Genetic Control of Protein Synthesis 69 



E. coli can vary roughly a thousand-fold up to a maximum of 

 some 6 per cent of the cellular protein if a jS-galactoside, an 

 "inducer," is present in the growth medium. Mutants have been 

 isolated which have lost this control of the rate of enzyme syn- 

 thesis. Jacob and Monod (13) have divided these "constitutive" 

 mutants into two genetically and functionally distinct classes 

 designated i^ and o^ By studying the dominance properties of 

 these mutations in partially diploid strains carrying both i+ and 

 i~ and both 0+ and o*" (both inducible and constitutive genetic 

 structures) Jacob and Monod have developed a model of the 

 control process (Fig. 5). This model proposes that a "repressor" 

 material is made under the control of the i gene, which they call 

 a regulator gene, and that this repressor binds to a site near the 

 structural gene, the O or operator gene, preventing formation of 

 the structural messenger. 



Requlator Operator jtructural 



Gene bene Genes 



Repression or Tnduction Proteins 



Fig. 5. The model proposed by Jacob and Monod for the mechanism controlling 

 the rate of action of the structural gene. A repressor material is made under the 

 control of the DNA of the regulator gene. This repressor material acts (after 

 possible metabolite activation) by binding to a DNA site adjacent to the struc- 

 tural gene or genes subject to rate control by the repressor and preventing the 

 formation of structural messengers. 



In this model the regulation is negative; genes are noimally 

 functional and are turned off by a repressor. Similar analysis of 

 the system controlling alkaline phosphatase synthesis (18) has 



