BACTERIA WITH HIGH LEVELS OF SPECIFIC ENZYMES 



97 



0.8 



- 06 



o 

 LU 



§04 



o 

 on 



02 



12 3 4 



TRYPTOPHAN CONCENTRATION (;xg/X) 



Figure 1. Growth-rate constants tor two tryptophan-requiring strains (B/1, t) of E. coli 

 as a function of the concentration of tryptophan. 



concentration of the substrate if this concentration is too low to main- 

 tain the maximum rate of enzyme formation. Here a fall in concentra- 

 tion of the substrate means not only a decrease in the rate at which the 

 cell encounters substrate molecules; it means also a decrease in the 

 fraction of the encountered substrate molecules which are captured. 

 How rapidly the growth rate falls with decreasing concentration of 

 substrate depends on how much the rate of formation of the capturing 

 enzyme decreases. 



In the case where the capturing enzyme is inducible, several kinds 

 of mutations to fitter strains can be expected. A strain in which the 

 substrate is a more eflFective inducer of the enzvme would be favored, 

 because it would have a higher level of capturing enzyme at a given 

 low concentration of substrate. But a constitutive mutant would be 

 even more favored, since it would have maximum levels of the captur- 

 ing enzyme independent of the concentration of substrate. The mecha- 

 nism of selection in this case can be understood from Figure 2. Here two 

 hypothetical curves for growth rate versus concentration of substrate 

 are given— one for an inducible and one for a constitutive strain. When 



