58 CONTROL MECHANISMS IN CELLULAR PROCESSES 



diminish through dikition, in the course of cell multiplication, or 

 through destruction ) . 



As a control mechanism of enzyme formation, though, repression, 

 as emphasized above, is rapidly acting. In this capacity, repression 

 is effective in avoiding the production of temporarily unnecessary 

 enzymes. Such control not only provides substantial savings of en- 

 ergy and materials but also tends to prevent a possible harmful over- 

 production of enzymes. 



If the pace-setting phenomenon (Fig. 2-2) indeed reflects a 

 regulation of the formation of enzyme-forming sites, this (probably 

 fast-acting) phenomenon would represent another control mecha- 

 nism with which the repression process can cooperate. One can 

 thus visualize a hierarchy of regulatory mechanisms that operate at 

 the levels of (a) the synthesis of small-molecule metabolites, (b) the 

 synthesis of enzymes, and (c) the synthesis of enzyme-forming sys- 

 tems. These control devices have in common the property of being 

 responsive to endogenous or exogenous substances of relatively low 

 molecular weight. In this context, it is noteworthy that additional 

 regulation can be provided by repressible permeases ( Vogel, 1960a ) 

 which would be involved in the transfer of exogenous materials into 

 the cell. The complementary nature of enzyme repression and in- 

 duction as control mechanisms has been discussed elsewhere ( Vogel, 

 1957b). 



The high degree of usefulness to the cell of control by repression 

 is attested to by the latter's broad distribution with respect to met- 

 abolic systems. It seems evident that repressibility of enzymes is 

 a property that must have been positively selected in the course of 

 evolution (Vogel, 1957a). Consistent with this thought is the ap- 

 parently widespread occurrence of repression with respect to or- 

 ganisms. Because of the experimental advantages that they offer, 

 bacteria have received more attention in this connection than have 

 other organisms. However, in view of the possible significance of 

 the repression and induction processes to the development and dif- 

 ferentiation of higher forms of life (Vogel, 1958), the indicated 

 existence of these processes, particularly of repression, in mammalian 

 systems is of considerable interest. 



DeMars ( 1958 ) has studied the antagonism of glutamine to the 

 formation of glutamyl transferase in cultures of human cells. The 

 general character of this antagonism was suggestive of repression. 

 In addition to glutamyl transferase activity, the enzyme preparations 



