CONTROL BY REPRESSION 41 



It will be recalled that Pardee et al. ( 1959) concluded that in the 

 /3-galactosidase system induction represents an antagonism of an 

 exogenous inducer to an endogenous repressor. Gorini (196()a), in 

 carefully controlled experiments with double mutants, examined 

 ornithine transcarbamylase formation as a function of the interaction 

 of an exogenous repressor (arginine) and an exogenous antagonist 

 (ornithine) of this repressor. He found that ornithine reverses the 

 repressive effect of arginine within certain concentration ranges; 

 however, ornithine was without effect under conditions of complete 

 repression or derepression. Thus, ornithine can, under appropriate 

 conditions, have an inducing effect on the formation of the trans- 

 carbamylase by antagonizing arginine repression. Various relation- 

 ships between induction and repression have been the subject of 

 well-documented reviews by Monod ( 1959b ) and Halvorson ( 1960) . 

 Interesting thoughts on repressor and inducer action have been pre- 

 sented by Brenner ( 1959 ) . 



Features of Repression Relevant to Its Mechanism. At this point, 

 it would seem advisable to list some of the biochemical and genetic 

 data, discussed above, that have become available since 1957 and 

 bear rather directly on the problem of mechanism. Any such con- 

 siderations should, of course, be viewed against the background of 

 current knowledge of protein synthesis which has been admirably 

 summarized in recent reviews by Cohen and Gros ( 1960 ) , Fincham 

 ( 1960 ) , and Yanof sky ( 1960 ) . 



Among the predominantlv biochemical features are the kinetics 

 of repression and derepression ( Fig. 2-1 ) , the pace-setting phenom- 

 enon in derepression (Fig. 2-2), coordinate (and non-coordinate) 

 repression (Fig. 2-3), the effects of intracellular repressor concen- 

 tration (Fig. 2-4), the promptness of repression and derepression, 

 and the relatively large differential rates of derepression of some 

 systems. Among the predominantlv genetic features are structural 

 and regulatory determinants, including operator-type genes, and 

 their linkage and dominance relationsliips (cf. Fig. 2-5). Addi- 

 tional results of relevance to mechanism will be cited in the context 

 of the discussions below. 



"Active" Repressors. The regulator hypothesis contemplates that 

 repressors mav act as "active" derivatives (Vogel, 1957b). This 

 thought implies that the functional repressor is a bipartite molecule 

 made up of a small-molecule repressor ( such as arginine ) in combi- 



