THE FUNCTIONAL ORDER 



elusion is based on the consideration of a heterozvgote containing 

 two sets of chromosomes, one i + z+, the other i^z + . In the absence 

 of an inducer, no enzyme, no /?-galactosidase, is produced, despite 

 the presence of the gene i~ responsible for constitutivity. The 

 hypothesis is that the gene i+ produces a repressor, and the 

 expression of the structural gene z+ of the chromosome i~z+ is 

 blocked, despite the presence of the constitutive gene i~. Thus, 

 no enzyme is formed in the absence of an inducer. The gene i+ 

 imposes the inductive situation. The inductive gene i+ is said 

 to be dominant, and the constitutive gene i~, recessive (Figure 21). 

 The repressor produced thanks to the gene i+ acts on the "con- 

 stitutive" chromosome carrying the constitutive gene i~ or on 

 the enzyme-forming system produced by the "constitutive" 

 chromosome. 



The problem of the site of action of th,e repressor was solved 

 by the discovery of a new category of mutants. An extensive study 

 has revealed that in some mutants constitutivity, instead of being 

 recessive, is dominant. When constitutivity is dominant, the hetero- 

 zygotes i + z + , i~z+ manufacture the /3-galactosidase in the absence 

 of inducer, despite the presence of the inductive gene i + . An 

 elaborate series of experiments has led to the conclusion that when 

 a bacterium mutates from recessive to dominant constitutivity, a 

 specific gene is involved which has been called the operator and 

 desiraated by the letter o. 



The dominant inductive would have the constitution i+o+z + , 

 the 0+ gene being "sensitive" to the repressor. The dominant con- 

 stitutive would be i~o'^'z + , the o'' gene being "insensitive" to the 

 repressor. It was found that the mutation o+ ^ o"^ affects not only 

 the gene z (for the /?-galactosidase) but also the gene v (for the 

 permease). But it affects only those genes located on the same 

 chromosome, that is, in position cis (Figure 22). 



Thanks to recombination experiments, it is known that the genes 

 y and z are adjacent. Close to them is the operator gene o, and next 

 to it the regulating gene i. The structural gene contains the informa- 

 tion for the synthesis of the specific enzymes. The regulating gene 

 produces — or does not produce — a specific repressor. The func- 

 tional unit formed by the operator gene and the structural gene has 

 been called an "operon." 



The repressor formed by the combination of the aporepressor and 



[ 55 ] 



