130 THE BIOSYNTHESIS OF PROTEINS 



establishing some kind of specific inhibition. Since in the second type of 

 crossing inducibiHty (as opposed to constitutivity) takes some time to be 

 established, it would seem that the development or the operation of the 

 inhibitory mechanism is rather slow. On the other hand, an exogeneous 

 inducer is able to release the inhibition rapidly. 



These genetical studies do not indicate the nature of the inhibitory 

 system. They are not incompatible with the general induction theory. The 

 i+ gene might be responsible for the production of an enzyme D which 

 destroys the endogeneous inducer; since this endogeneous inducer must be 

 present in the constitutive cytoplasm, the z+ gene would be expressed as 

 soon as introduced; to the contrary, the expression of the dominant i+ gene 

 would take the time required for the enzyme D to reduce the steady state 

 concentration of the endogeneous inducer below the induction threshold. 

 A completely different interpretation of the experimental results can also 

 be considered, which rejects the generalized induction theory. It may be 

 assumed that the i+ gene (inducibility) controls the production of a sub- 

 stance which blocks specifically the synthesis of galactosidase, and the 

 action of which is antagonized by exogeneous inducers. The delay in the 

 establishment of the 'inducible' character in the second type of crossing is 

 then explained by the time required for reaching the threshold concentra- 

 tion of the repressing agent. 



Recent data by Pardee and Prestidge (1959) indicate that the i+ gene 

 is able to express itself under conditions which prevent protein synthesis 

 almost completely. If this conclusion is correct, the first model can be dis- 

 carded. Besides, these experiments indicate that the i+ gene does not con- 

 trol the structure of any protein, and — adopting the second of the models 

 discussed above — that the repressing agent is not a protein. 



The generalized repression theory departs from the generalized induc- 

 tion theory in that it leaves all the structural information to the structural 

 gene, the inducer and the repressor are mere regulators of the enzyme 

 producing systems. The generalized repression theory unites in a single 

 concept the phenomena of induction and of repression of enzyme synthesis, 

 which will now be examined briefly. 



2. Repression of Enzyme Synthesis in Micro-organisms 



Galactose inhibits the synthesis of constitutive ^-galactosidase in E. colt. 

 Tryptophan and certain of its analogues inhibit the formation of tryptophan 

 synthetase in the same organism (Monod and Cohen-Bazire, 1953). The 

 specificity of these inhibitions indicated that the substances interfere with 

 the process of induction of the enzymes (Cohn and Monod, 1953). More 

 cases of such inhibitions have been observed and further analysed. Thus 

 Gorini and Maas (1957) and Vogel (1957), who coined the name repression 

 for this phenomenon, observed that the synthesis of each enzyme of the 



