332 



H. H. ROBERTS, R. J. BRITTEN, AND B. J. MCCARTHY 



Time in minutes 



Fig. 24. The incorporation of C'Meucine (10" 3/) into protein in a culture of 

 E. coli ML30. C'-leucine was added to a final concentration of 10 ' M at 1 minute. 



direct evidence for these processes can be obtained by observation of the 

 kinetics of incorporation of amino acids into ribosomes. 



These experiments suffer, however, from two technical difficulties. A 

 part of the newly formed protein found associated with ribosomes is not 

 destined to remain in the ribosomes but is nascent protein which is sub- 



•a. 



5 

 O 



20 30 40 



Fraction number 



Fig. 25. Sedimentation analysis of three total cell extracts prepared from cells 

 labeled with P" for four generations and C"-leucine for 1 minute followed by (a) 10 

 seconds, (b) 2 minutes, (c) 7 minutes, 45 seconds in C'Meucine (see Fig. 24). Cells 

 washed and extracts prepared in tris-HCl 0.01 M, pH 7.4, containing MgCL lO"' M. 

 Centrifugation 165 minutes at 37,000 rpm, 4°C. P'= shows the location of the SOS 

 and SOS ribosomes. 



