Control of Bacterial Cell Growth and Composition 339 



has been clearly established that the cell contains more than 

 one type of RNA, and that the "soluble" RNA, which in 

 bacterial cells constitutes about one-tenth of the total 

 RNA, plays an important part in the activation of amino 

 acids for protein synthesis (Gros, 1958, personal communica- 

 tion). The bulk of the RNA exists in the form of ribonucleo- 

 protein particles (Schachman, Pardee and Stanier, 1952), and 

 it has been postulated that this material plays the role of a 

 template in protein synthesis (Bounce, 1953): it orders the 

 activated amino acids before they are polymerized to a 

 protein molecule. The last step in protein synthesis is the 

 removal of the protein molecule from the RNA tem^plate, and 

 it is this process which repressors and inducers are presumed 

 to affect. The rate of synthesis of an enzyme would depend 

 on the ratio of the intracellular levels of its repressor and its 

 inducer, which in turn would depend on the activity of this, 

 as well as of other enzymes. Adjustments in enzyme levels 

 would, therefore, not require the synthesis or destruction of the 

 RNA template ; the cell would at all times possess the machin- 

 ery to form all the enzymes which it is genetically capable of 

 producing. Even the drastic changes in the rates of synthesis 

 of certain enzymes, which occur when the major carbon and 

 energy source of the cell is changed, should not reflect the 

 synthesis of new RNA templates, but rather the acceleration 

 and retardation of protein synthesis by different existing 

 enzyme -forming units. 



We undertook to test this hypothesis by exploring the 

 relationship of protein synthesis to RNA synthesis during the 

 adjustment of A. aerogenes to changes in its environment.* 

 In these experiments a mutant strain, 5-P-14, was used which 

 required the amino acid, arginine, and the purine, guanine, for 

 growth; it was therefore possible to limit the growth of the 

 organism by restricting the supply of either of the two 

 nutrilites, or of glucose, the major source of carbon and 



* The experimental work described in this section was carried out by one of 

 us (F.C.N.) at the Department of Bacteriology and was supported by U.S. 

 Public Health Service Grant RG-3554. 



