24 CONTROL MECHANISMS IN CELLULAR PROCESSES 



(Moiiod and Cohen-Bazire, 1953a), the very enzyme which had 

 proved so useful in the elucidation of the induction phenomenon. 

 Repression and induction were brought into still closer juxtaposition 

 when it was proposed (Vogel, 1957a, 1957b) that they are related 

 not onlv throu2;h their function in the cell but also through the un- 

 derlying molecular events (in terms of the interference with or 

 promotion of the dissociation of template product from template). 

 As detailed in another section, strong evidence has indeed been ob- 

 tained that the induction of /3-galactosidase represents an antagonism 

 to the action of an endogenous repressor (Pardee, Jacob, and 

 Monod, 1958, 1959). 



Since 1953, repression has been observed upon the addition to 

 growing organisms of any of a wide variety of substances, including 

 amino acids, purine and pyrimidine compounds, carbohydrates, suc- 

 cinate, and phosphate (Table 2-1). The various small-molecule re- 

 pressors may not function as such, but rather as "active" derivatives 

 (see the section Mechanism of Repression). A broad range of en- 

 zymes are subject to repression, and this phenomenon appears to 

 occur regardless of the type of catalytic activity of the enzyme in- 

 volved; additionally, permease formation can be repressible ( Vogel, 

 1960a). With respect to the organisms in which it is found, repres- 

 sion is also widespread : it occurs in a variety of bacteria ( with which 

 the bulk of the work on repression has been carried out) as well as 

 in mammalian cells (DeMars, 1958; Walker, 1959, 1960) and prob- 

 ably in fungi ( Yura and Vogel, 1959; Horowitz et at., 1960). 



Frequently, as Table 2-1 reveals, the repressors are "end prod- 

 ucts" of the pathwavs in which the respective repressible enzymes 

 occur, and the repressors usually seem to be quite specific in their 

 action (Monod and Cohen-Bazire, 1953b; Vogel, 1957a). These 

 properties of repressors would appear to be of particular relevance 

 to the control of cellular function. 



Basic Picture and Kinetics of Repression 



Definition of Repression. Enzyme repression has been defined 

 as a relative decrease, resulting from the exposure of cells to a given 

 substance, in the rate of synthesis of a particular apoenzyme ( Vogel, 

 1957a). This definition was worded in analogy with that proposed 

 for enzyme induction by Cohn et al. ( 1953b ) . The notion of re- 

 pression has been extended to include enzyme-like substances such 



