A6 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



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Time in minutes 



Fig. 2-6. Induction of /i-galactosidase following "thymine saturation." A 

 mutant strain of 'EschencWyo co// that requires thymine (T), arginine (A), and 

 uracil (U) was incubated in the presence of thymine, but in the absence of 

 arginine and uracil, for the achievement of the thymine-saturated state. Argi- 

 nine and uracil were added at zero time; inducer, methyl-thio-/i-D-galactoside 

 (TMG), was added at the times indicated. Enzyme activity is presented in arbi- 

 trary units. Upon addition of inducer at zero time, enzyme formation proceeds 

 after a pronounced lag; a later addition (at 10 minutes) results in a shorter lag. 

 If the inducer is supplied after a sufficiently long period of incubation, lag is 



