98 



MOLECULES, VIRUSES, AND BACTERL^. 



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the original strain is inducible, the concentration of substrate in the 

 chemostat growth tube will be relatively large. It can be seen that a 

 constitutive mutant would have a much higher growth rate and that 

 only a relatively short time would be required for its selection. This 

 procedure provides a method which has been used many times, in this 

 laboratory and in others, for the isolation of mutant strains able to 

 make /?-galactosidase constitutively. This system of selection should be 

 applicable in the case of any inducible enzyme, under conditions in 

 which the growth rate depends on the level of the enzyme. 



The question remains: What mechanism gives one constitutive 

 strain the advantage over another, as in the case of the tryptophan- 

 requiring strains? Does the later strain simply have a higher level of 

 the capturing enzyme, or does it have an enzyme with a higher affinity 

 for the substrate or a higher turnover number? 



For example, in a discussion Ephraim Racker raised the question 

 of what happens upon continued operation of a chemostat in which 

 bacteria are grown on limited rations of lactose. After the selection of 



LACTOSE CONCENTRATION 



Figure 2. Hypothetical relationship between the growth-rate constants and the concen- 

 tration of substrate when the growth-rate-limiting enzyme is constitutive and when it is 

 inducible by the substrate. 



