330 



CELL HEREDITY 



0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 

 IGP-Dehydrase Specific Activity 



2 4 6 8 10 12 14 15 18 



lAP-Transaminase Specific Activity 



(h) 



0.4 0.5 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 

 Histidinol Dehydrogenase 

 Specific Activity 



ic) 



4 6 8 10 12 14 

 Glutamic Dehydrogenase 

 Specific Activity 

 (d) 



FIGURE 11.13. Coordinate repression by histidine of the formation of enzymes con- 

 cerned with histidine biosynthesis in Salmonella. Four enzymes catalyzing consecutive 

 steps in histidine biosynthesis are (o) imidazoleglycerol phosphate dehydrase,- (b) 

 imidazole acetol phosphate transaminase; (c) histidinol phosphate phosphatase; and 

 (d) histidinol dehydrogenase. The enzyme glutamic dehydrogenase is on another path- 

 way and was assayed in these experiments as a control. The specific activity of each 

 enzyme was measured in wild-type and mutant cells grown under various conditions of 

 internal histidine concentration, resulting in a wide range of enzymatic activities per 

 cell. In each instance, the relative activity of all four enzymes of the histidine pathway 

 correlated very closely, indicating a coordinated control of the rates of their forma- 

 tion. This correlation was not observed in the case of glutamic dehydrogenase, which 

 is not directly on the histidine pathway (from Ames and Garry, 1959, Proc. Nof/. Acad. 

 Sci.Wos/i., 45:1453). 



