MECHANISMS OF GENE ACTION 



275 



-*- A 



Enzyme 

 synthesis 



Gene 1 



1 



-^B 



(Essential 

 metabolite) 



(a) 



Competing reactions 



Coupled reaction 



Functioning 

 metabolite 



Enzyme 

 degradation 



Inhibitor or competing 

 substrate 



(b) 



FIGURE 10.2. Diagram of a metabolic block in the conversion of A to 8 resulting from 

 mutation. 



(o). Simplified scheme. Mutation in gene 1 leads to inactivation of enzyme f, . 



(b). Other possibilities. Inability to form 6 could result from a mutation altering 

 any one of the reactions which influence the conversion of A to 6 (from Wagner & 

 Mitchell 1955, Genetics and Metabolism, New York, Wiley, p. 219). 



equivocal guide to the identity of the relevant enzyme. And so even at 

 the cellular level, the correlation of genotype and phenotype is made 

 difficult by the highly integrated state of intracellular metabolism. 



Nonetheless, altered enzymatic activity following mutation has been 

 demonstrated for about 50 different enzymes in microorganisms, hi most 

 of these systems, only a quantitative decrease or loss of activity has been 

 shown. One cannot safely infer a mechanism of gene action from these 

 examples, because, as just discussed, decreased enzyme activity may 

 occur for many reasons in addition to alteration of the specific enzyme 

 in question. The only systems in which the action of the gene can be 

 attributed unequivocally to a direct effect upon an enzyme are those 

 in which the mutated strain produces a qualitatively altered enzyme, 



