Development | 65 



The product of regulator genes is a cytoplasmic repressor sub- 

 stance, perhaps the RNA transcription of the gene. It is postulated 

 that the repressor substance tends to associate reversibly with a 

 particular operator, and the combination of operator and repressor 

 prevents the transcription of an entire operon. Protein synthesis is 

 thus blocked. The repressor is also viewed as reacting reversibly 

 with small molecules, called effectors, in the cytoplasm as well as 

 with the operator. In certain systems, only the unaltered repressor 

 can associate with the operator and block the operon. Presence of 

 the effector will then eliminate the effect of the repressor and release 

 the operon from repression. In other systems, only the reactant of the 

 repressor and the effector can combine with the operator. Transcrip- 

 tion of the structural gene is thus prevented by the presence of the 

 effector. A simplified diagram of this model is given in Fig. 4.1, 

 which should be compared with Fig. 1.5. 



The details of the Jacob-Monod model are beyond the scope of 

 this book. It is necessary only to add that microbial genetics pro- 



Fig. 4.1 j A model of gene function. R, cytoplasmic repressor substance; 

 E, effector substance; R', repressor modified by association with 

 effector; E', effector modified by association with repressor. ( Adapted 

 from Jacob and Monad, 1961, Cold Spring Harbor Symp. Quant. Biol. 

 26.) 



Operon 



