Control of Bacterial Cell Growth and Composition 341 



three cell constituents; guanine deficiency severely restricts 

 the formation of the nucleic acids, but permits the protein to 

 increase by about 30 per cent ; exhaustion of glucose results in 

 the cessation of RNA increase while protein and DNA increase 

 by about 30 and 50 per cent, respectively. The net increase in 

 protein after the exhaustion of the glucose from the medium, 

 occurs at the expense of the accumulated products of glucose 

 metabolism, which have been described earlier. This increase 

 must represent in part the synthesis of new enzymes necessary 

 for the further metabolism of these products, a synthesis 

 which had been repressed during glucose degradation. 



A similar situation occurs when the cells are grown in a 

 medium that contains glucose as well as another compound 

 capable of serving as a source of carbon and energy : as shown 

 by Monod (1947), cells in such a medium grow first on the 

 glucose, which suppresses the formation of enzymes required 

 for the attack on the second carbon compound; a period of 

 diminished growth, during which these enzymes are synthe- 

 sized, separates the phase of growth on glucose from the phase 

 of growth on the other compound. 



The behaviour of the nucleic acid and protein fractions of 

 the cell during the lag separating the two phases of this 

 diauxic growth was investigated with the results shown in 

 Fig. 2. During this period the cells made the transition from 

 exponential growth on glucose (with a mean generation time 

 of 45 minutes) to exponential growth on mz/o-inositol (with a 

 mean generation time of 100 minutes). It can be seen that 

 DNA synthesis continues at its original rate during the first 

 part of the lag period, and then decreases to the new rate 

 characteristic of growth on mi/o-inositol. Protein synthesis 

 proceeds throughout the lag period, though at a slightly 

 slower rate than DNA synthesis. On the other hand, no net 

 increase in RNA could be detected throughout the period of 

 the diauxic lag; RNA began to increase again only after the 

 transition from glucose metabolism to mi/o-inositol metabolism 

 had been accomplished. The failure to observe any net 

 increase in RNA during the diauxic lag could conceivably be 



