422 F. GROS 



nucleic acids. Evidence of the stability of the nucleic acids was obtained 

 from Hershey's early experiments with E. coli 101 and more recently from 

 those of Manson, 102 of Siminovitch and Graham, 103 and of Koch and Levy. 91 



Nevertheless, several observations indicate that under suitable conditions 

 some of the cellular RNA can undergo partial or complete turnover. For 

 instance, in a mutant of E. coli requiring both thymine and uracil, signifi- 

 cant incorporation of radioactive precursors takes place in RNA in the 

 absence of the two pyrimidines. 104 The same observation has been made 

 with another uracil-dependent mutant of E. coli. 10b Another possible exam- 

 ple of RNA turnover comes from the fact that the purine analog, 8-aza- 

 guanine, is incorporated either into a fraction of the RNA or on to an 

 appropriate site on the RNA in an apparently reversible manner, since 

 the incorporated analog can be rapidly displaced by the corresponding 

 natural base 106 " 108 and released in the medium. 



These few cases of metabolic renewal of the RNA may concern only the 

 soluble fraction of this nucleic acid, since it is known that the sRNA can 

 reversibly fix nucleotides in the terminal position. 109 Moreover, during in- 

 corporation of adenine-C 14 or P 32 by growing cultures of E. coli, the sRNA 

 becomes labeled much earlier than do the RNA particles. 



Finally, a few special cases, in which bacterial RNA manifests an ex- 

 tremely active renewal, must be mentioned. For instance, the RNA which 

 is synthesized in the presence of chloramphenicol, undergoes very rapid 

 destruction after the removal of the inhibitor. 110, U1 Also, after infection 

 of E. coli by phage T2 or T6 no net RNA synthesis occurs, but P 32 and radio- 

 active bases are still actively incorporated into the "RNA pool." The rela- 

 tive rates of incorporation into the nucleotides of the RNA suggest the 

 reversible formation of a new RNA species, the base composition of which 

 is specific for the infecting phage. 112 



It has recently been shown that bacteria grown in a medium of very low 



101 A. D. Hershey, J. Gen. Physiol. 38, 145 (1954). 



102 A. L. Manson, J. Bacterial. 66, 703 (1953). 



103 L. Siminovitch and A. F. Graham, J . Histochem. and Cytochem. 4, 508 (1956). 



104 H. D. Barner and S. S. Cohen, Biochim. el Biophys. Acta 30, 12 (1958). 



105 A. B. Pardee, J. Baeteriol. 69, 233 (1955). 



106 H. G. Mandel, J. Biol. Chern. 225, 137 (1957). 



107 R. E. F. Matthews and J. D. Smith, Nature 177, 271 (1956). 



108 H. Chantrenne and S. Devreux, Exptl. Cell Research, Suppl. 6, 152 (1958). 



109 L. I. Hecht, P. C. Zamecknik, M. L. Stephenson, and J. F. Scott, J. Biol. Chem. 

 235, 954 (1958). 



110 F. C. Neidhardt and F. Gros, Biochim. el Biophys. Acta25, 513 (1957). 



111 F. E. Hahn, M. Schaechter, W. S. Ceglowski, H. E. Hopps, and J. Ciak, Biochim. et 

 Biophys. Acta 26, 469 (1957). 



112 E. Volkin and L. Astrachan, Virology 2, 149 (1956). 



