322 



CELL HEREDITY 



280 



480 



FIGURE 11.8. Induction of galactoz/mase in /east cells in the presence and absence 

 of a nitrogen source. Yeast cells grown on glucose have no galoctozymase activity, but 

 following the addition of galactose, enzyme formation begins. Within 4 hours, con- 

 siderable enzyme is formed both in the presence and absence of a nitrogen source. 

 These results indicate that a new enzymatic activity can appear in nondividing or 

 scarcely dividing cells (after Spiegelman, 1946, C. S. H. Symp. Quanf. Biol., 1 1 :256). 



gene action, and regulatory mechanisms of metabolic control at the 

 cellular level. In what follows, we shall attempt neither a chrono- 

 logical development of the subject nor a comprehensive summary of 

 the field, but rather a sketch of those findings and related concepts 

 which appear to be central to the analysis of mechanisms of genetic 

 control. 



The most extensive, systematic, and diversified analysis of enzyme 

 formation has been performed by Monod and collaborators over a period 

 of many years at the Pasteur Institute, focusing primarily upon the 

 |3-galactosidase enzyme of E. coli. Other contributions of central im- 

 portance have come from Spiegelman's studies with yeast, and Pollock's 

 investigations of penicillinase formation in the bacterium Bacillus 

 cereus. 



Consider a population of yeast cells growing in liquid culture with 

 glucose as their carbon source. They are washed, and half of them are 

 resuspended in a growth medium containing galactose as sole carbon 

 source, the other half in a medium with galactose but no nitrogen source 

 to prevent growth. The resulting increase in activity of the galactose- 

 fermenting enzyme, galactozymase, is shown in Figure 11.8. Almost as 



