BIOLOGICAL ORDER 



increases the fitness between the bacterium and its environment. 

 It is why this process has been, for a long time, called enzymatic 

 adaptation. It is now described as an induced biosynthesis of enzyme. 

 Let us now consider the reverse process. 



Repression. The bacterium Escherichia coli is able to grow in a 

 synthetic medium devoid of any amino acid. Such a bacterium is 

 therefore able to synthesize all its building blocks, or essential me- 

 tabolites, and, among them, the amino acid methionine. The last step 

 in the synthesis of methionine is catalyzed by an enzyme called 

 methionine synthase. 



When growing in a synthetic medium, the bacteria contain 

 methionine synthase. Of course they have to. For in the absence 

 of methionine synthase, they are unable to synthesize methionine. 

 And in the absence of methionine, they are unable to synthesize 

 proteins, that is, to grow. The enzyme methionine synthase is 

 actually present (Figure 12). What happens when the growth 



© 



cysteic acid— H* cysteine " "t" ^ homosenne 



C SO, C SH C S C C— OH 



II 



C NHg C NH2 ^ ^ _ . 



COOH COOH COOH C NHj C — NHj 



COOH 



C S — CH3 



I 



c 



I 



C — NH, 



I 



COOH 



homocystine methionine 



METHIONINE SYNTHASE 



Figure 12. The Synthesis of the Amino Acid Methionine. 



[38] 



