426 F. GROS 



TABLE III 



Transfer of a Radioactive Amino Acid from the RNA 



Amino Acid Pool to the Protein Fraction" 



Experiment 



No. 1 



Control 



Chloramphenicol 

 No. 2 

 No. 3 



° A culture of the auxotroph E. coli M-191 (tnethionine-histidine) is starved for 4 hours at 32°C. Methio- 

 nine-S' 5 (2 X 10" 6 .W) is then added and allowed to label the "RNA pool" for 5 minutes in the absence of histi- 

 dine; at the end of this period, a large excess of nonradioactive methionine (4 X 10~».lO plus some histidine are 

 added to the culture. The total radioactivity of the "RNA pool" and of the proteins is measured at the 

 time of addition of the unlabeled amino acid and 8 minutes later. 



b. Amino Acid Incorporation into "Soluble" Protein and into the Ribosomes 



Many experiments with whole animals or tissue slices have established 

 that after incorporation of a radioactive amino acid, the label first appears 

 in the proteins of the microsomal fraction (see Chapter 37 and the review 

 by Loft Held' 15 ). When labeled microsomes are fractionated, the highest 

 specific radioactivity is associated with the fraction insoluble in deoxy- 

 cholate, that is the ribonucleoprotein. These observations have often been 

 quoted as showing that the protein-forming site is a ribonucleoprotein. 



This question has been examined in bacteria by Roberts et al. bl The re- 

 sults show that if individual variations in amino acid content are taken into 

 consideration, no protein fraction, not even that of the ribosomes, exhibits 

 a higher specific radioactivity than any other. Two explanations are pos- 

 sible: 



(1) The first highly radioactive proteins formed have been stripped off 

 the ribosome during the fractionation procedure. This is improbable how- 

 ever since even the crude microsome pellet does not contain any one pro- 

 tein of high specific radioactivity. 



(2) The time of exposure to the tracer (4 minutes) may still be too long 

 because of the very high growth rate of E. coli. A calculation shows 51 that 

 if ribosomes are specific templates for protein synthesis, and if polypeptide 

 chains of 150 residues are formed, each of the 10,000 ribosomes present in 

 a cell of E. coli must produce a polypeptide chain every 10 seconds accord- 

 ing to the observed growth rate of the organism. Therefore, if only one poly- 



115 R. B. Loft field, Progr. in Biophys. and Biophys. Chem. 8, 347 (1958). 



