BIOLOGICAL ORDER 



this hypothesis implies that the repressor is unstable and that its 

 half-life is short, let us say a few minutes. 



The question will immediately be asked: How does /?-galacto- 

 sidase behave in a constitutive bacterium? Some bacteria are consti- 

 tutive because they do not produce a repressor, some others because 

 the "operator" (explained in the next section) is insensitive to the 

 repressor. In both cases, whether or not lactose is present, the bac- 

 terium contains yff-galactosidase. Estimates show that the consti- 

 tutive bacteria contain about 30% more enzyme than the inductive 

 ones. Enzyme synthesis is of course limited by something, let us 

 say by the availability of amino acids. Anyhow, in a medium devoid 

 of lactose, the enzyme is useless. 



When adaptive and constitutive bacteria are inoculated together 

 into a medium devoid of lactose, the constitutive bacteria are out- 

 grown by the adaptive ones and disappear, owing to natural selec- 

 tion. The growth rate of the constitutive bacteria is obviously lower 

 than the growth rate of the adaptive ones. This is obviously due to 

 the fact that part of the building blocks is utilized for the manu- 

 facture of a useless enzyme. 



Evolution has thus ended in a remarkable device. When lactose 

 is absent, no enzyme for its utilization is produced. When lactose 

 is present, the enzyme is synthesized and the synthesis adjusted to 

 the need. 



Before any further discussion, a preliminary attempt at a general- 

 ization seems to be necessary. It might be stated that the synthesis 

 of the enzymes required for the metabolism of the energy source 

 is controlled by the over-all available energy sources. Moreover, 

 the synthesis of the enzymes necessary for the synthesis of any 

 given essential metabolites is controlled by the "level" of this 

 essential metabolite. "Level" here means intracellular concentra- 

 tion, whether the essential metabolite is extrinsic or endogenous. 

 The effector of the regulatory mechanism is a repressor. How 

 does the repressor act? 



The operator. The structural gene carrying the information for 

 the synthesis of a given enzyme will not express itself if a repressor 

 is present. How does the repressor repress? Inductivity and con- 

 stitutivity have to be considered anew. 



In the cases studied so far, inductivity is "dominant." This con- 



[54] 



