Control of Bacterial Cell Growth and Composition 847 



system, but not high enough to saturate the RNA-forming 

 system. Consequently, any change in the composition of the 

 medium which increases the level of the activated amino 

 acids in these rapidly growing cells will cause an immediate 

 increase in the rate of RNA synthesis; the rate of protein 

 synthesis will not be directly affected by the higher level of 

 the activated amino acids, but will increase gradually with the 

 rising RNA content of the cell. 



The assumption that the RNA-forming system has less 

 affinity for amino acids than the protein-forming system 

 would also explain the observation, illustrated in Figs. 1 and 

 2, that after the exhaustion of glucose, protein synthesis 

 continues at a diminished rate, while RNA synthesis ceases. 

 It has been shown by Mandelstam (1958) that in Escherichia 

 coli the intracellular concentration of amino acids decreases 

 markedly during the transition of the cell from the exponential 

 phase of growth on glucose to the stationary phase ; a point 

 will be reached at which the level of the amino acids will be 

 sufficient for protein synthesis, but too low to permit the 

 synthesis of RNA to proceed at a measurable rate. 



We may summarize the concepts of the regulation of cell 

 growth and composition discussed in this essay in four state- 

 ments. 



(1) The genome, i.e. the DNA fraction of the cell, possesses 

 all the qualitative information required for the synthesis of 

 every protein that the cell is capable of making. 



(2) This information is transmitted to the actual protein- 

 forming machinery, which consists of ribonucleoprotein 

 particles; these act as templates for the construction of the 

 protein molecules. The cell contains at all times an equal 

 number of templates for every species of protein that it is 

 genetically capable of making. However, the rate of synthesis 

 of all templates depends on the intracellular concentration of 

 the amino acids required for protein synthesis (or of their 

 activated derivatives). Consequently, a cell growing in a rich 

 medium will have a greater capacity for the synthesis of all 

 proteins, than a cell growing in a poor medium. 



