the 

 immediate precursors of Bi are clearly de- 

 fective for the enzyme that catalyzes the last 

 step in B, synthesis. Stocks of such cultures 

 are made from samples growing in the pres- 

 ence of B,. To test for and localize the 

 genetic basis for the B, auxotrophy, each of 

 these haploid strains is crossed to a haploid 

 strain normal for B, synthesis. The diploid 

 hybrid is formed and undergoes meiosis (pp. 

 124-126). producing a sac containing eight 

 haploid ascospores. Each spore is removed 

 and grown on a Bi-supplementcd minimal 

 medium. If the haploid strain under test 

 were Bi-deficicnt because of a mutation, 

 transplants of each of the eight haploid cul- 



- Based upon work of G. W. Beadle and E. L. 

 Tatum. 



