BIOLOGICAL ORDER 



medium devoid of arginine, the bacterium synthesizes arginine. If 

 arginine is supplied, the enzymes cease to be produced. The end 

 product of the chain of biosynthesis blocks the synthesis of all the 

 enzymes involved in the chain of arginine biosynthesis. Thus, if 

 an excess of arginine is produced, the synthesis of the enzymes 

 involved in its synthesis is specifically repressed, and less arginine 

 will be produced. The same mechanism is operative in a number of 

 biosynthetic pathways, such as nucleic bases, histidine and tryp- 

 tophan. Only the case of tryptophan will be discussed here. 



COOH 



A, 



HO- Y °" 



OH 

 shikimic acid 



<d) ® ® 



COOH 



NHg— ••B.D.C. 



/^ 



serine 



HO— C— C— COOH 

 H I 

 NHg 



C — C COOH 



I 

 NH, 



® ® 



■n (D 



anthranilic acid 



indole 



tryptophane 



TRYPTOPHANE SYNTHETASE 



Figure 20. The Biosynthesis of the Amino Acid Tryptophan. 



In a synthetic medium devoid of tryptophan, tryptophan is 

 synthesized by a system of enzymes (Figure 20). If tryptophan is 

 added, the synthesis of the enzymes is stopped, that is, repressed. Let 

 us admit that the synthesis is blocked by a repressor. This repressor 

 is specific; it is known to act only on the tryptophan enzymes and 

 not on the synthesis of any other system, as shown in Table III. 

 From the original repressed bacterium, it is possible to isolate mu- 

 tants that are called derepressed. In these mutants, the synthesis of 

 the tryptophan enzymes is not blocked any more by tryptophan. The 

 bacteria produce much more tryptophan than needed, and this 

 excess tryptophan is excreted. If tryptophan is added to the medium, 

 the synthesis of the enzymes responsible for the synthesis of tryp- 

 tophan is not altered in the derepressed strain, whereas it is inhibited 



[50] 



