REGULATION 131 



anabolic chain which makes arginine starting from acetylornithine is in- 

 hibited by arginine. VaHne represses the synthesis of vahne transaminase 

 (Adelberg, 1953), uracil represses the synthesis of at least three constitutive 

 enzymes of the metabolic chain leading to orotic acid (Yates and Pardee, 

 1957); similar repression phenomena were found in the pathways of syn- 

 thesis of the purines (Magasanik, 1957, 1958), of methionine and cysteine 

 (Cohn et al., 1953, Wyesundera and Woods, 1953, 1960; Bourgeois et al, 

 1959, 1960) and histidine (Ames and Garry, 1959). Inorganic phosphate 

 represses the synthesis of alkaline phosphatase in E. coli (Horiuchi et al., 

 1959; Torriani, 1960). 



Inhibition of the synthesis of certain enzymes by glucose had been 

 known for a long time, especially in cases of induced enzyme synthesis 

 (Dienert, 1900; Stephenson and Yudkin, 1936; Gale, 1943). This glucose 

 effect is well illustrated by diauxie (Monod, 1942). When bacteria are grown 

 in a medium containing two carbon sources, e.g. glucose and maltose, they 

 use glucose first ; growth slows down very much when most of the glucose 

 has been used up, and it is only then that the synthesis of the enzyme which 

 attacks maltose is permitted. The utilization of the new carbon source 

 makes possible a restoration of growth, resulting in the familiar diauxie 

 growth curve. The glucose effect, which had not been analysed very much 

 because it was observed for many enzymes and therefore seemed to be 

 unspecific, is a case of repression (Neidhardt and Magasanik, 1956, 1957; 

 Magasanik, 1957; Magasanik et al, 1959). 



Repression is thus as common a phenomenon as induced synthesis of 

 enzymes. As a rule, enzyme synthesis is induced by the substrate and 

 repression is caused by the product of reaction or by the end product of the 

 anabolic chain in which the enzyme is involved. Induction and repression 

 of enzyme synthesis thus afford a remarkable adjustment system which 

 selects within the genetic potentialities of the cell an enzyme assortment 

 adapted to the conditions of the medium. Induction has an immediate or 

 very rapid elTect on the level of the enzyme ; repression changes much less 

 rapidly the level of enzyme: it simply stops further production of an 

 enzyme which was being made, the level of the enzyme usually decreases as 

 it is being diluted in the increasing cell mass of the growing bacterial 

 population. 



3. Mechanisms of Repression and Induction 



The formal antagonism of inducer and repressor suggests that these 

 substances might both act in opposite ways on the same controlling system ; 

 there might exist between them a relation similar to that of the substrate 

 of an enzyme and a competitive inhibitor. Recent experiments by Gorini 

 (1960) indeed showed that ornithine can reverse the repressive eff^ect of 

 arginine on the formation of ornithine transcarbamylase, and induction by 



