CHAPTER V 



Regulation of Protein Synthesis 



A. ENZYMIC ADAPTATION 



The rate of protein synthesis, like that of all the anabolic processes, 

 depends on temperature, pH, energy production, availability of building 

 blocks, etc. . . . More interesting is the specific control of the rate of syn- 

 thesis of the individual proteins, exerted by external or internal agents. 



The assortment of proteins actually made by a cell is conditioned by the 

 genetic material ; the presence of the gene, however, confers but a potential- 

 ity, the expression of which depends on specific controlling systems. Let us 

 ignore for a moment the problem of differentiation and consider only 

 micro-organisms ; even for these relatively simple cells, the assortment of the 

 proteins actually produced does not depend on the genetic material alone : 

 it can be influenced by changes of the medium composition. Such mould 

 will start producing a protease when transferred on to a medium which 

 contains milk. A bacterium which makes a phosphatase in given surround- 

 ings may stop to manufacture the enzyme if inorganic phosphate is added to 

 the medium. Certain substances can thus specifically induce a cell to make 

 certain proteins, others can specifically repress the production of an enzyme 

 without affecting the synthesis of the other proteins. Induction and re- 

 pression of enzyme synthesis have been studied mostly in bacteria and 

 yeasts; comparable effects have been observed in plants and animals, but 

 they are much more complex especially in animals where homeostatic 

 mechanisms oppose changes of conditions and where superposed hormonal 

 actions make the results extremely intricate (see p. 170). 



It is not the purpose of the present chapter to discuss thoroughly the 

 problem of enzymic adaptation and repression; many excellent reviews 

 and discussion have been published on the subject. Reading them in the 

 chronological order gives an extremely vivid picture of the detours of 

 discovery in the field, with a succession of bright hypotheses, blind alleys 

 and break through, resulting in the slow elaboration of a more and more 

 accurate picture of the phenomenon (Wortman, 1882; Duclaux, 1900; 

 Dienert, 1900; Karstrom, 1936; Stephenson and Yudkin, 1936; Yudkin, 

 1938; Monod, 1947, 1956, 1958, 1960; Spiegelman, 1950; Stanier, 1951; 

 Mandelstam, 1956; Spiegelman and Campbell, 1956; Pollock and Mandel- 

 stam, 1958; Vogel, 1958; Pollock, 1956, 1959; Pardee et al, 1959). Only a 



126 



