274 CELL HEREDITY 



Methods were developed, as mentioned first in (Chapter I initially by 

 Beadle and Tatum and then by many other investigators, for isolating 

 mutant strains, now called auxotrophs, each of which, as the result of a 

 single gene mutation, required a particular growth factor such as an 

 amino acid, vitamin, purine, or pyrimidine, not recjuired for growth 

 by the wild-type strain. Large numbers of such mutants were found, 

 and with many of them, the particular biosynthetic step blocked by 

 mutation was identified. The use of these mutant strains greatly facili- 

 tated the analysis of the biosynthetic pathways of many of the amino 

 acids, nucleotides, and cofactors of intermediary metabolism. 



These investigations firmly established the principle that gene muta- 

 tions lead to blocks in specific step reactions, and, by inference, that 

 wild-tvpe genes act by making possible those reactions which the mutant 

 genes block. Similar proposals had been made numerous times in the 

 past, for example, by the Dutch microbiologist Beyerinck in 1903, and 

 the English physician Garrod in 1923. Nonetheless it is the studies 

 carried out since 1941 with the auxotrophic mutants of Neurospora, E. 

 coli, and other microorganisms that have provided a vast body of evidence 

 indicating that virtually all biosynthetic reactions which are under 

 enzymatic control are thereby under gene cojitrol. 



QUALITATIVELY ALTERED ENZYMES 



But what do these investigations tell us about the mechanism of gene 

 control? There has been a widespread tendency to assume that, if a par- 

 ticular step is blocked following mutation, then, a priori, the enzyme 

 mediating that step has been altered or lost. Upon further scrutiny, this 

 assumption has proved to be invalid in many instances. The blocking of 

 a reaction following mutation may result from (a) a qualitative or quan- 

 titative change in the enzyme controlling that reaction step, or (b) a 

 change elsewhere which indirectly affects the reaction. Since branching 

 and reversibility are common features of the pathways of intermediary 

 metabolism, many of the observed blocks in particular reactions follow- 

 ing mutation actually result from the indirect effects of other systems. 

 In such instances, the enzyme of the blocked reaction may be quite 

 normal. A number of such possibilities are indicated in Figure 10.2. 



An example is the tyrosinase-forming system in Neurospora, briefly 

 described in Chapter 1. Two different loci, ty-1 and ty-2, have been 

 identified, each of which totally blocks tyrosinase formation except under 

 special nutritional conditions; then the enzyme is formed. Thus the 

 identification of the step blocked by mutation does not provide an un- 



