GENETIC CONTROL OF CELL INTEGRATION 



329 



Theoretical for 

 £„,. = 5.3 



Cell Divisions 



FIGURE 11.12. The role of arginine in controlling the rate of synthesis of ornithine 

 transcarbamylase in wild type cells of E. coli. Enzyme activity per cell is plotted against 

 time in cell divisions to demonstrate rapid fluctuations in response to arginine. Curve A 

 is based upon induction kinetics of the type shown in Figure 1 1 . 1 0. Curve C shows the 

 total absence of enzyme activity in the presence of arginine, which is a repressor of 

 ornithine transcarbamylase formation. In the absence of arginine (curve B), enzyme 

 activity levels off at about 2 units per cell, after a sudden initial rise to more than 

 5 units per cell. The overshoot results from a temporary absence of intracellular 

 arginine, soon followed by establishment of a pool which regulates the enzyme activity 

 at about 2 units per cell (from Gorini and Maas, 1958, in: McElroy and Glass, (eds.). 

 Chemical Basis of Developmenf, Baltimore, Johns Hopkins Press, p. 463). 



of several amino acids, of the purines and pyrimidines, the end product 

 can repress the formation of all the enzymes along the pathway. An 

 extensive analysis of histidine biosynthesis has shown that the relative 

 rates of formation of four of the enzymes of this pathway are closely 

 correlated, and all are controlled by the histidine concentration of the 

 cells, not by the concentrations of any intermediate substrates. An ex- 

 ample of these data is shown in Figure 11.13. 



Now we may ask: What is the relation between induction and re- 

 pression? The existence of an intimate connection of some sort can al- 



