THE FUNCTIONAL ORDER 



of the corepressor would act on the operator gene. When the in- 

 ductive bacterium i+ is o + , the operator is sensitive to the repressor. 

 When the repressor is present, the operator gene is in position stop, 

 and the two structural genes z+ and v+ are "blocked" and do not 



OPERON 



regulating gene operator 



structural genes 



+ 



Y + 



^____^ enzynrie forming 

 ~ ~ systems 



/3-galactosidase permease 

 co-repressor 



lactose 



metabolism 



Figure 22. The Operon. 



The structural genes z+ and v"^ "manufacture," respectively, /3-galactosidase 

 and /3-galactoside permease only in the absence of the repressor. The repressor 

 is produced from an aporepressor synthesized by the gene i+ and a corepressor 

 that is a product of metabolism of the energy source — glucose, for example. 

 Lactose, the inducer, competes with the corepressor and prevents the forma- 

 tion of the repressor. When no repressor is present, the operator gene o+ is in 

 position go, and the structural genes are active. When a repressor is present, 

 the operator gene is in position stop, and enzyme synthesis is repressed. 



produce the enzyme-forming system. W' hen the repressor is absent, 

 the operator gene is in position go. There is a new problem now, and 

 the question must be asked: How does the operator gene act? We 

 have every reason to believe that the operator does not send messages 

 to the cytoplasm. One hypothesis is that it acts by inducing a 



[57] 



