346 B. Magasanik, et al. 



enzymes are no longer made when they have become super- 

 fluous or redundant. The same economy seems to apply in 

 part to the production of RNA. The cell growing on myo- 

 inositol (Fig. 3, point 4), which is not metabolized rapidly 

 enough to give maximal growth, uses a smaller proportion of 

 its resources to make RNA than a cell growing on glucose 

 (Fig. 3, point 8) ; a larger portion of the energy and the building 

 blocks produced by the degradation of m?/o-inositol, is, there- 

 fore, available for protein synthesis. 



The observation that the composition of the growth medium 

 affects the RNA content of the cell suggests that the rate of 

 RNA synthesis is sensitive to the concentration of some inter- 

 mediary product(s) of metabolism. Several lines of evidence 

 suggest that the rate of RNA synthesis is controlled by the 

 concentration of the amino acids in the cell: (1) cells deprived 

 of an amino acid required for protein synthesis immediately 

 cease to synthesize RNA. This well known fact is illustrated 

 in Fig. 1, where it can be seen that arginine deprivation 

 prevents the synthesis of RNA as well as of protein. (2) The 

 synthesis of RNA without concomitant synthesis of protein, 

 which occurs in chloramphenicol-inhibited bacteria, depends 

 on the presence of all of the amino acids required for protein 

 synthesis (Pardee and Prestidge, 1956; Gros and Gros, 1956; 

 Yeas and Brawerman, 1957). (3) The addition of a mixture of 

 amino acids to cultures of ^. aerogenes growing at a slow rate 

 on m?/o-inositol or glycerol (Fig. 3, points 4 and 6), increases 

 their rate of growth and their RNA content to a level which 

 seems maximal for this organism (Fig. 3, points 9 and 10). 



Let us therefore assume that amino acids, or perhaps their 

 activated derivatives, are essential not only for the synthesis 

 of protein, but also for the synthesis of RNA and, furthermore, 

 that the protein-forming system has greater affinity for these 

 metabolites than the RNA-forming system; a ready explana- 

 tion for the observed relationship between the growth rate and 

 the RNA content of the cell is then at hand. In a rapidly 

 growing cell (k > 0-6, see Fig. 3), the level of activated amino 

 acids is sufficiently high to saturate the protein -forming 



