Control of Bacterial Cell Growth and Composition 343 



were harvested at the end of the lag phase and their nucleic 

 acids were fractionated. The molar radioactivity of the RNA- 

 guanine and of the DNA-guanine was determined and com- 

 pared with the molar radioactivity of the guanine present in 

 the growth medium (Table III). It is evident that propor- 

 tionally much less guanine was incorporated into RNA than 



Table III 



[8-^*C]GUANINE INCORPORATION INTO THE NUCLEIC ACIDS OF 



Aerobacter aerogenes during a diauxic lag 



RNA-Guanine DNA-Guanine 



Radioactivity of the isolated guanine 

 relative to that in the medium 

 (97,000 cpm/[xM) 0-103 0-330 



Increase calculated from the radio- 

 activity 11% 49% 



Increase calculated from colorimetric 



measurements 7 % 40 % 



Cells of strain 5-P-14 were grown in a medium containing glucose and myo- 

 inositol as major carbon sources with arginine and guanine present as essential 

 growth factors exactly as described for Fig.2. [8-i*C]Guanine was added at the 

 beginning of the lag phase and the entire culture harvested one hour later. 

 Extraction of the acid-soluble pool and of the lipid fraction of the cells was 

 followed by alkaline hydrolysis of the RNA, precipitation of the DNA and 

 protein with acid, and then acidic hydrolysis of the DNA. The RNA-guanine 

 was isolated as guanylic acid by electrophoresis, and the DNA-guanine was 

 isolated as guanine by paper chromatography. 



into DNA; the failure of the RNA to increase proportionally 

 with DNA and protein during the diauxic lag thus appears to 

 be due to an impairment of RNA synthesis, and not to an 

 enhancement of RNA degradation. This result indicates that 

 the synthesis of new types of protein, which is essential for the 

 transition of the cell from one chemical environment to 

 another, can proceed without the synthesis of an equivalent 

 amount of RNA. Such a finding is in harmony with the con- 

 cept that the synthesis of enzynies is regulated by the control 

 of the action, and not of the formation of the protein-forming 

 machinery. 



The regulatory mechanism we have discussed so far serves 

 to apportion the building blocks available for protein synthesis 



