GENETIC CONTROL OF CELL INTEGRATION 327 



but attempts to analyze these curves are hopelessly complicated by the 

 changing metabolic state of the cells. Ideally, the kinetics of enzyme 

 formation should be studied against a background of homogeneity, both 

 of cell type and metabolic state, in every respect other than formation 

 of the new enzyme. Furthermore, since the internal concentration of in- 

 ducer may be xery important, it would be useful to employ an inducer 

 which is not metabolized. This ideal state has been appro.ximated 

 in the j8-galactosidase system by the use of TMG as inducer, with rapidly 

 growing young cultures. Under these circumstances, the differential rate 

 of formation of the enzyme is constant over time, from the moment of 

 induction, as can be seen in Figure 11.10. The increase in enzyme is 

 plotted against the increase in total protein; the resulting linearity 

 demonstrates that the amount of enzyme being synthesized is a constant 

 fraction of the total cell protein being formed. It is under conditions 

 in which the differential rate of enzyme synthesis remains constant, or is 

 altered in a controlled manner, that the process of enzyme formation can 

 best be analyzed. 



It is also useful in some studies to plot the data in the form of 

 enzyme content per cell against time when the effects of environmental 

 changes are being investigated. Plotted in this way, at a constant dif- 

 ferential rate of synthesis, enzyme concentration per cell will approach 

 a constant maximum value in about four generations after induction. 

 Studies of arginine synthesis under conditions which violated these 

 kinetics provided some of the first evidence that the control of enzyme 

 formation involved more parameters than just the presence or absence 

 of an inducer in an otherwise competent cell. 



Let us consider the pathway of arginine biosynthesis, shown in Figure 

 11.11. Several of the enzymes of this pathway have been studied; here 

 we shall discuss ornithine frans-carbamylase (OTC-ase). When wild- 

 type cells of E. coli, strain W, are grown on a minimal medium, the 



Glutamate 



N"-acetylornithine >- ornithine >■ citrulline *► arginine 



/ 



CO2 + NH3 + ATP ^ carbamyl phosphate 



\ 



aspartate 

 FIGURE 11.11. Pathway of arginine biosynthesis in E. coli. 



