28 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



The repressed rate of acetylornithinase formation in strain 39A- 

 23R3 cannot be further repressed by increasing the initial arginine 

 concentration used by factors up to 100. The derepressed rate, how- 

 ever, of this strain is susceptil^le to variation, as discussed below. 



Pace-setting Phenomenon in Enzyme Derepression. The de- 

 repression of acetylornithinase formation was studied after varying 

 periods of repression by arginine (Vogel, 1960c). This was accom- 



I2r 



10 



-^8 



H 



uj 6 

 < 



o 



_l 

 >- 

 I- 



o 



< 



4 - 



2 - 



± 



± 



40 80 120 



TOTAL PROTEIN (Mg /ml) 



160 



Fig. 2-2. Formation of acetylornithinase following early (A) or late (B) onset 

 of derepression (data from Vogel, 1960c). After preliminary cultivation in a 

 glucose-salts medium supplemented with a growth-limiting amount of arginine, 

 strain 39A-23R3 of Escherichia coli was grown in a glucose-salts medium con- 

 taining N'^-acetyl-L-ornithine, 50 /xg/ml, and L-arginine hydrochloride, either 

 5 /xg/ml (Culture A) or 15 /xg/ml (Culture B). The two cultures were incubated 

 anaerobically at 37° C and, after establishment of second-phase growth {i.e., of 

 derepressive conditions, see the text), were sampled at intervals. Organisms 

 from the samples taken were harvested, disrupted, and assayed for acetyl- 

 ornithinase (see the legend for Fig. 2-1). The differential rates of synthesis of 

 the enzyme for A and B are 0.129 and 0.063 units per /j,g protein, respectively. 



