BIOLOGICAL ORDER 



in the normal strain. Let us admit that the original wild type 

 possesses the gene R+try, which is able to manufacture the repressor, 

 and that the derepressed bacterium carries a mutated gene R~ try, 

 unable to manufacture the repressor. R+ and R" bacteria are 

 allowed to conjugate. A zygote is thus formed, carrying both R+try 

 and R~try genes. In this heterozygote, the synthesis of the tryptophan 

 enzyme is blocked. A repressor is obviously produced by the R+ 

 gene, which is dominant over the R~ gene (Table IV). 



Table IV. Enzyme Synthesis as Affected by Tryptophan in a 

 Repressed and a Derepressed Bacterium and in the Heterozygote. 



The derepressed bacteria contain three to seven 

 times more enzymes than the repressed one. The 

 addition of tryptophan is without effect on the en- 

 zyme level in the derepressed strain, whereas it blocks 

 enzyme synthesis in the repressed one. The hetero- 

 zygote behaves like the repressed strain: the character 

 "repressed" is dominant. 



The study of the tryptophan repression leads to an important 

 conclusion. In the derepressed bacterium, tryptophan is produced 

 in excess; nevertheless, there is no inhibition of enzyme synthesis. 

 This means that in the normal strain, it is not tryptophan itself 

 which is the repressor, but something produced with the help of 

 tryptophan. 



The hypothesis has been proposed that the regulating gene R + 

 produces an inactive aporepressor. Tryptophan is the corepressor 

 that combines with the aporepressor to form the repressor proper. 



[52] 



