32 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



employment of different conditions permitted an interesting kinetic 

 study of the effect of intracellular repressor concentration on the 

 formation of ornithine transcarbamylase, which is repressible by ar- 

 ginine (Gorini and Maas, 1957, 1958). As shown in Fig. 2-4, when 

 cells of E. coli, strain W, grown in the presence of arginine are inocu- 



CELL DIVISION 



Fig. 2-4. Ornithine transcarbamylase synthesis in flask cultures of Escherichia 

 coli, strain W. Washed cells from cultures growing exponentially in the pres- 

 ence of arginine were inoculated into an arginine-free lactate-salts medium and 

 incubated at 37" C with shaking. For the assay procedure and enzyme unit, see 

 Gorini and Maas (1957). The specific activity is expressed in units of enzyme 

 per milligram of dry weight of bacteria. The cell division time was 60 minutes. 

 (Courfesy, L. Gorini and W. K. Maas.) 



lated into arginine-free minimal medium, there is a relatively rapid 

 initial rise in enzyme level, followed by a drop to an approximately 

 steady level. The initial rise is attributed to enzyme synthesis occur- 

 ring under conditions of low endogenous arginine concentration, i.e., 

 under conditions of derepression. The ornithine transcarbamylase 

 and other enzymes in the arginine pathway thus formed are consid- 

 ered to allow increased svnthesis, and hence an accumulation, of 

 arginine, which in turn exerts a repressive effect. 



Derepression as de novo Protein Synthesis. Implicit in kinetic 

 studies, such as those presented above, is the notion that the appear- 



