444 



F. GROS 



0.2 0.4 



RNA/PROTEIN 



Fig. 5. Relationship between the growth rate and the RNA content of a bacterium 

 grown on different media [B. Magasanik, A. K. Magasanik, and F. C. Neidhardt, in 

 "The Regulation of Cell Metabolism," A Ciba Symposium, p. 334. Churchill, London, 

 1959]. 



as a fundamental constituent), but with the following restrictions: when 

 the carbon source is not very efficient or, more generally, when the composi- 

 tion of the medium is not optimal, (K between 0.2 and 0.6), it is the cellular 

 content in "building blocks" (amino acids "free" or "activated") and not 

 that of the RNA which limits the growth rate. On the contrary, when the 

 composition of the medium is rich enough to provide protein precursors 

 at a very high rate, the RNA content becomes the limiting factor for the 

 rate of protein synthesis. 



If a cell can synthesize protein at a rate proportional to its RNA content, 

 one must expect that RNA destruction leads to almost complete cessation 

 of the protein synthesizing abilities of the cell. 



Two different techniques have been used selectively to alter or destroy 

 RNA in vivo. The first is based on a selective labeling of the RNA by very 

 highly radioactive P 32 . The results of such an experiment have already been 

 reported. They show that the capacity to synthesize protein and enzymes 

 declines regularly with the number of phosphorus atoms which have under- 

 gone decay in the RNA. This decline is, however, 4 times slower than when 

 both DNA and RNA are labeled. 6 * • 17s 



Complete destruction of the cellular RNA by ribonuclease should lead in 

 principle to more easily interpetable results and this constitutes the second 

 approach which has been followed extensively by Brachet and his school 181 



