328 CELL HEREDITY 



concentration of this enzyme and the rate of its formation have constant 

 values which we shall consider the norm. When these cells are grown 

 with arginine supplied from the medium, formation of the enzyme stops 

 abruptly and its concentration falls by dilution during subsequent 

 growth. When those arginine-grown cells are washed and resuspended 

 in minimal medium, they begin to synthesize OTC-ase again, as if it were 

 being induced. The phenomenon resembles induction in reverse: the 

 presence of arginine inhibits enzyme formation, and the absence of 

 arginine induces it. 



These observations have a number of interesting aspects; let us begin 

 with the role of arginine. The effect of arginine in the medium upon the 

 content of OTC-ase per cell is shown in curves B and C of Figure 11.12. 

 Cells grown with arginine contain little or no enzyme, and cells grown 

 without arginine level off at about two units per cell, after an unexpected 

 overshoot. The expected kinetics for induction at a constant differential 

 rate are shown in curve A. Analysis of the overshoot indicated that it re- 

 sulted from a temporary very low level of endogenous arginine within the 

 cells. It was shown that withdrawal of arginine from the medium led to 

 the rapid appearance of all the enzvmes of the arginine biosvnthetic chain, 

 which began to make arginine. As long as very little arginine accumu- 

 lated within the cells, enzyme synthesis continued, and soon excess 

 endogenous arginine was being formed. It was postulated that this ex- 

 cess served to repress further enzyme formation, and that bv the third 

 cell division the system had equilibrated. The most direct evidence that 

 endogenous arginine itself (or a derivative) has a controlling role in 

 synthesis of the enzymes of the arginine pathway comes from studies 

 with the chemostat. Using an arginine-requiring mutant, blocked before 

 ornithine and capable of making the enzyme OTC-ase, it was shown that 

 the content of enzyme per cell under equilibrium conditions was a func- 

 tion of the intracellular arginine concentration (as set by the growth rate 

 of the cells on limiting arginine). At its slowest growth rate, the 

 mutant contained 25 to 50 times the enzyme concentration of the wild- 

 type cells grown without arginine. At various intermediate growth rates, 

 corresponding enzyme concentrations were found. As a control, wild- 

 type conditions could be simulated by using a histidine-requiring mutant 

 growing on limiting histidine in the chemostat. These cells were found 

 to contain the same OTC-ase concentration as do wild-type cells. 



It is thus evident that enzyme formation can be repressed as well as 

 induced. Furthermore, the cell is able to regulate enzyme formation 

 with great sensitivity within the wide range between zero and maximal 

 rate. Similar lines of evidence of enzyme repression have been estab- 

 lished with a number of different systems. In the biosynthetic pathways 



