CONTROL BY REPRESSION 29 



plished with the aid of strain 39A-23R3 and the use of acetylor- 

 nithine-arginine supplements, in which the initial concentration of 

 acetylornithine was held constant and that of arginine was varied; 

 in this manner, it was possible to effect a swift transition from re- 

 pression to derepression, without interruption of cultivation, and the 

 point of transition was easily fixed by the arginine concentration in 

 the supplement emplo\'ed. In the two cases shown in Fig. 2-2, the 

 enzyme and the total protein are seen to be formed in a constant 

 ratio; for Culture B, this ratio is smaller than it is for Culture A. The 

 lower differential rate of acetylornithinase synthesis in Culture B is 

 associated with the higher initial arginine concentration (Fig. 2-2), 

 and consequently with the longer cultivation in the presence of 

 added arginine and the later termination of repression. Since the 

 constant differential rates of derepression can be shown to start, 

 within the accuracy of the methods used, from the respective points 

 of onset of derepression (cf. Fig. 2-1), these rates can be viewed 

 as reflecting a "pace-setting" phenomenon. The differential rates 

 usually maintain their constancy until shortly before growth stops 

 (Vogel, 1960b). The pace-setting effect is interpreted in terms of 

 a repression-like antagonism to the formation of a component, or 

 components, of the acetylornithinase-forming system. This effect 

 is of possible relevance to an understanding of the molecular basis 

 of repression and will be discussed further in the section Mechanism 

 of Repression. 



Coordinate Repression and Derepression. Very large quanti- 

 tative variations in derepression have been observed for different 

 enzymes. For example, in the case of aspartate transcarbamylase, 

 the differential rate of synthesis upon derepression can exceed the 

 repressed rate by a factor of more than 1000 ( Shepherdson and Par- 

 dee, 1960a, 1960b). In contrast, the corresponding factor for acet- 

 ylornithinase, depending on the point of onset of derepression, is of 

 the order of magnitude of 10. These two enzymes, of course, occur 

 in different biosynthetic pathways. A revealing analysis of the de- 

 repression of enzymes occurring in a single pathway has been re- 

 ported by Ames and Garry ( 1959 ) and Ames et al. ( 1960 ) . These 

 authors studied the repression by histidine of the last four enzymes 

 in the histidine pathway of Salmonella typhimurium. Unlike the 

 acetylornithinase in strain W of E. coli (see above), these histidine 

 enzymes of S. typhimurium are maximally repressed when the wild- 



