THE FUNCTIONAL ORDER 



If an inducer is added after the synthesis has stopped, the produc- 

 tion of enzyme starts anew. 



The genetic analysis has thus revealed a number of important 

 facts. The genes z. y, and i are independent units of function and 

 therefore are different genes. Thus, the small sector of the bac- 

 terial chromosome that controls the synthesis of the enzymes 

 responsible for the metabolism of lactose, and more generally of 

 ^-galactosides, is composed of three units. Two of them, z and y, 

 carry the information for the synthesis, respectively, of galactosidase 

 and permease. They are called structural genes. The other, i, deter- 

 mines the constitutive as opposed to the inductive type of synthesis 

 of both enzymes. It is called a regulating gejie. Things happen as 

 if the gene i+ controlling the inductive state corresponded to the 

 synthesis of a repressor, the mutation i+ ^ i~ to the loss of the 

 ability to form a repressor, hence to a constitutive situation. The 

 repressor has not yet been identified. But a number of data are in 

 favor of the hypothesis that it is a ribonucleic acid. 



The Ways of Repression 



Repressor, aporepressor, corepressor, and inducer; the con- 

 trol OF enzyme syntheses. For the bacterium, lactose is a carbon 

 source and a source of energy. It is part of the food. The bac- 

 terium may be fed with other organic compounds, such as glycerol 

 or pyruvic acid. The food that a bacterium may find depends on 

 the hazards of its daily life. The bacterium obviously cannot afford 

 to manufacture a large number of useless enzymes. And it is quite 

 natural that the synthesis of the contingent enzymes is regulated. 

 Let us consider the enzymes responsible for the synthesis of the 

 essential building blocks and especially of amino acids. 



Amino acids. It has already been stated that if the amino acid 

 methionine is provided, the bacterium stops synthesizing methionine 

 synthase, the enzyme responsible for the last step in the biosyn- 

 thesis of methionine. The product of enzymatic activity stops the 

 synthesis of the enzyme. This mechanism is known to be quite 

 general, and a few cases have been studied extensively. One, for 

 example, is the biosynthesis of the amino acid arginine. In a 



[49] 



