438 F. GROS 



These observations have been confirmed by several authors. 111 It is in- 

 teresting to note that, in B. cereus, azaguanine gives rise to an accumulation 

 of RNA without protein synthesis and that the RNA thus formed is meta- 

 bolically stable. 108 Similarly, the RNA accumulated during methionine 

 starvation in certain mutants of E. coli K12, does not breakdown when 

 growth is resumed on addition of methionine. 158 



The metabolic instability of the chloramphenicol RNA might be related 

 to its peculiar physicochemical properties. 42, 43 In fact, treatment of bacteria 

 by chloramphenicol results in a drastic change in the distribution of the 

 cytoplasmic particles. Extracts of treated cells contain very little of the 

 50 *S and 30 S component but appreciable amounts of a new type of particle, 

 called 15 S which contains 75 % RNA and 25 % protein as compared with 

 60% and 40% respectively, in the normal particles. Since the 15 S compo- 

 nent contains 70 to 80 % of the RNA built up after chloramphenicol addi- 

 tion, its RNA is probably associated with preexisting protein derived from 

 the normal 30 S or 50 S particles. Unlike the normal particles, the 15 S is 

 very sensitive to ribonuclease probably on account of insufficient protection 

 of the RNA by the protein envelope. This ribonuclease sensitivity seems to 

 be concerned with the metabolic instability of the chloramphenicol RNA 

 reported above. 



It may well be that the chloramphenicol RNA, or more precisely the 15 £ 

 particle, is an artifact resulting from the cessation of protein synthesis, or 

 that it is a natural "free" intermediate of the particle, which would accumu- 

 late when its protein counterpart could not be formed. There are good 

 reasons to believe that the 15 S particles of Nomura and Watson are iden- 

 tical with the natural intermediate of the ribosomes discovered by Roberts 

 el al., bl which consists of free, highly polymerized RNA associated with a 

 small amount of protein, and which accumulates after chloramphenicol 

 addition. 



4. Necessity of Amino Acids for the Synthesis of RNA 



Since RNA synthesis can take place in the presence of chloramphenicol 

 it might be expected also to occur when inhibition of protein formation 

 results from another cause, such as the omission of an essential amino acid. 

 Contrary to expectation, an amino acid auxotroph, when starved of its 

 essential amino acid, does not synthesize any nucleic acid. 27,28162 This 

 observation, which has been confirmed for a large variety of auxotrophs, 

 has been interpreted as a proof that amino acids are necessary for the syn- 

 thesis of RNA at a stage where they are not yet assembled into a peptide 

 chain. This interpretation is strengthened by the following experiment: if, 



162 M. K. Sands and R. B. Roberts, J. Bacteriol. 63, 505 (1952). 



