38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 



429 



10,000 



^ 5,000 



I 



/* \Adenine-C 1 * 



t 



Adenine - C 1i 



50 



100 



Fig. 4. Rates of incorporation of adenine-C 14 into the "soluble" and "particulate" 

 RNA; E. coli ML 308, growing at 18°C. in mineral medium plus glucose, at a given 

 time adenine-C 14 (10 jug. /ml. and 300 c. p.m. /jug.) is added. Samples of the culture are 

 taken at intervals, and cooled rapidly at 0°C. Bacteria are washed in dilute Tris 

 buffer (0.005 M) pH 7.6 and ground with alumina in 0.002 M Tris + MgCl 2 5 X 10" 3 

 M at pH 7.4. The extract is freed of DNA and the sRNA is separated from the "RNA 

 particles" by ultracentrifugation for 2 hours at 100,000 g. 



The direct or indirect conversion of sRNA to pRNA [mechanism (2)] is 

 an unlikely process, since after starvation of uracil in a particular mutant 

 of E. coli, the sRNA does not disappear as would be expected since its re- 

 synthesis would be prevented. 41 



Mechanisms (1) or (3) are more probable and the possibility of their 

 existence is strengthened by the observation 122 that part of the radioactivity 

 found in the sRNA after adenine-C 14 incorporation by growing bacteria, 

 can be released as radioactive adenosine on submitting the labeled sRNA 

 to mild alkaline hydrolysis. 



On the other hand, since mechanism (1) has been demonstrated to occur 

 in vitro, the possibility of its occurrence in vivo is not unreasonable. It can 

 be assumed therefore, that the sRNA is not a precursor of the pRNA and 

 is not involved in its synthesis. 



/. The Cell Membrane as the Site of RNA Synthesis 



Spiegelrhan 117 found that when intact protoplasts of E. coli are incubated 

 with uridine-C 14 in a medium supporting growth, the most rapidly labeled 

 fraction of the RNA was associated with the membranes, followed by the 



