THE FUNCTIONAL ORDER 



Whether the tryptophan has been manufactured by the bacterium 

 or pumped from the medium into the bacterium, the result is the 

 same. 



Carbon and energy source. How does the aporepressor-repressor 

 hypothesis apply to the regulation of enzymes involved in the 

 metabolism of the carbon and energy source? 



When lactose is present as the sole carbon and energy source, the 

 bacterium "needs" galactosidase in order to grow, and it starts 

 synthesizing it, provided it possesses the corresponding genetic 

 information. 



How does lactose induce the synthesis of the enzyme? It is perhaps 

 simpler to consider first the inhibition of the synthesis. When bac- 

 teria are growing in a medium containing glucose and lactose, no 

 ^-galactosidase is produced by the z+ bacteria. Glucose is used up 

 first, and the synthesis of /^-galactosidase starts, but only after a 

 negative phase that lasts around 30 minutes. Things happen as if in 

 the presence of glucose a repressor has been produced. The theory 

 is that a product of the metabolism of glucose acts as a corepressor 

 \\'hich combines with an aporepressor. As long as this repressor 

 is present, no /^-galactosidase is synthesized. When it has disap- 

 peared as a result of the cellular metabolism, the synthesis of the 

 enzyme can start. 



Why then is lactose necessary? Glucose has been used up in the 

 first phase of the bacterial development. But when it is used up, 

 metabolism continues at the expense of the reserves that the bac- 

 terium has stocked. A corepressor perhaps continues to be pro- 

 duced, but it has less affinity for the aporepressor than the product 

 of glucose metabolism. Lactose competes for the aporepressor with 

 this hypothetical corepressor of low affinity and thus prevents 

 the formation of the repressor. The ;8-galactosidase is synthesized 

 and functions: lactose is hydrolyzed into glucose and galactose. 

 The latter are used up in the metabolic processes. If glucose is used 

 up entirely, a corepressor is not produced or is not available. But 

 if an excess of /^-galactosidase is synthesized, an excess of glucose is 

 produced as a result of its activity. A corepressor will be formed 

 and, consequently, a repressor. The synthesis of the enzyme respon- 

 sible for the metabolism of lactose will be repressed. In view of 

 the relatively rapid adjustment of enzyme synthesis to the need, 



[53] 



