78 MOLECULES, VIRUSES, AND BACTERLV 



an enzyme produced, the second the rate at which each enzyme mole- 

 cule functions. Mechanisms to regulate both amounts and activities 

 have been found within the past half-dozen years. We will provide ex- 

 amples of these processes. Then an attempt will be made to correlate 

 the two sorts of regulatory processes with information regarding the 

 functioning of metabolic pathways, in order to synthesize a model of 

 metabolic regulation. This subject was extensively reviewed at an 

 earlier date ( Pardee, 1959 ) . 



Almost all of the work to be discussed here was done with Escheri- 

 chia coll; unless otherwise stated, it will be assumed that this organism 

 was employed. 



Enzyme induction 



That the quantity of an enzyme in a living cell can vary consider- 

 ably under different environmental conditions has been known for 

 some time. In particular, the increase in the specific rate of enzyme 

 synthesis upon addition of some nutrient, usually the substrate of the 

 enzyme, has been studied extensively under the name of enzyme induc- 

 tion or adaptation (Pollock, 1959). Tryptophan, for instance, can in- 

 crease the specific rate of synthesis of tryptophanase in Escherichia coli 

 almost immediately by several hundred times ( Figure 1 ) . Data of this 



E 

 \ 



CO 



lij 

 < 

 < 



X 

 CL 

 O 



2 - 





 10 20 30 40 



MINUTES AFTER INDUCTION 



Figure 1. Induction of tryptophanase and the effect of glucose. 



