38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 



415 



c p.m. /ml 

 8.000 h 



6.000 - 



4000 



2.000 - 



4 5 



ml. SAMPLED 



Fig. 1. Distribution of the "RNA bound" radioactivity after ultracentrifugation 

 of an extract of E. coli K12 labeled with S 35 . S 35 -labeled bacteria are converted to 

 protoplasts by treatment with lysozyme in the presence of sucrose. The extract ob- 

 tained after osmotic shock is freed of DNA and ultracentrifuged for 2 hours at 100,000 

 g. Aliquots are drawn off separately with a syringe, and the "RNA bound" radioac- 

 tivity is determined according to the principle described in Table II. 



amino acid protects its corresponding specific site from such an inactivation, 

 a possible interpretation of this result is that the sRNA of E. coli does not 

 naturally contain any amino acid attached to it. Preiss et at. consider rather 

 that the amino acid may have been split off during the process of isolation 

 and purification, since sRNA from other sources usually contains amino 

 acids when isolated in the "native form." 45 



Moreover, if E. coli is cultivated in an "amino acid free" medium plus 

 S 35 , which is converted into radioactive methionine and cystine, the total 

 RNA extracted from the bacteria is highly labeled. By ultracentrifugation 

 (Fig. 1) it can be shown that most of the radioactivity is bound to the 

 nonsedimentable fraction of the bacterial RNA. Therefore in E. coli a 

 fraction of the endogenously formed amino acids is attached to the sRNA. 

 Furthermore, only a very low concentration of amino acid is required to 



45 G. Acs, (1. Hartmann, H. G. Boman, and F. Lipmann, Federation Proc. 18, 178 



(1059). 



