348 B. Magasanik, et al. 



(3) The overall rate of protein synthesis depends on the 

 amount of RNA the cell contains, and on the intracellular 

 concentration of the activated amino acids. 



(4) The rate at which an individual protein is formed 

 depends on the activity of the corresponding units of protein- 

 forming machinery; this activity is controlled by the inter- 

 action of the RNA template with specific repressors and 

 inducers, which are in general products and substrates, 

 respectively, of the enzyme-protein whose production they 

 regulate. This automatic adjustment provides the cell with 

 the enzymic composition best suited for growth in a particular 

 environment. 



It is hardly necessary to stress that most of the evidence on 

 which these concepts are based is preliminary and indirect, 

 and will require confirmation by decisive experiments with 

 isolated cell fractions. However, it would seem to us that the 

 formulation of a unified hypothesis for the regulation of 

 cellular growth and composition at this time may be useful in 

 planning future experiments. The clear recognition of the 

 feebleness of the foundations on which the hypothesis rests 

 should enable us to pursue this work with an open mind. 



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