38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 



423 



200 



2 ^ 100- 



9 10 



minutes 



Fig. 2. Incorporation of methionine-S 35 into the RNA of E. coli K12 growing 

 exponentially. Dilution by unlabeled methionine. Temperature 20°C; DL-methionine- 

 S 35 (2.10 -4 M), specific radioactivity (2/ac/yimole). 



phosphate content, can degrade up to 30% of their RNA, whereas DNA 

 synthesis continues. 113 



2. Rate of Incorporation of Radioactive Precursors into Proteins 



and Nucleic Acids 



Since the incorporation of single radioactive precursors into proteins 

 and nucleic acids can under certain circumstances be an irreversible process, 

 the use of radioactive tracers should reveal not only the absolute rate of 

 synthesis of these macromolecules but also the nature of possible interme- 

 diates in their synthesis. 



a. Amino Acid Incorporation into the " Intracellular Pool" and into the 

 RNA 



By measuring the comparative rates at which radioactive amino acids 

 label the pool, the RNA, and the protein, one should be able to determine 

 whether amino acids are fixed to an acceptor RNA before being built into 

 a peptide chain. Figure 2 presents a typical example of the kinetics of this 

 type of incorporation obtained by adding a radioactive amino acid to a 

 growing culture of E. coli. The results are compatible with the idea that the 

 pool of free amino acids and the amino acid-RNA complexes are protein 

 precursors. These two fractions become labeled according to a typical 

 saturation curve while the label in the protein increases exponentially. 



J13 T. Horiuchi, S. Horiuchi, and D. Mizuno, Biochim. et Biophys. Acta 31, 570 (1959). 



