BIOLOGICAL ORDER 



negative feedback mechanism: in this case the control is corrective. 



Things are different when the energy source is concerned. When 

 glucose is present, a repressor is produced, thanks to a corepressor 

 that is a by-product of glucose metabolism. The repressor prevents 

 the synthesis of ^-galactosidase. The activity of the enzyme re- 

 sponsible for the metabolism of one energy source prevents the 

 synthesis of a system involved in the metabolism of another energy 

 source. This mechanism could be considered as a feedback, but it is 

 in fact a preventive, or preclusive, counteraction. 



The corrective feedback deals with the synthesis of an essential 

 metabolite, the preclusive feedback with the metabolism of an energy 

 source. In the first case, the corepressor is the end product of the 

 activity of the enzymatic system submitted to repression, namely, 

 the tryptophan zymon. In the other case, the repressor is one of 

 the end products of a system of enzymes different from the system 

 submitted to the repression. Obviously, many more systems have 

 to be analyzed before any attempt at a generalization can be made.* 



Conclusion 



The study of enzyme synthesis has thus revealed that the cell is 

 endowed with an elaborate dual mechanism which controls the 

 activity and the synthesis of enzymes. The organism has to cope 

 with the variation of the environment. In order to survive, it must 

 adjust its enzymatic equipment according to the nature of its food 

 and to the nature of its needs. And also, the machine must be 

 regulated in such a way that each one of the four nucleic bases and 

 each one of the twenty amino acids is manufactured in just the 

 right amount and proportion. 



The organism, the bacterium, has been compared to a factory. 

 In a factory, one person, or one group of persons, directs the activity 

 of all the others. Some readers may be tempted to ask the question: 

 Who commands in a bacterium? Obviously, in a microorganism, no 

 single molecule or group of molecules can be held responsible for 



*The reader interested in the control of protein synthesis and in the prob- 

 lem of regulation is referred to the excellent review of Jacob and Monod, 

 /. MoL BioL (1961), 3, 318-356, to the last issue of the Cold Spring Harbor 

 Syinposia on Quantitative Biology (1961), 26, and to Jacob and Wollman, 

 Sexuality and the GeJietics of Bacteria, Academic Press, New York, 1961. 



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