GENETIC CONTROL OF CELL INTEGRATION 



333 



cumulation and for its reversal. When the intracellular accumulation of 

 TMG is plotted against bacterial growth, a linear relationship is found, 

 similar to that observed in induced enzyme formation (Figure 11.15). 

 From these and related observations, it has been inferred that the 

 entrance of galactosides into E. coli requires the induced formation of 

 a "permease," which is considered on indirect evidence to be a protein. 

 This galactoside-permease is referred to in the literature as Y. 



In this system, the autocatalytic curves found at low inducer concen- 

 trations (Figure 11.14) result from the special properties of permease 

 formation, and do not reflect directly the kinetics of j8-galactosidase 

 formation. The most direct demonstration that /3-galactosidase kinetics 

 are constant at all inducer levels is provided by studies of a mutant 

 strain which lacks the galactoside-permease. 



This mutant, called "cryptic," does not grow on lactose, but when 

 grown, for example, on succinate, will respond to certain galactosides, 

 such as TMG, by forming /3-galactosidase in high titer. Cells which are 

 full of the enzyme still do not grow on lactose although, when broken, 

 they hydrolyze lactose very well. Clearly, external lactose does not reach 

 the internal enzyme. With these cells, there should be no effect upon 



50 



100 



150 

 Minutes 



200 



250 



FIGURE 11.15. Uptake and release of inducer showing ability of E. coli cells to con- 

 centrate TMG intracellularly. The uptake of the inducer methyl-|S-D-thiogalactoside-S^^ 

 (TMG)-S ^ was measured in induced cells, which may concentrate it up to 4 per cent of 

 their dry weight. Subsequently, addition of the competitive inhioitor phenyl-/3-D- 

 thiogalactoside (TPG) displaced the TMG rapidly from the cells. Because of the uptake 

 of high concentrations of inducer, the permease is considered a catalyst rather than a 

 binding site (from Monod, 1956, Enzymes: Units of Biological Strucfure and Function, 

 New York, Academic Press, pp. 7-28). 



