414 F. GROS 



TABLE II 



Nature of the Radioactivity Bound to the RNA Fraction After Incorporation 



of a Radioactive Amino Acid" 



Property of RNA fraction 



Amount of fraction 



(%) 



Precipitable by TCA (5% in the cold) 90 



Extractable in hot TCA (20 minutes at 100° C.) 95 



Dialyzable against water or urea (12 hours at 4° C.) 5-10 



Dialyzable after ribonuclease treatment (digestion for 3 70 



hours at 30° C. with 0.1 mg./ml.) 



Lability in the presence of alkali (0.05 N at 20° C. for 20 80 



minutes) 

 Electrophoretic mobility in agar As for RNA 



a Bacteria are labeled during growth for a few minutes in the presence of a radioactive amino acid. The 

 pool of the free amino acids is removed by extensive washing with 5% TCA in the cold. Bacterial residues are 

 then washed in alcohol and in alcohol plus ether. RNA is extracted by two successive treatments in 10% NaCl 

 at 100°C. for 20 minutes, and precipitated from the extracts by 60% alcohol at — 10°C. 



In' these experiments the bulk of the RNA was extracted together with the 

 RNA amino acid complex. This explains the relatively low content of amino 

 acids. For six amino acids (valine, phenylalanine, arginine, methionine, 

 leucine, and isoleucine) the average value is 0.2 mumole per milligram RNA; 

 this indicates one amino acid for every fifty nucleotides if all the twenty 

 amino acids are represented in the RNA amino acid pool. There are a few 

 important exceptions: the RNA proline, and the RNA tyrosine pools are 

 much higher in amino acid content than the other amino acid RNA pools. 

 Either these two amino acids are linked to RNA fractions other than sRNA 

 itself, or more than one molecule of these amino acids is bound (specifically 

 or not) per molecule of sRNA. Work on this question is in progress. 



Some general properties of the amino acid RNA complexes of E. coli are 

 presented in Table II. These properties are very similar to those of the 

 amino acid RNA complex from liver tissue. 29 The question arises as to 

 whether the RNA amino acid complex isolated after incorporation of a 

 radioactive amino acid by intact bacteria is merely a biological artifact. 

 It might, for instance, be formed only when the intracellular concentration 

 of free amino acid is very high due to the functioning of the permease. It 

 is doubtful in fact whether the sRNA of E. coli growing in the absence of 

 external amino acid contains amino acids. An experiment by Preiss et al. 4i 

 suggests that it does not : sRNA was isolated from E. coli which had grown 

 in a simple mineral medium unsupplemented with amino acids. All the bio- 

 logical sites of this RNA could be inactivated by periodate. Since a bound 



44 J. Preiss, P. Berg, E. J. Ofengand, F. H. Bergmann, and M. Dieckmann, Proc. 

 Natl. Acad. Set. U. S. 45, 319 (1959). 



