26 CONTROL MECHANISMS IN CELLULAR PROCESSES 



as permeases ( Vogel 1960a ) . ( For further extensions of the repres- 

 sion concept, see the section Genetic Aspects. ) 



Kinetics. The ]3asic features of the repression phenomenon can 

 perhaps best be ilkistrated in kinetic terms. Fig. 2-1 shows the for- 

 mation of acetylornithinase in a single culture of strain 39A-23R3 of 

 E. coli, first under conditions of repression by arginine and then 

 under derepressed conditions, i.e., under conditions of repression 

 release (Vogel, 1957a). The results are presented as a "differential 

 plot" (Monod et al., 1952) of amount of enzyme versus amount of 

 total protein produced. Strain 39A-23R3, a reisolate of strain 39A-23 

 (cf. Vogel, 1953a), has an early block in the ornithine-arginine path- 

 way (Vogel, 1953b) and gives a growth response to acetylornithine 

 or to arginine (or to arginine precursors following acetylornithine 

 in biosynthetic sequence ) ; mixed supplements of acetylornithine and 

 arginine, in suitable proportions, give diphasic growth ( Vogel, 1953a, 

 1957a): arginine is used preferentially, at wild-type growth rate, in 

 the first phase, and acetylornithine, due to restrictive uptake ( Vogel, 

 1960a), is used at a lower exponential growth rate in the second 

 phase. This growth behavior on appropriate mixed supplements ap- 

 pears to reflect conditions of intracellular arginine concentration that 

 also underlie the kinetics of acetylornithinase formation shown in 

 Fig. 2-1. The first growth phase (during which an excess of arginine 

 is available ) corresponds to repression, and the second growth phase 

 (during which the intracellular arginine level is restricted) cor- 

 responds to derepression. The change in the differential rate of en- 

 zyme synthesis occurs relatively abruptly and nearly simultaneously 

 with the sharp transition from first-phase to second-phase growth. 

 It will be seen that under conditions of repression as well as of de- 

 repression, linear differential rates of enzyme synthesis are obtained, 

 i.e., the enzyme is synthesized as a constant fraction of new protein 

 produced. Under derepression, this fraction is greater than it is 

 under repression. The relative promptness of onset of derepression 

 is noteworthv; the onset of repression of acetylornithinase can also 

 be shown to be very rapid. 



Acetylornithinase formation by the W strain of E. coli (from 

 which strain 39A-23R3 was derived) is also illustrated in Fig. 2-1; 

 strain W was grown, without added arginine, in the same general 

 manner as was strain 39A-23R3. Strain W can be seen to give a 

 linear differential rate indicative of a partial repression. This partial 



