132 THE BIOSYNTHESIS OF PROTEINS 



ornithine occurs only under conditions of partial repression by arginine. 

 The effects of ornithine, the inducer, and of arginine, the repressor, seem 

 to be competitive. 



Sulphate represses tyrosinase formation in Neurospora (Horowitz et ah, 

 1960) and aromatic amino acids release the repression. Tyrosinase thus 

 behaves as an adaptive or as a constitutive enzyme depending on the con- 

 centration of sulphate in the medium. Here it is difficult to visualize how 

 sulphate and aromatic amino acids could compete for the same site; it 

 should be realized that there is no reason to think that the exogeneous 

 inducers and repressors act as such on the protein making machinery. The 

 necessity for the transformation of the added inducer into the actual induc- 

 ing agent has been postulated in several cases of enzyme induction. Clayton 

 (1960) provided evidence for the elaboration of an intermediary substance 

 responsible for induction of catalase by the exogeneous inducers oxygen 

 or hydrogen peroxide. New words will be needed to distinguish between 

 the inducers and repressors that the experimenter adds to the cell sus- 

 pension and the resulting active intracellular agents which operate the 

 regulatory mechanism. Competition probably takes place between these 

 'elaborated' inducers and repressors. 



Since induction and repression are highly specific, inducers and repres- 

 sors must exert their action on the system which controls the specificity of 

 synthesis of the individual proteins. Three sites of action of the regulators 

 can be envisaged at present: the structural gene, the genetic messenger 

 which carries the information to the enzyme making system, and the 

 template itself. Presently available data do not permit to decide. Enuclea- 

 tion experiments should make it possible to establish whether induced 

 enzyme synthesis can take place in the absence of the gene. Unfortunately, 

 the type of cells which can be easily enucleated, so far proved unfavourable 

 to enzyme induction studies. An increase of catalase activity was caused by 

 hydrogen peroxide in anucleate as well as in intact Acetahiilaria mediter- 

 ranea (Brachet and Chantrenne, 1953), but it could not be clearly established 

 whether catalase was synthesized in the process (Chantrenne, 1955b). 

 Induced enzyme formation in yeast is not inhibited by very high doses of 

 ionizing radiation, which cause such damage to the DNA of the cell that it 

 cannot be precipitated with acid after alkaline extraction (Chantrenne and 

 Devreux, 1959). Induced enzyme synthesis has been observed in bacterial 

 preparations from which most of the DNA had been removed by salt 

 extraction or destroyed by deoxyribonuclease (Spiegelman, 1956). These 

 facts suggest that the site of action of the inducer is not the DNA; unfor- 

 tunately, the results are not entirely convincing, because the nature of the 

 damage caused to DNA by radiation is not clearly understood, and the 

 disrupted bacterial system from which most of the DNA was extracted 

 synthesized new DNA while making enzymes. 



