332 



CELL HEREDITY 



200 



4 X 10 M 



50 100 



Increase in Bacterial Mass ^^g per ml) 



(a) (h) 



FIGURE 11.14. Comparative kinetics of ^-galactosidase formation by wild-type and 

 cryptic mutant cells of E. coli in response to induction by methyl-)3-D-thiogalactoside. 

 Wild type cells exhibit a constant diflPerential rate of enzyme formation at high but not 

 at low inducer concentrations. The cryptic mutant shows a constant differential rate 

 at all effective inducer concentrations. The diflPerence in response of the two strains 

 results from the presence in wild-type cells of a permease which must be induced before 

 enough inducer can enter the cell to initiate /3-galactosidase formation. Consequently, 

 the kinetics of both permease and /3-galactosidase formation are reflected in graph 

 (a), but only the latter enzyme in graph (b), since the cryptic mutant lacks permease 

 (from Cohn, 1958, in: McElroy and Glass, (eds.). The Chemical Basis of Developmenf, 

 Baltimore, Johns Hopkins Press, p. 458). 



nomenon has involved the demonstration of the role of permeabiHty in 

 induction. 



When cells are preinduced by growth in the presence of a galactoside, 

 and then washed and grown in the presence of radioactive TMG-S °, 

 they rapidly accumulate TMG to a concentration more than 100 times 

 that of the surrounding medium. This TMG is held within the cells 

 but can be released, as demonstrated by adding an unlabeled galacto- 

 side and following the loss of radioactivity (Figure 11.15). The ac- 

 cumulation apparently involves a stereospecific kind of active transport. 

 Energy is required, and so is the ability of the cell to synthesize 

 protein. A high order of stereospecificity is required both for ac- 



