THE FUNCTIONAL ORDER 



OPERON 



structural genes 



reguloting gene 



operator 



L^^' 



enzyme forming 

 systems 



Qd O/i Oy 6^ enzymes 



Figure 23. Control of the Activity and Synthesis of "Tryptophan 

 Enzymes." 



The activity of the systems of enzymes involved in the synthesis of tryptophan 

 (tryptophan zymon) is under a dual control. 



1. The activity of the first enzyme is specifically inhibited by tryptophan, 

 the end product of the tryptophan zymon activity. 



2. The synthesis of the tryptophan zymon is specifically inhibited by a 

 repressor. The repressor is the result of the interaction of an aporepressor 

 produced by a regulating gene and of tryptophan, which is a corepressor. The 

 yet-hypothetical operator is in position go in the absence of repressor and is 

 switched to stop by the repressor. 



The term zyvion is proposed to save time and money. Zymons belong to 

 two categories: anazymons, which are responsible for anabolism, that is, syn- 

 theses; and catazymons, which deal with catabolism or breakdown. The system 

 considered here is the tryptophan anazymon, whereas /3-galactoside permease 

 and /3-galactosidase constitute the lactose catazymon. 



bacterium synthesizes an excess of tryptophan, either because of a 

 surplus of enzymes or because of a relative deficiency in the tryp- 

 tophan consumption. The substance in excess, tryptophan, enters 

 the constitution of a repressor. The operator, normally in position 

 go, receives the signal stop. Tryptophan, the end product of a chain 

 of enzymes, stops the synthesis of these enzymes. This is a true 



[59] 



