30 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



type organism grows on "minimal medium. ' The fact that these 

 enzymes are repressed becomes evident under conditions of repres- 

 sion release. Such conditions were realized with the aid of the 

 "leaky" histidine-requiring mutant strain hisE-11 of S. typhimurium. 

 When this strain is cultivated in the presence of histidine, repressed 

 enzyme levels are obtained. Upon exhaustion of the added histi- 

 dine, the mutant strain continues to grow, albeit more slowly, due to 

 its leaky block, and the conditions are now derepressive with respect 

 to the four enzymes of histidine synthesis. As shown in Fig. 2-3, 





 HOURS 



Fig. 2-3. The specific activities of four histidine biosynthetic enzymes and 

 glutamic dehydrogenase during the growth of the "leaky" histidine-requiring 

 mutant strain hisE-ll of Salmonella typhimurium. The amount of histidine that 

 was added (0.03 mM) was exhausted during the growth period (at zero time), 

 and the mutant strain then grew at a lower rate that was limited by the small 

 amount of histidine it was capable of making. A liter culture was grown in a 

 two-liter flask, agitated at 37° C in a rotary shaker, and optical density readings 

 were taken at various times. The solid triangles and circles on the growth curve 

 represent two separate experiments run under identical conditions. At the times 

 indicated by the arrows, 100-ml aliquots were harvested for protein and enzyme 

 assays (see Ames and Garry, 1959). The specific activities, in normalized units, 

 of imidazoleglycerol phosphate dehydrase (D), imidazoleacetol phosphate 

 transaminase (T), histidinol phosphate phosphatase (P), and histidinol dehy- 

 drogenase (H) are presented; specific activities of glutamic dehydrogenase 

 (circled G) are included for comparison. {Courtesy, B. N. Ames and B. Garry.) 



