THE FUNCTIONAL ORDER 



medium is supplemented with methionine? A labeled methionine 

 is utilized, a methionine with a hot sulfur atom. The labeled methi- 

 onine is taken up. And, after a few hours, the bacteria are assayed 

 for methionine synthase: the enzyme has disappeared. This means 

 that in the presence of extrinsic methionine, the synthesis of the 

 enzyme is blocked. This phenomenon, the inhibition of enzyme 

 synthesis, is called repression. 



Suppose now that the bacterium is growing in the absence of 

 methionine, and that it produces an excess of methionine. This excess 

 can be due either to an excess of enzyme or to a decreased rate of 

 utilization of methionine. The result is the same, that is, an excess of 

 methionine. But, as a result of this excess, the synthesis of the re- 

 sponsible enzyme is repressed, and as a consequence the synthesis of 

 methionine decreases. A balanced state is reached which corresponds 

 to an equal production and consumption of methionine. 



Repression of enzyme synthesis thus increases the fitness between 

 the bacterium and its environment. Whether we deal with the 

 utilization of the carbon and energy source, namely the sugar lactose, 

 or with the synthesis of an essential building block, namely the 

 amino acid methionine, we observe an adjustment of the enzymatic 

 system. The cell tends toward a physiological balance. Our problem 

 is to learn what the responsible mechanism is. 



The development of genetics and the chemical identification of 

 the genetic material have been possible because of the existence of 

 hereditary variation called mutations. The development of our 

 knowledge concerning the functional order has been possible, 

 thanks to the existence of nonhereditary, not transmissible variation, 

 such as the induced synthesis of enzymes or the repression of en- 

 zyme synthesis. Let us consider next the problem of the interactions 

 of genes and of enzymes. 



The Genetic Control 

 of Bacterial Metabolism 



Metabolism of lactose. Escherichia coli will be taken as a tool. 

 The bacterium is surrounded by a cell wall whose function is mainly 

 to counteract the internal osmotic pressure. The cytoplasm is 

 limited by a thin membrane. Among the remarkable properties of 



[39] 



