424 F. GROS 



Hoagland 29 has obtained similar results in measuring the kinetics of amino 

 acid incorporation in the whole animal. It must be stressed, however, that 

 in the experiment described here the amino acid-RNA pool includes 

 amino acids attached to both the sRNA and the RNA particulate. 



The RNA amino acid pool is saturated very rapidly (in 4 to 5 minutes 

 at 20°C.) and a little earlier than the "free" pool which is saturated in about 

 10 minutes. This may be accounted for by the extremely small size of the 

 pool of the RNA bound amino acids; a low internal concentration of free 

 amino acids is sufficient to saturate the activating enzymes. The free amino 

 acids, and those bound to the RNA, undergo very fast renewal. Thus, if 

 bacteria are labeled with a radioactive amino acid, dilution of this tracer 

 by an excess of the corresponding unlabeled amino acid quantitatively dis- 

 places the radioactive amino acid in both the "free" pool and in the amino 

 acid-RNA complex. 



In the case of the "free" pool, this renewal should depend on both the 

 activity of the permease, 9 and the utilization of the free amino acids to 

 make protein. However, if the external concentration of the radioactive 

 amino acid is sufficient to saturate the permease, the rate of its uptake for 

 protein synthesis is very small compared with the rate of its penetration 

 into the bacterial cell. Therefore, the renewal of the free amino acid pool 

 is practically independent of the rate of protein synthesis. 10 In order to ex- 

 plain the renewal of the RNA amino acid pool, one might be led to various 

 interpretations following that one considers this pool as composed mostly 

 of the sRNA bound amino acids or of the RNA particulate bound amino 

 acids. In the first case, the factors involved in the renewal of the bulk of 

 the RNA amino acid pool can be of at least two types: 



(1) A reversible fixation of the activated amino acids in the sRNA 



a 



Adenyl amino acid + sRNA ^ sRNA amino acid + AMP 



b 



(2) A reaction or a series of reactions leading irreversibly to the use of 

 these RNA bound amino acids for protein synthesis 



sRNA amino acids — » protein + sRNA 



Experiments show that when protein synthesis is inhibited (reaction c) 

 for instance, by chloramphenicol, the rate of renewal of the RNA bound 

 amino acids considerably diminishes as the rate of incorporation of a radio- 

 active amino acid into this fraction (Fig. 3) and the rate of its displacement 

 by a nonradioactive amino acid are both reduced by about 80%. Conse- 

 quently, the extent of protein synthesis could be a limiting factor in the 

 renewal of the amino acids bound to the sRNA, contrary to the situation 

 for the free amino acids. 



