344 B. Magasanik, et al. 



(presumably activated amino acids) among the different units 

 of enzyme-forming machinery (presumably ribonucleoprotein 

 particles). The overall rate of protein synthesis, and therefore 

 of growth, must depend on the intracellular concentration of 

 the activated amino acids and on the RNA content of the cell. 



It has been known for many years that the RNA content of 

 a cell is not constant, but varies with the growth rate of the 

 cell, although different authors have described this relation in 

 quantitatively different terms (Brachet, 1955). We investi- 

 gated therefore the relationship between RNA content and 

 growth rate in A. aerogenes, the organism used in our studies 

 of the regulation of enzyme formation. The results of a series 

 of experiments in which we determined the RNA content 

 (expressed as the ratio of RNA to protein) of cells growing at 

 widely different rates are presented in Fig. 3. It can be seen 

 that in rapidly growing cells (k = 0-6 to k = 1 -1), the rate 

 of growth, and therefore the rate of protein synthesis, is 

 proportional to the total RNA content of the cell ; however, in 

 cells growing more slowly (k = 0-2 to k = 0-6), there is little 

 variation of the RNA content with a change of growth rate. 



It would, therefore, appear that when the rate of growth is 

 severely restricted by the composition of the growth medium, 

 it is the low concentration of activated amino acids, and not 

 the insufficiency of the machinery, which limits the rate of 

 protein synthesis : the cell is capable of increasing the rate of 

 protein synthesis threefold (from k = 0-2tok = 0-6) without 

 a substantial increase in its RNA content. On the other hand, 

 an increase in the rate of protein synthesis from k = • 6 to 

 k =1-1, seems to involve an expansion of the protein-forming 

 systems of the cell. The proportionality of the rate of protein 

 synthesis and of the total RNA content of the cell in this 

 range would seem to imply that the concentration of the 

 activated amino acids is sufficiently high to saturate the 

 protein-forming machinery; the rate of protein synthesis is, 

 therefore, a function only of the amount of protein-forming 

 machinery which the cell contains. 



It is physiologically advantageous for a cell to be able to 



