BIOLOGICAL ORDER 



functional change in the structural genes, but nothing more is 

 known at the present time. 



Co-ordinated control. The group of enzymes that is responsible 

 for the synthesis of a given essential metabolite or more generally 

 for a given sequence of reactions will be called a zymon. All the 

 members of a zymon work in harmony. One enzyme acts on the 

 product of the activity of the preceding one. And the activity of 

 each enzyme depends on the activity of all the members of the 

 zymon. The zymon is a functional whole. The most efficient and 

 economical system of control would be a single system controlling 

 the synthesis of all the members of the zymon. This would be the 

 case if the genetic information for the zymon is a unit, if all the 

 individual genes are linked and belong to the same operon. 



The study of bacteria has revealed that the structural genes 

 carrying the information corresponding to one zymon are some- 

 times linked. This is the case, for example, for the genes of the 

 tryptophan zymon. This is all right. Sometimes, however, the genes 

 corresponding to a zymon are not linked. And for this no explana- 

 tion has yet been found. But it will be found. 



Refinement of regulation; inhibition of enzymatic activity. 

 When the end product of a biosynthetic process is produced in 

 excess, a repressor is formed which blocks the synthesis of the 

 corresponding enzymes. But the enzymes are there, and if they 

 continued to function, the accumulation of the end product would 

 continue. It happens that this accumulation is prevented by a very 

 simple device. The end product of enzymatic activity inhibits, or 

 blocks, the functioning of the first enzyme of the biosynthetic path- 

 way. The other enzymes involved in the particular chain of bio- 

 synthesis are consequently deprived of their substrate, and the 

 activity of the zymon is reduced or blocked (Figure 23). This con- 

 dition is known to exist in the arginine and the tryptophan zymons. 



Thus a dual mechanism controls the production of amino acids, 

 and more generally of essential metabolites. When the output ex- 

 ceeds the consumption, that is, the need, the synthesis of the re- 

 sponsible enzymes is repressed, and the activity of the already 

 existing enzymes is depressed or stopped. 



Preclusive and corrective feedback mechanisms. Let us 

 consider first the synthesis of an essential metabolite: tryptophan. A 



[58] 



